What does a leaf develop from? The structure of the leaf of a plant, the types of arrangement of leaf plates, photosynthesis and transpiration
LEAF - LATERAL SHOOTING organ
General characteristics of the sheet
Sheet- flattened lateral organ of the shoot with bilateral symmetry; it is laid in the form of a leaf tubercle, which is a lateral protrusion of the shoot. The leaf rudiment increases in length due to the growth of the apex and in width due to marginal growth. In seed plants, apical growth quickly stops. After the deployment of the kidney, there is a multiple division of all leaf cells (in dicotyledons) and an increase in their size. After differentiation of meristem cells into permanent tissues, the leaf grows at the expense of the meristem at the base of the leaf blade. In most plants, the activity of this meristem ends quickly, and only in a few, such as clivia, amaryllis, continues for a long time.
In annual herbaceous plants, the life span of the stem and leaf is almost the same - 45-120 days, in evergreens - 1-5 years, in conifers, such as fir - up to 10 years.
The first leaves of seed plants are represented by the cotyledons of the embryo. The next (true) leaves are formed in the form of meristematic tubercles - primordians, arising from the apical meristem of the shoot.
The leaf performs three main functions: photosynthesis, gas exchange and transpiration. In addition, it can be an organ of protection (scales, spines), attachment to a support (antennae), a supply of nutrients and water, as well as vegetative propagation.
The main functions of a leaf are photosynthesis, transpiration and gas exchange.
leaf morphology.
The main part of the sheet is leaf blade. The lower part of the leaf, articulated with the stem, is called basis sheet. Quite often, a stem-like cylindrical or semicircular in section is formed between the base and the plate. petiole sheet. In this case, the leaves are called petiolate, Unlike sedentary leaves without petiole. The role of the petiole, in addition to supporting and conducting, is that it retains the ability to intercalate growth for a long time and can regulate the position of the plate, bending towards the light.
The base of the sheet can take a different shape. Sometimes it is almost imperceptible or looks like a slight thickening ( leaf pad), for example, sour. Often the base grows, covering the entire node and forming a tube called vagina sheet. The formation of a sheath is especially characteristic of monocots, in particular for cereals, and from dicots, for umbrellas. The sheaths protect the interstitial meristems at the base of the internodes and the axillary buds above the nodes.
Often the base of the leaf gives paired lateral outgrowths - stipules. The shape and size of the stipules are different in different plants. In woody plants, stipules usually look like membranous scaly formations and play a protective role, constituting the main part of the kidney integument. At the same time, they are short-lived and fall off when the buds expand, so that stipules are not found on an adult shoot in fully developed leaves (birch, oak, linden, bird cherry). Sometimes the stipules are green in color and function along with the leaf blade as photosynthetic organs (many legumes and rosaceae).
All representatives of the buckwheat family are characterized by the formation bells. The bell is formed by the fusion of two axillary stipules and surrounds the stem above the node in the form of a short membranous tube.
The main part of the assimilating leaf is its lamina. If a leaf has one plate, it is called simple. At difficult leaves on one petiole with a common base there are two, three or more separate plates, sometimes with their own petioles. Individual records are called leaflets complex sheet, and the common axis carrying the leaves is called rachis. Depending on the location of the leaves on the rachis, they distinguish pinnate- and palmately complex leaves. In the first, the leaves are arranged in two rows on both sides of the rachis, which continues the petiole. The palmately compound leaves do not have rachis, and the leaflets extend from the top of the petiole. A special case of a complex sheet - ternary.
Rice. Sheet parts (diagram): 1 - petiolate leaf; 2 - sessile leaf; 3 - sheet with a small pillow at the base; 4 - vaginal leaves; 5 - leaf with free stipules; 6 - leaf with stipules adhering to the petiole; 7 - leaf with axillary stipules; Pl- plate; os- base; Vl- vagina; Etc- stipules; H- petiole; PP- axillary kidney; THEM- intercalary (intercalary) meristem.
Rice. Compound leaves (diagram): A - unpaired pinnate; B - paired pinnate; B - ternary; G - palmately complex; D - doubly paroperistoslozhny; E - twice unpaired pinnate; 1 - leaflet; 2 - petiole; 3 - rachis; 4 - petiole; 5 - stipules; 6 - rachis of the second order.
The process of forming a complex leaf resembles branching, which can go up to the second or third order, and then twice and thrice pinnate leaves. If the rachis ends with an unpaired leaf, the leaf is called unpaired pinnate if a couple of leaves - paired pinnate.
When characterizing a leaf blade, a number of features are taken into account: the general outlines (contours) of the leaf, the shape of the base and top, the shape of the edge, venation, the nature of the surface, texture, and other features.
The leaf blade or leaflet may be whole or dismembered more or less deep blades,shares or segments, located at the same time pinnate or palmately. Distinguish pinnate- and palmate-lobed,pinnate- and palmately divided and pinnate- and palmately dissected leaves . There are twice, thrice and repeatedly dissected leaf blades.
The forms of whole leaf blades and dissected leaves in general outline are distinguished depending on two parameters: the ratio between length and width and in which part of the blade its greatest width is located.
Rice. Forms of leaf blades: 1 - needle; 2 - heart-shaped; 3 - kidney-shaped; 4 - swept; 5 - spear-shaped; 6 - crescent.
When describing, also pay attention to the shape of the top, base and edge of the plate. .
Rice. The main types of tops, bases and edges of leaf blades: A - tops: 1 - sharp; 2 - pointed; 3 - dull; 4 - rounded; 5 - truncated; 6 - notched; 7 - pointed; B - bases: 1 - narrow wedge-shaped; 2 - wedge-shaped; 3 - wide wedge; 4 - descending; 5 - truncated; 6 - rounded; 7 - notched; 8 - heart-shaped; B - edge of the sheet: 1 - serrate; 2 - doubly serrate; 3 - gear; 4 - crenate; 5 - notched; 6 - solid.
Classification of leaves with their petioles
During leaf fall, the leaves of a compound leaf fall first, and then the rachis (the legume and rosaceae families).
Among simple leaves Distinguish leaves with a whole and dissected leaf blade. simple leaves with whole The leaf blade is characterized by:
The shape of the leaf blade is round, ovoid, oblong, etc.;
The shape of the base of the leaf is heart-shaped, spear-shaped, swept, etc.;
The shape of the edge of the leaf blade is serrated, serrated, pitted, etc.
simple leaves with dismembered leaf blade, depending on the venation (fingered or pinnate) and the degree of depth of dissection, are divided into:
On palmate-lobed, or cirro-lobed, if the dissection of the leaf blade reaches 1/3 of the width of the blade or half-blade;
Palmate-separate, or pinnatipartite, if the dissection of the leaf blade reaches 1/2 of the width of the blade or half-blade;
Palmately dissected, or pinnately dissected, if the degree of dissection of the leaf blade reaches its base or central vein.
Rice. Simple leaves with entire leaf blade
compound leaves are ternary, consisting of three leaves (strawberry), and palmate, consisting of many leaves (chestnut). In these types of compound leaves, all leaflets are attached to the top of the rachis.
In addition, some compound leaves have leaflets along the entire length of the rachis. Among them, there are paired-pinnate, if they end at the top of the leaf blade with a pair of leaves (sowing peas), and odd-pinnate (common mountain ash), ending with one leaflet.
Rice. Compound and simple leaves with dissected leaf blade
Venation
One of the important descriptive features of a leaf is the nature of the venation.
Venation- this is a system of conducting bundles and tissues accompanying them, through which the transport of substances in the sheet is carried out.
Vein The leaf is represented by a vascular fibrous bundle and performs conductive and mechanical functions. The veins entering the leaf from the stem through the base and petiole are called the main ones. depart from the main lateral veins of the first, second and subsequent orders. Between themselves, the veins can be connected by a network of small veins - anastomoses.
dichotomous venation (the main vein branches forked) is characteristic of most ferns, and of gymnosperms - ginkgo. In this case, there are no anastomoses, and the endings of the veins approach the edge of the leaf blade.
Arc and parallel venation more common in monocotyledonous plants. With arc venation, non-branching veins are arranged in an arcuate manner and converge at the top and base of the leaf blade (lily of the valley). With parallel venation, the veins of the leaf blade run parallel to each other (cereals, sedges).
Palmate venation - several main veins of the first order (in the form of fingers) enter the leaf blade from the petiole. Veins of subsequent orders depart from the main ones (typical for dicotyledonous plants, for example, for the Tatar maple).
Pinnate venation - the central vein is expressed, coming from the petiole and strongly branching in the leaf blade in the form of a feather (typical for dicotyledonous plants, for example, for a bird cherry leaf).
A variety of pinnate venation - mesh venation, when many veins are connected by anostomoses, forming a pattern resembling a grid.
Rice. Venation types: a- arc; b- parallel; in- finger; G- pinnate
leaf formations. Within the shoot, the leaves are not the same. When a plant is grown from a seed, the first leaves of the embryo appear - the cotyledons (they are usually very simple outlines). Then in the middle part of the shoot develop middle leaves, which are colored green, as they carry the function of assimilation. They are characterized by the largest size and degree of leaf dissection - into a base with stipules, a petiole and a leaf blade.
riding leaves develop in the area of the inflorescence. These are the covering leaves of flowers - bracts. They are underdeveloped and poorly dissected.
The consistency is often filmy, the color is green. Often, upper leaves perform an additional function - attracting pollinating insects, then their color is bright white, pinkish, red, lilac, etc. 
Lateral shoots usually develop from axillary buds. The kidneys are protected from the outside by grassroots leaves - kidney scales. In form, they are very simple, as they represent a wide base of a leaf devoid of a plate, petiole and stipules.
Grassroots the leaves are initially white, but turn brown as they age, and black when they die. They are adapted to perform the function of protection or reserve, or both together (lily).
Heterogeneity(heterophylly) - in a broad sense, this is a difference in the shape, size and structure of leaves on one plant. The leaf formations described above are a manifestation of heterophylly. In a narrower sense, heterophylly is the difference between the leaves of the middle formation within the plant, usually associated with the influence of the external environment. Heterophylly is especially well expressed in aquatic plants (arrowhead, handbill, water ranunculus). Their underwater leaves are ribbon-like or repeatedly filiformly dissected, the surface leaves are entire or lobed.
Three leaf formations of May lily of the valley:1 - grassroots; 2~ median; 3 - riding
The anatomical structure of the leaf blade
The meristem cells of the leaf rudiment differentiate into the primary integumentary tissue - the epidermis, the main parenchyma and mechanical tissues. The layers of procambium, which originated from the middle meristematic layer of the leaf rudiment, differentiate into vascular bundles.
The structural features of the leaf are determined by its main function - photosynthesis. Therefore, the most important part of the sheet is mesophyll where chloroplasts are located and photosynthesis takes place. The remaining tissues ensure the normal functioning of the mesophyll. Epidermis, covering the leaf, regulates gas exchange and transpiration. branched system conductive beams supplies the leaf with water necessary for the normal course of photosynthesis, and ensures the outflow of assimilates. M mechanical fabrics provide strength to the sheet.
Mesophyll occupies the entire space between the upper and lower epidermis, excluding conductive and mechanical tissues. The cells of the mesophyll are quite uniform, most often rounded or slightly elongated in shape. Cell walls remain thin and non-lignified. The protoplast consists of a parietal layer of cytoplasm with a nucleus and numerous chloroplasts. There is a large vacuole in the center of the cell. Sometimes the cell walls form folds that increase the surface of the parietal layer of the cytoplasm and allow the placement of a larger number of chloroplasts.
In most plants, the mesophyll is differentiated into palisade(columnar) and spongy fabrics. The cells of the palisade mesophyll, usually located under the upper epidermis, are elongated perpendicular to the leaf surface and form one or more layers. The cells of the spongy mesophyll are connected more loosely, the intercellular spaces here can be very large compared to the volume of the cells themselves. The increase in intercellular spaces is often achieved by the fact that the cells of the spongy mesophyll form outgrowths.
The palisade tissue contains about three-quarters of all leaf chloroplasts and performs the main job of carbon dioxide assimilation. Therefore, the palisade tissue is located in the best light conditions, directly under the upper epidermis. Due to the fact that the cells are elongated perpendicular to the leaf surface, light rays penetrate deeper into the mesophyll more easily.
Gas exchange occurs through the spongy mesophyll. Carbon dioxide from the atmosphere through the stomata, located mainly in the lower epidermis, penetrates into the large intercellular spaces of the spongy mesophyll and diverges freely inside the leaf. Oxygen released during photosynthesis moves in the opposite direction and exits through the stomata into the atmosphere. The location of stomata mainly on the underside of the leaf is explained not only by the position of the spongy mesophyll. The loss of water by the leaf during transpiration is slower through the stomata located in the lower epidermis. In addition, the main source of carbon dioxide in the atmosphere is "soil respiration", i.e., the release of CO 2 as a result of the respiration of numerous living creatures that inhabit the soil.
The thickness of the palisade and spongy tissue and the number of cell layers in them are different depending on the lighting conditions. Even within the same individual, leaves grown in the light ( rice. 4.59), have a more developed columnar mesophyll than leaves grown under shading conditions ( rice. 4.60).
In shade-loving forest plants, the palisade mesophyll consists of a single layer of cells that have the characteristic shape of wide-open funnels ( rice. 4.61). Large chloroplasts are located in them so that they do not obscure each other. Spongy mesophyll also consists of one or two layers. On the contrary, in plants of open habitats, the palisade mesophyll has several layers of cells and has a significant total thickness ( rice. 4.62).
Leaves in which the palisade tissue is located on the upper side of the plate, and the spongy tissue on the lower side, are called dorsoventral.
If the underside of the leaves receives enough light, then a palisade mesophyll will form on it ( rice. 4.63). Leaves with the same mesophyll on both sides are called isolateral.
In pine needles, the assimilation part of the leaf is represented by a folded chlorenchyma located around the central axial cylinder. The structure of such leaves is called radial.
Not in all plants, the mesophyll is differentiated into palisade and spongy tissues, often (especially in monocots) the mesophyll is completely homogeneous ( rice. 4.64).
Rice. 4.62. Cross section of a camellia leaf: 1 - upper epidermis; 2 - columnar mesophyll; 3 - spongy mesophyll; 4 - cell with a druze; 5 - sclereid; 6 - conducting beam; 7 - lower epidermis; 8 - stomata.
In the mesophyll of the leaves, cells with calcium oxalate crystals are often found, the shape of the crystals plays an important role in the diagnosis of medicinal plant materials.
The integumentary tissue of the leaf is always epidermis. Variations in its structure depend on habitat conditions and are expressed in the thickness of the cuticle and wax formations, in the presence of different types of trichomes, in the nature, number and placement of stomata. On leaves oriented with their upper side towards the light, stomata are more often located in the lower epidermis ( hypostomatic leaves). With uniform illumination of both sides, stomata are usually present on both sides ( amphistomatic leaves). Stomata can be located exclusively on the upper side, for example, in leaves floating on the surface of the water ( epistomatic leaves).
Conductive tissues in leaves united in closed collateral bundles. The xylem is turned to the top, and the phloem is turned to the underside of the leaf. With such an organization, the conductive tissues of the stem and leaves form a single continuous system. Conductive bundles with their surrounding tissues are called veins. Large veins often protrude strongly above the surface of the leaf, especially from the underside. Smaller bundles are completely immersed in the mesophyll. The veins usually form a network with closed cells, but the smallest of them may have blind endings in the mesophyll. The conductive elements of the bundles do not directly come into contact with mesophyll cells and intercellular spaces. In larger bundles they are surrounded by sclerenchyma, and in small bundles they are tightly closed. lining cells. Parietal cells differ from neighboring mesophyll cells in larger sizes, they are often devoid of chloroplasts. The parietal cells, similarly to the endoderm of the axial organs, regulate the short-range transport of substances in the leaf. 
Rice. 4.66. Cross section of a corn leaf in the region of a large conductive bundle: 1 - cuticle; 2 - upper epidermis; 3 - sclerenchyma; 4 - mesophyll cells; 5 - chloroplasts; 6 - parietal cells; 7 - xylem; 8 - phloem; 9 - lower epidermis; 10 - air cavity.
mechanical fabrics sheets play the role of reinforcement and resist its rupture and crushing. These are sclerenchyma fibers, individual sclereids and strands of collenchyma. Combining with living elastic cells of the mesophyll, mechanical elements form something like reinforced concrete. Securely interconnected, the cells of the epidermis play the role of an external binding that increases the overall strength of the sheet. Sclerenchymal fibers most often accompany large vascular bundles. They surround conductive tissues from all sides or only from above and below ( rice. 4.66). Collenchyma is more often present near large tufts or along the edge of the leaf, protecting it from tearing. Sclereids of various shapes are found in the mesophyll of some plant species with dense leathery leaves (pod, camellia). Leaf strength can be very high. In many palm trees, the leaves reach a length of several meters, but, despite the wind, heavy rains, etc., they retain their shape and position in space.
According to the anatomical structure, isolateral, dorsoventral and radial leaves are distinguished.
Leaf structure of the dorsoventral structure
Above and below the sheet is covered with a living single-layer epidermis. The upper epidermis, compared to the lower epidermis, is represented by larger cells and is covered with a cuticle. Often the upper epidermis is covered with wax, which enhances the protective function of the leaf from water loss. The cells of the epidermis are tightly closed, which is facilitated by their sinuous outlines. Epidermal cells play a significant role in the formation of trichomes. Trichomes can be of various shapes: unicellular, multicellular, branched, in the form of bristles, stellate. In the cells of trichomes, the protoplast dies, the contents are filled with air; their main function is to protect against water loss, overheating, and being eaten by animals.
Stomata are located in the epidermis. They are more common in the lower epidermis, but can also be found on both sides; aquatic plants with floating leaves have stomata only in the upper epidermis. If in dicotyledonous plants the stomata are located quite freely throughout the epidermis, then in monocotyledonous plants with linear leaves they are in even rows, and the stomatal gaps are oriented along the leaf axis. Stomata are always accompanied by air cavities through which transpiration and gas exchange take place.
Under the upper epidermis in 1-3 layers is placed columnar mesophyll(columnar chlorenchyma). Its cells are cylindrical in shape, with the narrow side adjoining the epidermis. This highly specialized tissue is involved in photosynthesis. The cylindrical shape of the cells ensures the safety of chlorophyll in chloroplasts. Being most of the time on elongated radial walls, lenticular chloroplasts are not exposed to direct sunlight. The rays slide along them, evenly illuminating the chloroplasts, without destroying the chlorophyll. All this contributes to the active flow of photosynthesis.
Below lies spongy mesophyll, characterized by loosely arranged rounded cells with large intercellular spaces. The spongy mesophyll, like the columnar mesophyll, contains chloroplasts, but their number in the cells is 2-6 times less than in the cells of the columnar chlorenchyma. The main functions of spongy tissue are transpiration and gas exchange, but it is also involved in photosynthesis.
Rice. Scheme of the structure of the dorsoventral sheet: 1 - upper epidermis; 2 - columnar chlorenchyma; 3 - sclerenchyma; 4 - core rays of xylem; 5 - xylem vessels; 6 - phloem; 7 - spongy chlorenchyma; 8 - air cavity; 9 - stomata; 10 - collenchyma; 11 - lower epidermis
Rice. Volumetric image of a part of the leaf blade: 1 - upper epidermis; 2 - glandular hair; 3 - covering hair; 4 - palisade (columnar) mesophyll; 5 - spongy mesophyll; 6 - collenchyma; 7 - xylem; 8 - phloem; 9 - parietal sclerenchyma of the bundle; 10 - lower epidermis; 11 - stomata
Large leaf veins are represented by a complete vascular-fibrous bundle, while small ones are incomplete. At the top of the complete vascular-fibrous bundle is the xylem, and below it is the phloem. As a rule, they are devoid of cambium, but in some dicotyledonous plants traces of cambium activity are visible, which stops its work early. In dicotyledonous plants, a sclerenchyma lining lies in a ring around the bundle, protecting the bundle from the pressure of growing cells of the leaf mesophyll. Above and below the bundle there is an angular or lamellar collenchyma adjacent to the epidermis and performing a supporting function. Small veins run in the thickness of the mesophyll under the columnar chlorenchyma. Sclerenchyma may be located in patches or around these veins.
The structure of the sheet of the radial structure
The structure of the leaves of coniferous plants on the example of pine needles. Epidermal cells are thick-walled, lignified, almost square in shape, covered with a thick cuticle layer. Under the epidermis is the hypodermis; it lies in one layer, and in the corners - several layers. The cells of the hypodermis become lignified over time and perform water storage and mechanical functions. On both sides of the leaf there are submerged stomata, under which lie large air cavities.
Rice. The general plan of the structure of the camellia leaf: 1, 7 - corner collenchyma; 2 - epidermis; 3 - bunch of lateral vein; 4 - bundle of the central vein; 5 - xylem; 6 - phloem
The organism of a flowering plant is a system of roots and shoots. The main function of aboveground shoots is the creation of organic substances from carbon dioxide and water using solar energy. This process is called air nutrition of plants.
The shoot is a complex organ consisting of a stem, leaves, and buds formed during one summer.
main escape- a shoot that develops from the bud of the seed germ.
side escape- an escape that appeared from the lateral axillary bud, due to which the stem branches.
Elongated escape- escape, with elongated internodes.
Shortened Escape- escape, with shortened internodes.
vegetative shoot- shoot bearing leaves and buds.
generative escape- an escape bearing reproductive organs - flowers, then fruits and seeds.
Branching and tillering shoots
branching- this is the formation of lateral shoots from axillary buds. A highly branched system of shoots is obtained when side shoots grow on one (“mother”) shoot, and on them, the next side ones, and so on. In this way, as much air supply medium as possible is captured. The branched crown of the tree creates a huge leaf surface.
tillering- this is branching, in which large side shoots grow from the lowest buds located near the surface of the earth or even underground. As a result of tillering, a bush is formed. Very dense perennial bushes are called tufts.
Shoot branching types
In the course of evolution, branching appeared in thallus (lower) plants; in these plants, the growth points simply bifurcate. Such a branch is called dichotomous, it is characteristic of pre-shoot forms - algae, lichens, liverworts and anthocerot mosses, as well as outgrowths of horsetails and ferns.
With the appearance of developed shoots and buds, monopodial branching, in which one apical bud retains its dominant position throughout the life of the plant. Such shoots are ordered, and the crowns are slender (cypress, spruce). But if the apical bud is damaged, this type of branching is not restored, and the tree loses its typical appearance (habitus).
The most recent type of branching in time of occurrence - sympodial, in which any nearest bud can develop into an escape and replace the previous one. Trees and shrubs with this type of branching are easy to pruning, crown formation, and in a few years they are overgrown with new shoots without losing their habit (linden, apple, poplar).
A kind of sympodial branching false dichotomous, which is characteristic of shoots with an opposite arrangement of leaves and buds, therefore, instead of the previous shoot, two grow at once (lilac, maple, mock orange).
The structure of the kidneys
Bud- a rudimentary, not yet unfolded shoot, at the top of which there is a growth cone.
Vegetative (leaf bud)- a bud consisting of a shortened stem with rudimentary leaves and a growth cone.
Generative (flower) bud- a bud, represented by a shortened stem with the rudiments of a flower or inflorescence. A flower bud containing 1 flower is called a bud.
apical bud- a bud located at the top of the stem, covered with young leaf buds overlapping each other. Due to the apical bud, the shoot grows in length. It has an inhibitory effect on the axillary kidneys; removing it leads to the activity of dormant kidneys. Inhibitory reactions are disturbed, and the kidneys open.
At the top of the embryonic stem is the growth part of the shoot - growth cone. This is the apical part of the stem or root, consisting of educational tissue, the cells of which are constantly dividing by mitosis and give the organ an increase in length. At the top of the stem, the growth cone is protected by bud scaly leaves; all elements of the shoot are laid in it - the stem, leaves, buds, inflorescences, flowers. The root growth cone is protected by a root cap.
Lateral axillary kidney- a bud that occurs in the axil of the leaf, from which a lateral branching shoot is formed. The axillary buds have the same structure as the apical bud. Lateral branches, therefore, also grow with their tips, and on each side branch the terminal bud is also apical.
At the top of the shoot, there is usually an apical bud, and axillary buds in the axils of the leaves.
In addition to apical and axillary buds, plants often form so-called adnexal buds. These kidneys do not have a certain regularity in location and arise from internal tissues. The source of their formation can be the pericycle, cambium, parenchyma of the medullary rays. Adnexal buds can form on stems, leaves, and even roots. However, in structure, these kidneys are no different from ordinary apical and axillary ones. They provide intensive vegetative renewal and reproduction and are of great biological importance. In particular, with the help of adventitious buds, root shoot plants reproduce.
dormant buds. Not all buds realize their ability to grow into a long or short annual shoot. Some buds do not expand into shoots for many years. At the same time, they remain alive, capable, under certain conditions, of developing into a leafy or flower-bearing shoot.
They seem to be sleeping, which is why they are called sleeping buds. When the main trunk slows down its growth or is cut down, dormant buds begin to grow, and leafy shoots grow from them. Thus, dormant buds are a very important reserve for the growth of shoots. And even without external damage, old trees can “rejuvenate” due to them.
Dormant buds, very characteristic of deciduous trees, shrubs and a number of perennial herbs. These buds do not develop into normal shoots for many years, often dormant throughout the life of the plant. Usually dormant buds grow annually, exactly as much as the stem thickens, which is why they are not buried by growing tissues. The stimulus for awakening dormant buds is usually the death of the trunk. When birch is felled, for example, stump shoots are formed from such dormant buds. Sleeping buds play a special role in the life of shrubs. A shrub differs from a tree in its versatility. Usually, in shrubs, the main maternal stem does not function for long for several years. When the growth of the main stem is attenuated, dormant buds awaken and daughter stems are formed from them, which overtake the parent in growth. Thus, the shrub form itself arises as a result of the activity of dormant buds.
mixed kidney- a bud consisting of a shortened stem, rudimentary leaves and flowers.
kidney renewal- wintering bud of a perennial plant, from which an escape develops.
Vegetative propagation of plants
| Way | Picture | Description | Example |
Creeping shoots |  | Creeping shoots or tendrils, in the nodes of which small plants with leaves and roots develop | Clover, cranberry, chlorophytum |
rhizome |  | With the help of horizontal rhizomes, plants quickly capture a large area, sometimes several square meters. At the rhizomes, older parts gradually die off and collapse, and individual branches are separated and become independent. | Lingonberry, blueberry, wheatgrass, lily of the valley |
tubers |  | When there are not enough tubers, it is possible to propagate by parts of the tuber, eye-buds, sprouts and tops of the tubers. | Jerusalem artichoke, potatoes |
bulbs |  | From the lateral buds on the mother bulb, daughter ones are formed - babies that are easily separated. Each daughter bulb can give rise to a new plant. | onion, tulip |
leaf cuttings |  | The leaves are planted in wet sand, and adventitious buds and adventitious roots develop on them. | Violet, sansevier |
layering |  | In the spring, bend the young shoot so that its middle part touches the ground, and the top is directed upwards. On the lower part of the shoot under the kidney, it is necessary to cut the bark, pin the shoot to the soil at the place of the cut and spud it with moist earth. By autumn adventitious roots are formed. | Currant, gooseberry, viburnum, apple tree |
shoot cuttings | A cut branch with 3-4 leaves is placed in water, or planted in wet sand and covered to create favorable conditions. Adventitious roots form on the lower part of the cutting. | Tradescantia, willow, poplar, currant |
|
Root cuttings |  | The root cutting is a segment of the root 15-20 cm long. If you cut off a piece of dandelion root with a shovel, adventitious buds form on it in the summer, from which new plants | Raspberry, rosehip, dandelion |
Root offspring |  | Some plants are able to form buds on their roots. | |
Grafting with a cutting |  | First, annual seedlings are grown from seeds - wildlings. They serve as a base. Cuttings are cut from a cultivated plant - this is a scion. Then the stem parts of the scion and rootstock are connected, trying to connect their cambium. This makes the tissue grow more easily. | Fruit trees and shrubs |
Kidney vaccination |  | A one-year-old shoot is cut from a fruit tree. Leaves are removed, leaving the petiole. An incision is made with a knife in the bark in the form of the letter T. A developed bud is inserted from a cultivated plant 2-3 cm long. The grafting site is tightly tied. | Fruit trees and shrubs |
tissue culture |  | Growing a plant from cells of educational tissue placed in a special nutrient medium. | Orchid, Carnation, Gerbera, Ginseng, Potato |
Modifications of underground shoots
Rhizome- an underground shoot that performs the functions of deposition of reserve substances, renewal, and sometimes vegetative propagation. The rhizome has no leaves, but has a well-pronounced metameric structure, the nodes are distinguished either by leaf scars and the remains of dry leaves, or by leaf scars and the remains of dry leaves, or by living scaly leaves and by the location of axillary buds. Adventitious roots may form on the rhizome. From the buds of the rhizome, its lateral branches and above-ground shoots grow.
Rhizomes are characteristic mainly of herbaceous perennials - hoof, violet, lily of the valley, couch grass, strawberry, etc., but are found in shrubs and shrubs. The life span of rhizomes varies from two to three to several decades.
tubers- thickened fleshy parts of the stem, consisting of one or more internodes. There are aboveground and underground.
Elevated- thickening of the main stem, side shoots. They often have leaves. Above-ground tubers are a reservoir of reserve nutrients and serve for vegetative propagation, they may contain metamorphosed axillary buds with leaf primordia, which fall off and also serve for vegetative propagation.
Underground tubers - thickening of the hypocotyl knee or underground shoots. On underground tubers, the leaves are reduced to scales that fall off. In the axils of the leaves are buds - eyes. Underground tubers usually develop on stolons - daughter shoots - from buds located at the base of the main shoot, look like very thin white stalks, bearing small colorless scale-like leaves, grow horizontally. Tubers develop from the apical buds of stolons.
Bulb- an underground, less often above-ground shoot with a very short thickened stem (bottom) and scaly, fleshy, succulent leaves that store water and nutrients, mainly sugar. Aerial shoots grow from the apical and axillary buds of the bulbs, and adventitious roots form on the bottom. Depending on the placement of the leaves, bulbs are scaly (onion), tiled (lily) and prefabricated or complex (garlic). In the sinus of some scales of the bulb there are buds from which the daughter bulbs develop - babies. Bulbs help the plant survive in adverse conditions and are the organ of vegetative reproduction.
Corms- outwardly similar to bulbs, but their leaves do not serve as storage organs, they are dry, membranous, often these are the remains of the sheaths of dead green leaves. The storage organ is the stem part of the corm, it is thickened.
Aboveground stolons (lashes)- short-lived creeping shoots that serve for vegetative propagation. They are found in many plants (drupe, bent grass, strawberry). Usually they lack developed green leaves, their stems are thin, fragile, with very long internodes. The apical bud of the stolon, bending upward, gives a rosette of leaves, which takes root easily. After the new plant takes root, the stolons are destroyed. The popular name for these aboveground stolons is mustache.
spines- shortened shoots with limited growth. In some plants, they form in the axils of the leaves and correspond to lateral shoots (hawthorn) or form on trunks from dormant buds (gleditsia). Characteristic for plants of hot and dry places of growth. They perform a protective function.
succulent shoots- above-ground shoots adapted for the accumulation of water. Usually, the loss or metamorphosis (turning into spines) of leaves is associated with the formation of a succulent shoot. The succulent stem performs two functions - assimilation and water storage. Typical for plants living in conditions of prolonged lack of moisture. Stem succulents are most represented in the cactus family, Euphorbiaceae.
Shoot growth occurs exogenously leaf rudiment. It is located below the top of the shoot and looks like an oval tubercle. The cells of the leaf rudiment divide in all directions, thus, the leaflet grows both in thickness and in height. As soon as the growth in thickness stops, the leaf takes on a flat appearance.
There are two parts of the leaf rudiment: apical(upper) and basal(lower). The apical growth of the leaf rudiment is limited and does not last long. When the top of the leaf stops growing, the base continues to grow. In other words, acropetal growth ends and basipetal growth begins. Thus, the apical meristem completes its growth function, and the intercalary meristems begin their development.
The leaf blade and petiole develop directly from the upper part of the leaf rudiment, and the base of the leaf and stipules develop directly from the lower part. Sometimes the laying of parts of the leaf is already formed in the kidney, and when it enters from the kidney, the already laid parts grow and their anatomical structures differentiate. One of the last to grow is the petiole.
Remark 1
It is worth noting that not all leaves have a petiole.
The leaf blade increases in size fairly evenly. The leaf is monosymmetrical in most plants. The leaf has two surfaces - dorsal (dorsal) and ventral (abdominal). The dorsal surface in the kidney is located inside, thus adjacent to the stem, and in the developed leaf - at the top. The abdominal, on the contrary, is located outside in the kidney, and below in the developed leaf.
Leaf modification (Metamorphoses)
The leaves are modified depending on the growing conditions of the plant, and in connection with the adaptation of plants to certain functions. Spines, scales, tendrils, phyllodes, the growth of hairs on the leaf are all a modification of the leaves.
Spines in plants perform two functions, less evaporation of water (cactus in the desert) and protection from animals. The spines have a different arrangement on the stem. For example, in barberry, the thorn is located under the leaf; in hawthorn, it is located in the axil of the leaf. In a cactus, the leaf blade has turned into a thorn. In the astragalus, the rachis of a complex leaf has changed into a thorn, in the acacia, the stipule has changed.
The shoots of vines have adapted for support in order to take a certain position in space. A similar function is performed by modified leaves into tendrils in peas, ranks, they help the plant move due to their tenacity.
Scales in bulbous plants play a special role, they accumulate nutrients. Also, the covering scales of the kidneys, bulbs, rhizomes perform a protective function.
Trapping devices, as a modification of leaves, are characteristic of insectivorous plants. The leaves are modified and resemble water lilies, urns, slamming sticky plates. The sticky hairs of the sundew nourish the plants, the insect gets on the sticky surface, the leaf closes and the decomposition of the animal begins, under the action of enzymes. This modification occurred due to the fact that the plant grew on soil with a deficiency of minerals.
Saccular modifications of leaves are found in an epiphytic plant of a humid tropical forest. Water and humus accumulate in such formations. As a result, adventitious roots are formed in the leaves, supplying the plants with moisture.
Phyllodes cover the shoots of the club moss, i.e. are outgrowths on the stem. They are green, and so on. they can photosynthesize, or they can carry sporangia in the form of sacs in which spores are formed. Phyllodes are also found in acacia. The petiole of acacia is turned into a flat leaf-like formation.
The hairy coat and waxy coating on the leaves is adapted to retain moisture and delay the evaporation process. The shiny surface of the ficus reflects the rays of light, which contributes to less evaporation of water by the plant.
The teeth along the edges of plants are adapted to express the processes of photosynthesis and transpiration. Thus, condensation occurs, which leads to the formation of dew.
Pheromones, poisons, aromatic oils, crystallization minerals produced by leaves can repel pests. Petals pollinate insects.
Remark 2
Thus, the modification of leaves is able to adapt plants to the environment, and have resistance to adverse conditions.
Leaf arrangement of leaves
Leaf arrangement of leaves, or phyllotaxis is the order in which the leaves are placed on the stem, thus reflecting the symmetry in the structure of the shoot. The arrangement of the leaf depends on the order of the laid leaf primordia on the cone of growth. In most plants, the leaves are located directly on the stems and branches, so that a general rule for their arrangement can be established. At first glance, the leaves seem to be arranged randomly. But if you look closely at the leaves, you will see that the leaves sit in pairs, one against the other, such an arrangement is called opposite. On some plants, the leaf pairs alternate so they are crossed over each other, this is called crossover. If there are three leaves on one node, which alternate with each other, and maybe even from $4-10$ or more leaves, then the arrangement is called ringed. If on stems with annular leaves, the leaves sit on top of each other, then several verticals and lines parallel to each other, which are called orthostichs, are obtained. If you draw a line from the very first bottom to the nearest sheet, then from the second to the nearest, etc. to the end, a spiral line is formed, then such a leaf arrangement is called spiral.
In each polynomial leaf arrangement, in addition to the main spiral, secondary steeper spirals are observed. They are called parastihas.
Picture 1.
If three or more leaves depart from the node, the leaf arrangement is called whorled. With a rosette leaf arrangement, the leaves are in a rosette, i.e. a bundle of leaves is arranged in a circle from one common center.
Leaf mosaic
Definition 1
Leaf mosaic- this is a kind of arrangement of the leaves of a plant in the same plane, in such a way as to ensure the least shading of each other's leaves. The leaves are directed perpendicular to the direction of the light rays. All this is the result of uneven growth of petioles and leaf blades that reach for the light and fill every lighted gap. A leaf mosaic is formed in absolutely any leaf arrangement - opposite, whorled, rosette, alternate or opposite opposite.
plant organ originally specialized for photosynthesis, i.e. nutrition of the organism, but in the course of evolution sometimes losing this function or acquiring additional functions. Of all the creations of nature, green, i.e. containing chlorophyll, the leaf is the most important structure for life on Earth. Without it, humans and other organisms could not exist. The atmospheric supply of oxygen is replenished by the continuous release of this gas from the leaf of green plants. Leaves absorb up to 400 billion tons of carbon dioxide per year, while binding 100 billion tons of carbon in organic compounds. It is these organic compounds formed in the leaves that serve as the primary source of food and vital vitamins for humans and all wild and domestic animals.
Leaves provide people with more than just oxygen and food. In the tropics, for example, people still live in huts covered with palm leaves. Throughout the world, one of the most important building materials is wood, which could not be formed without leaves on the trees. Aside from purely utilitarian needs, we should also remember that leaves make our life more pleasant and comfortable. Delicious and tonic drinks are prepared from them, for example, ordinary tea from the leaves of the tea bush or "mate" from the leaves of the Paraguayan holly - a shrub that grows along the banks of rivers in Argentina, Paraguay and southern Brazil. Smoking tobacco leaves Nicotiana tabacum) helps a lot of people relax. From the leaves of various plants, such as coca, foxglove, belladonna, potent medicines are obtained. Aloe leaves present ( Aloe vera) contain substances that cure some dermatitis, relieve pain from radiation and sunburn, and soften the skin. Some leaves, which have a pleasant aroma, are used directly as spices or serve as raw materials for the production of fragrant extracts. It is this application that finds, for example, basil leaves, laurel, marjoram, thyme, lavender and peppermint. From the leaves of sansevera cylindrical ( Sansevieria cylindrica) and sisal agave ( Agave sisalana) receive fiber for the manufacture of ropes, mats, bedspreads and hats are woven from the leaves of some other species.
Main parts and general characteristics.
A typical leaf consists of three parts: lamina, petiole, and stipules, small leaf-like structures at the base of the petiole. The main part is a plate, usually thin, flat and green. However, in some plants, its color is different, for example, dark red in Herbst's iresina, which is popular among flower growers ( Iresine herbstii), variegated in coleus (nettle), or silver in cypress santolina ( Santolina chamaecyparissus), also known as cypress grass. Sometimes the leaf surface is pubescent, i.e. covered with hairs - outgrowths of outer cells.
The petioles of some leaves, such as celery and rhubarb, are very large and are eaten. Sometimes there are no petioles at all, and the leaf blade is attached directly to the stem. Such leaves are called sessile. They are characteristic, in particular, of diervilla sessile ( Diervilla sessilifolia), belonging to the honeysuckle family. The stipules of most plants are small, but sometimes they are quite comparable in size to the leaf blade, like garden peas or Japanese chaenomeles. In some cases, such as white locust ( Robinia pseudoacacia), stipules are transformed into spines.
Leaf shape is one of the hallmarks of a plant species. A leaf can be simple or complex, i.e. consisting of several leaflets, depending on whether he has one plate or several. So, birches, beeches, elms, oaks and plane trees have simple leaves, while horse chestnuts, white acacia, wild rose, ailanthus and walnuts have complex leaves. Compound leaves are pinnately and palmately compound. In the first case, the leaves are arranged in two opposite rows along a common axis, as, for example, in white acacia and walnut, and in the second, they depart from one point, as, say, in horse chestnut or clover.
The size of the leaves varies widely depending on the taxon and even within the same plant species. They can reach a length of 20 m, for example, near a palm tree. Raphia ruffia growing in tropical Africa and Madagascar. Very small leaves in vegetable asparagus ( Asparagus officinalis var. Altilis), horsetail casuarina ( Casuarina equisetifolia) and tamarisk, or comb ( Tamarix spp.).
In most cases, the leaves are broad and flat, but sometimes they are cylindrical, like onions, needle-shaped, like pines, or scale-like, like cypresses. There are leaves linear (for cereals), rounded (for nasturtium), ovate (for the frame), heart-shaped (for linden), lanceolate (for willow), etc. Sometimes there is a so-called. heterophilia ("diversity") - on the same plant, leaves of different shapes are formed; for example, in sassafras officinalis, there are five variants.
Leaves with smooth edges are called entire. Among trees, such leaves can be seen, for example, in dogwood, lilac, rhododendron, eucalyptus, tiled, loose-leaved and virgin oaks. In many cases, the edges of the leaf blade are lobed, dissected, serrated, notched. For example, in red oak, the leaves are pinnately lobed with spiny protrusions of the veins at the tops of the lobes, and in white oak, the leaves are pinnately lobed or smoothly notched without sharp corners.
In most plants, the leaf arrangement is alternate, or spiral: leaves, like buds with lateral shoots, depart one by one from each node, either on one or the other side of the stem. An example is all birches, elms, oaks and nuts. In some species, in particular maples, viburnum and dogwood, leaves, buds and side shoots are opposite - on opposite sides of each node. When three leaves or more leave the node, the leaf arrangement is called whorled. In any case, the leaves move away from the stem so as to minimally obscure each other. They form a kind of “leaf mosaic” in space, designed to capture as much sunlight as possible falling on the plant.
Leaf plate.
A typical leaf blade consists of a thin layer of superficial cells - the epidermis and a layered inner tissue underneath - the mesophyll. The mesophyll is pierced by a system of veins. On a thin section of a leaf under a microscope, it can be seen that the outside of the epidermis is covered with a cuticle - a film consisting of a waxy cutin. This film is interrupted in places by inclusions of pectin-like substances. Through such areas, the leaf can absorb substances containing nitrogen, phosphorus, potassium and other elements necessary for the nutrition and normal life of the plant from the solutions that fall on its surface. The cuticle and epidermis protect the inner cells from rapid desiccation, and the thickness of these outer layers is often indicative of a species' adaptation to its environment. Thus, in pines and other narrow-leaved evergreens, a powerful cuticle is very effective in slowing down evaporation, especially in winter, when the frozen soil contains little water available to the roots.
The cuticle and epidermis are pierced by tiny holes - stomata, the number of which is not the same on both sides of the leaf. Each stomata is a gap between two bean-shaped guard cells, which, slightly changing shape, open or close it. This regulates the intensity of transpiration, i.e. plant water loss. When the stomata are open, water vapor escapes through them into the atmosphere and this ensures the upward movement of new portions of water with salts dissolved in it from the roots to the leaves and other parts of the shoots. Through the stomata, the plant also exchanges gases with the environment. Guard cells are sensitive to the level of illumination: when it rises, the stomata open wider, when it gets dark, the stomatal gap becomes narrower. Thus, stomatal gas exchange and transpiration are much more intense during the day than at night.
In the epidermis of the leaf there are also specialized stomata - hydathodes that release water in the form of droplets. This process is called guttation. Its intensity is maximum when a lot of water is absorbed, and evaporation is slow. Contrary to popular belief, the dew drops observed on the grass on a summer morning are the result of guttation, and not condensation of atmospheric moisture.
The main part of the leaf is the mesophyll. Directly under the upper (sometimes also under the lower) epidermis there are one or several layers of cylindrical, perpendicular to the surface of the sheet, the so-called. palisade cells - palisade parenchyma. Each of these cells contains numerous miniature bodies - chloroplasts, containing the green pigment chlorophyll, which captures solar energy and converts it into chemical energy. During this process, called photosynthesis, sugars are formed from atmospheric carbon dioxide and water coming from the soil. Under the palisade parenchyma are large cells that make up the spongy parenchyma. Free spaces between them (intercellular spaces) facilitate the diffusion of gases within the leaf. The spongy parenchyma contains fewer chloroplasts, and photosynthesis is not as intense here as in the palisade parenchyma.
Leaf-piercing veins, i.e. vascular-fibrous bundles that conduct water and nutrients are surrounded by a bundle sheath, or lining, of thin-walled, compactly arranged cells. The upper part of the vein consists of xylem, formed by vessels and tracheids, and the lower part of phloem, represented mainly by sieve tubes. Through the xylem, water with dissolved mineral salts moves from the roots to the leaf blades, and through the phloem from the leaf, the products of photosynthesis - organic substances - are sent to all organs of the plant.
There are two main types of leaf venation - mesh, when the veins branch and connect to each other, and parallel, when they run parallel to each other. The first type is characteristic of dicotyledonous flowering plants - geranium, tomato, maple, oak, etc.; the second - for monocots, i.e. iris, lilies, cereals (e.g. corn, bamboo, wheat), etc. There are various deviations from this scheme and transitional types of venation.
Photosynthesis.
The main function of the leaf is photosynthesis, during which sugars are formed from water and carbon dioxide due to solar energy. From these sugars in various organs of the plant, substances specific to them are formed, which are necessary, for example, for growth, lignification of cells, ripening of fruits and seeds, etc. Sugars are also stored in reserve so that they can be used if necessary. Thus, a green leaf is an organ on which the provision of plants with organic substances depends entirely. For growth, plants need the same organic substances as animals (proteins, fats, carbohydrates, etc.), but only photosynthesis makes it possible to obtain them from inorganic compounds. All non-photosynthetic living beings directly or indirectly depend on green plants for their nutrition.
The process of photosynthesis is very complex, and here we will consider it only in the most general terms. Typically, carbon dioxide enters the leaf from the atmosphere through stomata, spreads through the intercellular space, passes through the cell wall, and is absorbed by the cell-filling fluid. The carbon dioxide that got inside the chloroplasts and the water always present here enter into a series of reactions that give various intermediate products, ultimately sugars, in particular the water-soluble sugar glucose and its polymerization product, starch. Further, proteins are formed from sugars in the course of certain reactions with nitrogen and sulfur compounds (coming mainly from the soil). Ultimately, all other compounds necessary for the plant, such as cellulose, lignin, fats, oils, etc., are ultimately built from sugars.
Development and fall of leaves.
Leaves develop from areas of rapidly growing stem tissue - the meristem, located in the buds at the top of the stem and at the nodes of the shoot. Before the opening of the buds in them, the not yet dissected rudiments of the leaf are formed from the meristem in the form of a tubercle or roller on the cone of growth - the so-called. leaf primordia. As the bud opens, their cells begin to rapidly divide, grow, and specialize until the leaf is fully formed. As the leaf develops, in its axil, i.e. at the top of the angle between the leaf and the section of the stem going up from it, a new bud is almost always laid. From such axillary buds, new shoots may appear next year.
Unlike roots and stems, a leaf is a temporary organ. Having reached full development, after a while it dies and falls off. In deciduous species in the temperate zone, this occurs every autumn. Before this, the plant hormone abscisin II stimulates the formation at the base of the petiole of the leaf (or its plate, if the leaf is sessile) of a special layer of specialized tissue, the so-called. separating layer. It consists mainly of spongy parenchyma, i.e. thin-walled, loosely interconnected cells, therefore, under the influence of its own weight, as well as external influences, such a leaf is relatively easily broken off from the stem. In evergreen species, the foliage is also renewed, but each leaf lives for several years, and the leaves do not fall all at once, but in turn, so that outwardly these changes are invisible. This phenomenon is widespread in tropical plants, on which at any time of the year you can see leaves in different stages of development: some are ready to fall, others are just straightening out, and still others are at the peak of maturity and metabolic activity.
Autumn leaf color.
Leaves become especially brightly colored in autumn in some geographical regions, for example, in the northeast and northwest of the United States, southeast of continental Asia, and southwest of Europe. In Northern Europe, where winters are mild and rainy, the leaves become mostly dirty yellow and brownish before falling.
Autumn leaf color depends largely on the type of plant, but it is also influenced by weather conditions and soil type. Before the leaves fall, nutrients pass from them to the stems and roots. The formation of chlorophyll stops, and its residues are quickly destroyed by sunlight. As a result, yellow pigments, mainly xanthophylls and carotenes, become visible. They are present in the leaves throughout the growing season, but are masked by green chlorophyll in spring and summer.
Orange, red and purple tones of autumn foliage are due to other pigments - anthocyanins, which, unlike yellow pigments, appear only in autumn, and their amount depends on the weather. If the air temperature drops sharply to the level of 0–7 ° C, more sugars and tannins remain in the leaf, and as a result, the synthesis of anthocyanins is activated.
Thus, if the autumn is sunny, dry and cool, the leaves of many trees delight the eye with bright reds, yellows, oranges and crimson tones. If the autumn is cloudy and the nights are warm, then less sugar is synthesized in the leaves, and a significant proportion of it passes from them into the stem; under these conditions, the formation of anthocyanins is weak and the color of the foliage becomes predominantly dull yellow.
One of the most beautiful species in autumn is sugar maple ( Acer saccharum), the leaves of which turn dark yellow, golden orange and bright red. At the red maple ( A. rubrum) they turn red, and in Norway maples ( A. platanoides) and silver ( A. saccharinum) acquire a golden yellow color. The autumn crown of the resinous liquidambar, or amber tree, cannot but arouse admiration ( Liquidambar styraciflua): in the same tree, it can shimmer with different shades of purple, scarlet, yellow and green tones. Among the shrubs, the winged euonymus is famous for its bright autumn leaves ( Euonymus alatus), various types of barberry ( Berberis spp.) and American mackerel ( Cotinus americanus).
specialized leaves.
Leaves can specialize in various ways, losing their typical appearance, structure, and even functions. Examples of this are the tendrils of many legumes, which allow plants to cling to supports, the spines of cacti, in which the processes of photosynthesis have moved to green fleshy stems, the protective bud scales of trees, and the bracts - scaly covering leaves on the pedicels of many species. Sometimes the leaves surrounding the flowers and entire inflorescences are bright, conspicuous, such as the white or red spathes of the cobs in aronnikovye (calla, anthurium) or the red, white and pink apical leaves of the poinsettia ( Euphorbia pulcherrima). They are easily mistaken for petals, while the true flowers of these species can be relatively small and inconspicuous.
American agave ( Agave americana) the leaves are very thick and fleshy - they store water and nutrients. Among other plants with a pronounced storage function of leaves are various purslane (genus portulaca) and stonecrop (genus Sedum). Their leaves contain mucous colloidal substances that effectively bind water and slow down its evaporation in a dry habitat typical of these "leaf succulents".
At the so-called. Insectivorous plant leaves are turned into traps for small arthropods. So, in the Venus flytrap ( Dionaea muscipula) leaf halves, covered at the edges with spikes sticking up, can rotate relative to the midrib. When an insect lands on a leaf blade, these halves slam shut like a book, and the victim is trapped. Her body decomposes under the action of enzymes secreted by the glands of the leaf, and the decay products are absorbed by the plant. At the pitcher ( Nepenthes) there are modified leaves in the form of a jug. An insect crawling there cannot get out, drowns and digests in the liquid secreted by the glands at the bottom of the jug.
The leaves of many plants are fraught with danger to warm-blooded animals. So, sumac rooting ( Toxicodendron radicans) they contain an oily substance, which, once on the skin, causes severe inflammation (dermatitis). In the leaves of some species of astragalus (genus Astragalus) accumulates selenium poisonous to animals. Cattle that have eaten a large amount of these leaves fall ill with selenosis, from which they sometimes die. Leaves are poisonous, for example, in plants such as painted dieffenbachia ( Dieffenbachia picta), lily of the valley ( convallaria majalis), azaleas and rhododendrons (genus Rhododendron), Kalmia broadleaf ( Kalmia latifolia).
Features of the development of leaves in a palm tree
One of the central questions of modern biology is how does an embryo develop from a fertilized egg and then an adult animal or plant? The stages of development of a number of organisms and tissues have long been described in detail, but the study of the regulatory processes underlying them became possible only with the advent of molecular biology. And in order to understand the sequence of stages in the development of many tissues, methods such as electron microscopy were needed. It is impossible to understand the processes that make up the essence of individual stages without identifying these stages.
To make sure that until the path of development is clarified, it is impossible to judge its regulation, one can use the example of a leaf of representatives of one of the families of flowering plants - palm trees (Palmae). All flowering plants have three main organs: root, stem and leaf. The leaves are the most varied. For example, a leaf, in addition to its primary role as a photosynthetic organ, can mutate into the protective scales of a new bud, into a stem-climbing device, into reproductive organs, and even into an insect trap. The main part of an ordinary, unmodified leaf is a wide flat green formation, the tissues of which contain chlorophyll in a high concentration. This so-called leaf blade captures sunlight and functions as an organ for gas exchange. The blade is supported by a petiole. Through it, nutrients - products of photosynthesis - come from the leaf blade to the base of the leaf, which is connected to the plant stem. While performing a mechanical and nutrient transfer function to the stem (and thus to the plant as a whole), the base also serves as a protection for the leaf blade and petiole at that early stage when the future leaf is only part of the apical bud of the shoot. How does a leaf develop from a bud? I will deal with this process mainly in terms of palm trees, but it is also useful to pay attention to representatives of the Aroaceae family (Agaseae), which is relatively closely related to palms. In palms and aronniks, the leaves from the embryonic to the adult state go through completely different developmental paths, which until recently were poorly understood.
Plants develop in a completely different way than animals. In most animals, new organs are formed only in the early stages of embryonic growth. In plants, new organs arise continuously from growth centers - undifferentiated tissues consisting of cells capable of further differentiation. The centers of growth are the embryonic tissues of the top of the root and stem - the so-called apical meristem.
In a typical shoot, thanks to the meristem, the stem grows simultaneously with the leaves located along the stem in a certain geometric order. Due to the fact that stem growth and leaf development occur over a long period, it may seem that the shoot is a set of structurally identical units, like segments of the body of an earthworm. For example, it is believed that since the farther from the apical bud the leaf is, the “older” it is, the sequence of leaves on the shoot illustrates the stages of development of the leaf at a given position on the stem. In fact, this is true only when the plant has a stable growth, i.e. it can be shown that successive structural units of the shoot are identical. However, numerous examples are known when they noticeably change as the shoot grows. In this case, it is first necessary to choose one position of the leaf on the stem and study the development of that particular leaf.
The leaf arises as an outgrowth of the peripheral part of the apical bud,
Usually in the form of a flattened tubercle without any signs of differentiation. The base of the leaf and the leaf blade become the first to be distinguished. The petiole, if it develops at all, appears later as an insert between the base and the lamina.
The leaves of flowering plants vary in shape and size. Dissected, or compound, leaves are very common. The plate of a complex leaf is, as it were, cut into segments, or leaflets. From a developmental point of view, dissected leaves are of particular interest, as they clearly demonstrate how leaves that look very similar in appearance are formed in completely different ways. Giant palm leaves are the largest and most complex of the dissected leaves. As for tropical aronia (and in the aronia family, the leaves are very diverse in shape), then using their example, we will analyze two alternative ways of forming a dissected leaf.
The lamina of an adult leaf of the aronnik of the species Zamioculcas zamiifolia is pinnate: 4-5 pairs of segments of the lamina extend from an elongated rod, or rachis, like processes. If the young leaves are successively removed from the apical bud of Z. zamiifolia, a tiny domed structure about 100 µm in diameter is found. This is the apical meristem of the shoot: the youngest primordia, or embryonic leaves, arise from it. Primordia forms a small helmet over the apical bud of the shoot. As the primordium grows upwards, it completely covers the apical dome and the two free edges of its helmet are pressed tightly against each other.
As a result of the fact that different tissues grow at different speeds, future leaflets soon appear in the form of tubercles along the edges of the leaf. There are larger tubercles at the top of the leaf, and young small ones gradually appear towards the base. When the number of tubercles reaches 4-5 pairs, they grow and take the form of adult leaves.
In higher plants (whether simple vascular plants such as ferns; gymnosperms such as cycads; or higher flowering plants), the appearance of leaflets at the leaf margins is the most common way of developing a dissected leaf. Compound leaves can be dissected twice, thrice or even multiple times. Apparently, there are in principle no restrictions on the degree of dissection of the sheet.
And in such a representative of the Aronnikova as the philodendron with dissected leaves (one of the climbing genera of Monstera), the method of dissecting the leaf is completely different. This plant attracts attention with characteristic holes in the leaf blade, which makes it very popular as an ornamental. The size and shape of the holes are different, since different parts of the plate grow at different rates. So, the first holes appear closer to the edge of the plate. In the formed leaves, they are large and have an elliptical shape due to the fact that this part of the plate grows especially strongly in breadth. Holes that occur later are located closer to the midrib of the leaf. They are smaller and more rounded because the surrounding tissue grows much less laterally.
Even in the last century, it became known that the holes in the leaves of the philodendron are formed as a result of the death of cells in certain areas of the leaf blade. Using a scanning electron microscope, it was possible to observe this process in detail. First, slightly depressed rounded areas appear on the surface of the leaf blade. Indentation is a consequence of a decrease in the turgor of the affected cells. After the cells finally die, this section of the leaf simply dries up and falls off, and a hole remains in its place.
In many species of the genus Monstera, the marginal stripe, i.e. the tissue around the peripheral openings grows at a different rate than the rest of the leaf blade. As a result, the thin bridge of the marginal tissue usually breaks, and the perforated leaf blade turns into a lobed one. In some species (for example, M. subpinnata, M. tenuis and M. dilacerata), only wide elliptical holes are formed along the edges of the leaf blade. When the bridges of the marginal strip are torn, a pinnate lamina is obtained, strikingly similar to the pinnate leaves of Zamioculcas zami folia. If we did not know that the dissected leaf in Monstera is formed by the death of cells, and in Zamioculcas - by the formation of lobes, then it would be rather difficult to assume that their development programs are different.
Like aronnikovye, palms are widespread in the tropics. Palm leaves are usually very large. Palms of the genus Raffia have the largest leaves in the entire plant kingdom. The length of their leaflets can exceed 18 m. The process of formation of dissected leaves in palm trees is in some respects a combination of the two methods that we considered above: the formation of lobes and the death of cells. However, much is still unclear. Recently, I and my colleagues at the University of California at Berkeley conducted a study to determine what underlies the development of the palm leaf.
The palm leaf consists, like a typical leaf, of three parts: an elongated lamina, a petiole, and a base, which is usually tubular and completely encircles the stem. In palms, the base of the leaf plays a particularly important role. It not only supports the very large and heavy organs of photosynthesis - the lamina and the petiole - but also mechanically strengthens the shoot in the young parts of the stem (trunk) of the palm tree, where the internodes (i.e. individual sections) of the stem are still lengthening. That is why a network of vascular fibrous tubular bundles is very developed at the base of the leaf in palm trees, which ensure its strength and flexibility.
The leaf blades of palm trees are of two types, differing in how the growth is distributed in the process of cutting the leaf: pinnate and palmate. The pinnate leaf resembles a bird's feather, and the palmate leaf looks like a hand with spread fingers. Pinnate leaves have short petioles, while palmate leaves have long petioles. A feature of the development of leaves in palm trees is that the leaf blade first has a folded surface. Then, tissue breaks along some folds and the plate breaks up into separate leaves. In a fully formed palmate leaf, it is clearly seen that its segments were obtained from the folds of the leaf blade: by squeezing the leaflets, the leaf can be folded like a fan, moreover, each leaflet has a V-shape in cross section, i.e. as if cut out of a corrugated layer.
The cirrus leaf of a palm tree goes through a more complicated path in its development. For a better understanding, it is necessary to consider the entire process of development, starting from the moment the leaf was born. In a pinnate leaf, leaflets are located laterally along the central axial vein of the leaf, they are formed during the growth of the vein. The primordia of a pinnate palm leaf appears as a helmet-like outgrowth at the top of the shoot. That part of the primordia that protrudes higher and farther from the top of the shoot represents the future leaf blade. The base of the leaf appears as a collar around the peripheral part of the shoot apex, i.e. it is laid so that in the future it completely covers the stem. After a while, the primordia becomes more like a bonnet than a helmet. Its well-defined outer surface corresponds to the lower surface of the future leaf blade, and the narrow inner part will become its upper surface.
Whereas in Zamioculcas, the beginning of leaf growth entails the formation of lobes along the edge of the leaf blade, the future leaflets of the pinnate palm leaf appear as a series of bulges, or ridges, at some distance from the edge of the leaf. These folds are especially visible on the future lower surface of the plate, but they can also be seen on the upper surface. On a cut made at a right angle to one of the edges of the plate, it can be seen that the lower and upper ridges actually form one series of folds running along the entire plate in a dense zigzag, like a wave.
The peculiarity of this method of laying leaflets lies in the fact that the folds never extend to the edges of the plate, so that a narrow strip of fabric without folds stretches along the periphery of the leaf blade. Similarly, on the reverse side of the leaf blade, the folds extend only a short distance from the thickened axial vein of the leaf.
As the leaf continues to grow, new folds appear. They appear towards the base of the lamina, although a few folds may form towards the top of the lamina. The total number of folds usually corresponds to the number of leaflets in an adult leaf; once this number is reached, no more folds occur. At this stage, the folds remain tightly pressed together like the bellows of a camera.
When the folds become deep enough, the leaf blade separates into separate leaflets and the peripheral tissue that connected the tops of the leaflets disappears. First, adjacent folds are separated from each other. It is assumed that the separation is due to the destruction of the intercellular substance along the break lines. It has been established that cell death does not occur in this case. In most pinnate palms, the separation process occurs along the ridges of the underside of the leaf. Observations with a scanning electron microscope show how a depression appears and spreads along the top of the ridge, which then turns into a distinct crack. Note that, even if the leaflets are separated from each other almost along the entire length, their tops for some time - very briefly - remain attached to the strip of fabric running along the edge of the sheet. The leaf at this stage is like a horse's harness, and the loosely hanging strips are called reins.
The manner in which the palm leaf folds has been the subject of controversy among botanists for a century and a half. Two alternative hypotheses have been proposed. According to one of them, folds are formed due to the fact that, due to differentiated growth, the plate of a young leaf is bent.
According to another hypothesis, the formation of folds begins with the process of tissue rupture, and then differentiated growth follows. It is assumed that alternating gaps occur on the upper and lower surfaces of the leaf blade, because the gradual separation of the cells begins on the surface of the leaf blade and a furrow is obtained. As the cells separate from each other, the furrows deepen and the leaf surface becomes zigzag and very folded.
The second hypothesis is much more complicated than the first, since it requires some significant complications of cell differentiation. Thus, the development of furrows should divide the epidermis of the leaf into isolated areas. But according to observations, the sides of the furrows are also covered with a layer of epidermal cells. Then it should be assumed that they come from cells lying deep inside the leaf. However, inner cells usually do not transform into epidermal cells. If such a transformation takes place, then this is a unique case among higher plants.
Neither one nor the other hypothesis can be given final preference for a long time, for the reason that, in the absence of sufficiently powerful equipment, it was necessary to proceed only from the appearance of the folds. With the goal of investigating the occurrence of wrinkling in the leaves of several different palm species, we decided to use a scanning electron microscope to understand how the leaf blade develops in space. We also studied thin (1.3 µm thick) sections of plant tissue prepared using the transmission electron microscope method. (In a transmission electron microscope, unlike a scanning electron microscope, the electrons pass through the sample.) Such thin sections allowed us to obtain images with a very good resolution in the light microscope and eliminate many artifacts that interfered before.
According to both the tissue gap hypothesis and the differential growth hypothesis, the furrows separating the crests of the folds should deepen as the leaf develops. According to the first hypothesis, the furrows deepen on their own. According to the second - due to the growth of ridges upward from the base of the furrow. This means that if we calculate how many cells remained in contact in the lower and upper crests of the fold (either below or above the corresponding lines of the furrow), it will be possible to finally establish which of the two hypotheses is closer to the truth. If the matter is in progressive tissue rupture, then the number of contacting cells at the initial level of the “bottom” of the furrow should decrease over time, since as the furrow deepens, the cells separate. If the ridge grows upwards, as is assumed in the hypothesis of differentiated growth, then the number of contacting cells either increases (if the growth of the ridges occurs due to both cell division and elongation) or remains unchanged (if the cells only increase in size, but do not divide). ).
The scientists concluded that the furrows deepen not as a result of cell separation, but as a result of upward growth of the ridges. Although this process was not exactly the same for the lower and upper crests of the fold, we never found a decrease in the number of cells; sign of a progressive rupture. In the upper ridges, the number of contacting cells first increased, while in the lower ridges it almost did not change. Further research also supported the differential growth hypothesis. For example, it turned out that on the folds the layer of epidermal cells is continuous over the entire surface at all stages of development. We did not find any evidence of epidermal rupture or redifference at any stage. With pinnate and palmate leaves, essentially the same results were obtained. Therefore, we assume that, regardless of the lamina morphology, our conclusions are correct for all representatives of the palm family in general. However, another controversial issue has not been resolved. Why are the primordial leaf folds of some species of palms so shaped as to suggest tearing of the tissue? I wanted to understand what forces affect the shape of already formed folds.
In the 19th century it was believed that folds form in the primordia of a palm leaf because the space inside the apical bud is limited. This opinion was formed under the impression of the appearance of young leaves: in the crown of a palm they are collected in dense folds. Can we assume that the connection between folding and limited space, which is so obvious at the later stages of leaf development, also exists at the stage when the leaf primordia is still tiny, essentially microscopic, and it is difficult to speak of "crowding"? There is circumstantial evidence against such an assumption. Thus, if folding is considered simply as a result of lack of space, it would be logical to expect that the entire surface of the leaf blade will be folded. However, there are never folds along the edges of the plate. Moreover, when analyzing the relationship between the space inside the sheath of an older leaf and the shape and distribution of folds on the blade of an adjacent, younger leaf, no correlation is found between the space available for growth, on the one hand, and the configuration and degree of folding, on the other.
This does not mean, however, that the limited space in the kidney does not affect the shape of the folds at all. In those palms whose leaves are densely packed in buds, the outer surface of the leaf looks flat and the furrows of the folds resemble cracks. In palms, the leaves of which are less densely packed in the buds, the folds protrude noticeably, giving the impression that they arose as a result of bending of the leaf blade, and not a tear in the tissue.
It would be possible to elucidate the role of “crowding” in the formation of leaf folding by growing a separate leaf primordia on a nutrient medium, i.e. in conditions where space for growth is not limited. According to preliminary data, it is possible to grow such primordia in vitro starting from an early stage, when their length is less than 1 mm. However, we have not yet carried out a rigorous analysis of their development and do not know how comparable it is to the situation in vivo.
Our studies of the development of the palm leaf were inspired by the assumption that when the leaf blade is cut into leaflets, a process unique in the plant kingdom takes place - tissue rupture. However, experiments have shown that while tearing of the primary folds of the lamina into separate segments is indeed sometimes caused by the separation of tissues, the development of the folded lamina is only a relatively uncomplicated process - differentiated growth.
In essence, the same morphogenetic process is responsible for the dissection of the leaf in palm trees as in the aronnik Zamioculcas. The only difference is that in aronika, differentiated growth occurs near the free edge of the leaf blade, and in palm trees, in the inner part of the surface of the blade at some distance from its edge. It can be thought that in palms the leaf develops so difficultly because the leaves begin to form inside the plate, and not along the edge. From this point of view, the peculiarity of leaf development inherent in palm trees does not seem so anomalous.
Of course, one cannot confine oneself to cataloging the paths of plant development. The ultimate goal of a botanist should be to correlate such information with the results of research at the molecular level. Some molecular biologists believe that those regulatory systems that are open in the plants studied on this subject function in general in all higher plants. It is not at all necessary that this is the case. It is more likely that the differences in developmental pathways that we have examined here are related to profound differences in regulatory systems at the molecular level. The study of morphogenesis is valuable not in itself, but in combination with information obtained in other areas of biological research - only such an approach gives hope to understand the mechanisms of growth and development.
