The leaf is a very important organ of the plant. This is part of the shoot, the main functions of which are transpiration and photosynthesis. Features of the structure of the sheet are in its high morphological plasticity, large adaptive capabilities and a variety of forms. The base can expand in the form of stipules - leaf oblique formations on each side. In some cases, they are so large that they play a certain role in photosynthesis. Stipules are adherent to the petiole or free, they can be shifted to the inner side, and then they are called axillary.
External structure of the sheet
Leaf blades are not the same in size: they can be from a few millimeters to ten to fifteen meters, and even twenty meters from the palm trees. The structure of the leaf determines the lifespan of the vegetative organ, it is usually short - no more than a few months, although for some it is from one and a half to fifteen years. The shape and size are hereditary signs.
Parts of leaves
The leaf is a lateral vegetative organ that grows from the stem, has a growth zone and bilateral symmetry at the base. It usually consists of a stem (with the exception of sessile leaves) and leaf blade. In a number of families, the structure of the leaf suggests the presence of stipules. External organs of plants can be simple - with one plate, and complex - with several plates.
The leaf pad (base) is the part that connects the leaf with the stem. The educational tissue found here gives rise to petiole and leaf blade growth.
The scape is a narrowed part, with its base connecting the stem and leaf blade. He orients the sheet with respect to light, acts as the place where the interstitial educational tissue is located, due to which the vegetative organ grows. In addition, the scape weakens impacts on the leaf during rain, wind, hail.
The lamina is usually a flat, expanded part that performs the functions of gas exchange, photosynthesis, transpiration, and in some species also the function of vegetative reproduction.
Speaking about the anatomical structure of the leaf, it is necessary to say about stipules. These are leaf-shaped paired formations at the base of the vegetative organ. When you deploy a sheet, they may fall off or be saved. Designed to protect the axillary lateral buds and intercalary educational tissue.
Complex and simple leaves
The structure of a leaf is considered simple if it has one leaf plate, and complicated - if there are several or many plates with joints. Due to the last plates of complex leaves fall not together, but one by one. But some plants may fall completely.
Whole leaves in shape can be lobed, divided or dissected. In the blade leaf, the notches along the edge of the plate are up to 1/4 of its width. For the separate organ is characterized by a greater recess, its blades are called lobes. The dissected leaf at the edges of the plate has notches reaching almost the midrib.
If the plate is elongated, with triangular segments and lobes, the leaf is called strugo-like (for example, a dandelion). If the lateral lobes to the base decrease, they are unequal, and the final lobe is round and large, a lyre outer organ of the plant is obtained (for example, in a radish).
The structure of a sheet with several plates is significantly different. Allocate palchatofesional, trifoliate, pinomistema bodies. If a complex leaf contains three plates, it is called trifoliate, or triple-complex (for example, maple). Palatine sheet is considered when its petioles are attached to the main petiole at one point, and the plates diverge radially (for example, lupine). If the lateral plates on the main stem are on both sides along the length, the leaf is called peristosyllabic.
Forms of whole plates
Different plants have different forms of leaf blades in the degree of dissection, shape, type of base and apex. They can have round, oval, triangular, elliptical and other outlines. The plate is elongated, and its free end can be blunt, pointed, sharp or pointed. The base is drawn and narrowed to the stem, it is heart-shaped or rounded.
Attaching to the stalk
Considering the structure of a leaf of a plant, it is necessary to say a few words about how it is attached to the shoot. The attachment is carried out with the help of long or short stalks. There are also sessile leaves. In some plants, their bases grow together with the shoot (nibbling leaf), and it happens that the shoot escapes through the plate (pierced leaf).
Internal structure. Skin
The epidermis (upper skin) is a covering tissue located on the reverse side of the organ of the plant, often covered with cuticle, hairs, and wax. The internal structure of the sheet is such that outside it has a skin that protects it from drying, mechanical damage, penetration of pathogens to internal tissues and other adverse effects.
Peel cells are alive, they are different in shape and size: some are transparent, large, colorless, tight to each other, others are smaller, with chloroplasts giving them a green color, such cells can change shape and are arranged in pairs.
The skin cells can move away from each other, in which case a gap appears between them, which is called the stomatal. When the cells are saturated with water, the stoma opens, and when the fluid flows out, it closes.
The anatomical structure of the leaf is such that air flows through the stomatal gaps to the inner cells and gaseous substances go out through them. When the plants are not sufficiently supplied with water (this happens in hot and dry weather), the stomata close. So representatives of the flora protect themselves from draining, because with closed stomatal crevices water vapor does not come out and remain in the intercellular spaces. Thus, in the dry period, plants retain water.
The internal structure of the sheet does not do without the columnar tissue, the cells of which are in the upper side facing the light, fit snugly together, have a cylindrical shape. All cells have a thin membrane, nucleus, chloroplasts, cytoplasm, vacuole.
Another main fabric is spongy. Its cells are round in shape, loosely arranged, between them there are large intercellular spaces filled with air.
How the structure of a leaf of a plant will be, how many layers of spongy and columnar tissues are formed, depends on the lighting. In the leaves grown in the light of the leaves, the columnar tissue is much more developed than in those that grew in blackout conditions.
Forms of stem
On a cross-section, the petioles can be shaped: cylindrical, ribbed, flat, winged, grooved, etc.
Some plants (Rosaceae, legumes, etc.), except for the plate and petiole, have special growths - stipules. They cover the lateral buds and protect them from damage. Stipules can have the form of small leaves, films, thorns, scales. In some cases, they are very large and play an important role in photosynthesis. They are free or adhering to the stem.
Veins combine leaf with stem. This is the vascular fibrous bundles. Their functions: conductive and mechanical (veins serve as a support, protect leaves from breaking). Location, branching of the veins of the leaf blade is called venation. There is a venation from one main vein, from which the lateral branches diverge - mesh, pinnate (bird cherry, etc.), palmate (Tartar maple, etc.), or with several main veins that run almost parallel to one another –– arc (plantain, lily of the valley) and parallel (wheat, rye) venation. In addition, there are many transitional types of venation.
For most dicotyledons, they are characterized by pinnate, palmate, reticular venation, for monocotyledons - parallel and arc.
Leaves with straight veins are predominantly entire.
Simple the leaves have one leaf plate with a petiole, which can be whole or dissected. Simple leaves fall during the fall of the leaves completely. They are divided into leaves with a solid and dissected leaf blade. Leaves with a whole leaf plate are called whole.
The shape of the leaf blade is distinguished by a common contour, the shape of the top and base. The contour of the leaf blade can be oval (acacia), heart-shaped (linden), needle-like (coniferous), ovoid (pear), arrow-shaped (arrowhead), etc.
The tip (top) of the leaf blade is sharp, blunt, dull, pointed, notched, with a topscale, etc.
The base of the leaf blade may be rounded, heart-shaped, arrow-shaped, spear-like, wedge-shaped, unequal, etc.
The edge of the leaf blade can be entire or with notches (do not reach the width of the plate). According to the shape of the grooves on the edge of the leaf blade, there are toothed leaves (teeth have equal sides - hazel, beech, etc.), serrated (one side of the tooth is longer than the other - pear), warty (sharp notches, bulges stupid - sage), etc.
Challenging leaves have a common scape (rachis). Simple leaflets are attached to it. Each of the leaves can fall off independently. The complex leaves are divided into trifoliate, palmate and pinnate. Challenging trifoliate leaves (clover) have three leaves that are attached to the common petiole by short petioles. Palmatous leaves are similar in structure to the previous one, but the number of leaves is more than three. Peristocomplex leaves consist of leaflets located along the entire length of the rachis. There are paeroperisoslozhnye and odnarnopislosozlozhnye. Paternal perseverance leaves (pea) consist of simple leaves, which are arranged in pairs on the stem. Unparticulate leaves (wild rose, rowan) end with one unpaired leaf.
According to the method of division
The leaves are divided into:
1) lobed, if the division of the leaf blade reaches 1/3 of its entire surface, the protruding parts are called blades,
2) separate, if the division of the leaf blade reaches 2/3 of its entire surface, the protruding parts are called shares,
3) dissectedif the degree of articulation reaches the central vein, the projecting parts are called segments.
This arrangement is in a certain order of leaves on the stem. The leaf position is a hereditary trait, but during plant development, when adapting to lighting conditions, it can change (for example, in the lower part, the leaf position is opposite, at the top - the next one). There are three types of leaf arrangement: spiral, or the next, opposite and ringed.
In each node, two leaves sit opposite one another (lilac, maple, mint, sage, nettle, viburnum, etc.). In most cases, the leaves of two adjacent pairs depart in two mutually opposite planes, without shading each other.
More than two leaves depart from the node (elodea, raven eye, oleander, etc.).
The shape, size and location of the leaves are adapted to the lighting conditions. The mutual arrangement of the leaves resembles a mosaic, if you look at the plant from above in the direction of light (in hornbeam, elm, maple, etc.). This arrangement is called leaf mosaic. At the same time the leaves do not shade each other and use the light effectively.
The internal structure of the sheet
The internal structure of the sheet
Outside, the leaf is covered predominantly with a single-layer, sometimes multi-layered epidermis (skin). It consists of living cells, most of which are devoid of chlorophyll. Through them, the sun's rays easily fall to the lower layers of the leaf cells. In most plants, the peel secretes and creates a thin film of fat-like substances from the outside - the cuticle, which hardly lets water through. On the surface of some cells of the skin can be hairs, spines, which protect the leaf from damage, overheating, excessive evaporation of water. Plants that grow on land have a stomata on the underside of a leaf in the epidermis (in humid places (cabbage) - stomata on both sides of the leaf, in aquatic plants (water lily) whose leaves float on the surface, on the upper side, there are no stomatae plants that are completely submerged in water). Stomach functions: regulation of gas exchange and transpiration (evaporation of water by foliage). On average, there are 100–300 stomata per 1 square millimeter of surface. The higher the leaf is located on the stem, the more stomata per surface unit.
Between the upper and outer layers of the epidermis are the cells of the main tissue - the assimilation parenchyma. In most species of angiosperms, there are two types of cells in this tissue: columnar (palisade) and spongy (loose) chlorophyll parenchyma. Together they make up mesophyll sheet. Under the upper skin (sometimes - and above the bottom) contains a columnar parenchyma, which consists of cells of regular shape (prismatic), located vertically in several layers and tightly adjacent to one another. Loose parenchyma is located under the columnar and above the lower skin, consists of cells of irregular shape, which do not fit tightly to one another and have large intercellular spaces, filled with air. Intercellular spaces occupy up to 25% of the volume of the sheet. They connect with stomata and provide gas exchange and leaf transpiration. It is believed that photosynthesis processes are more intense in the palisade parenchyma, since its cells have more chloroplasts. In cells of a friable parenchyma of chloroplasts it is much less. They are actively stored starch and some other nutrients.
Through the tissue of the parenchyma pass the vascular fibrous bundles (veins). They consist of conductive tissue - vessels (in the smallest veins - tracheids) and sieve tubes - and mechanical. On top of the vascular fiber bundle is xylem, and on the bottom - phloem. Organic substances flowing through photosynthesis flow to all plant organs through sieve tubes. Through the vessels and tracheids water enters the sheet with mineral substances dissolved in it. Mechanical fabric gives strength sheet plate, support conductive fabric. Between the conducting system and the mesophyll is free space or apoplast.
Leaf modifications (metamorphosis) occur when performing additional functions.
Allow the plant (peas, vetch) to cling to objects and fasten the stem in an upright position.
Occur in plants that grow in arid places (cactus, barberry). In Robinia, pseudoacacia (white acacia) spines are modifications of stipules.
Dry scales (buds, bulbs, rhizomes) perform a protective function - protect against damage. Fleshy scales (onions) store nutrients.
In insectivorous plants (sundew) leaves are modified to trap and digest predominantly insects.
This is the transformation of the stem into a leaf-like flat formation.
The variability of the sheet due to a combination of external and internal factors. The presence of the same plant leaves of different shapes and sizes is called heterophilia, or variability. Observed, for example, in water yellow, arrowhead, etc.
Transpiration (from lat. trans - through and spiro - I breathe). This is the elimination of water vapor by the plant (evaporation of water). Plants absorb a lot of water, but use only a small part of it. Water evaporates all parts of the plant, but especially the leaves. Due to evaporation, a special microclimate arises around the plant.
Stomataltranspiration - this is the evaporation of water through the stomata. Stomach is the most intense. The stomata regulate the rate of evaporation of water. The number of stomata in different plant species is different.
Transpiration promotes the flow of new water to the root, raising water along the stem to the leaves (using suction force). Thus, the root system forms the lower water pump, and the leaves form the upper water pump.
One of the factors determining the rate of evaporation is air humidity: the higher it is, the less evaporation (evaporation stops when air is saturated with water vapor).
The value of water evaporation: reduces the temperature of the plant and protects it from overheating, provides an upward current of substances from the root to the aboveground part of the plant. The intensity of photosynthesis depends on the intensity of transpiration, since both of these processes are regulated by the stomatal apparatus.
This simultaneous dropping of leaves for a period of adverse conditions. The main causes of leaf fall is the change in the length of daylight, a decrease in temperature. This increases the outflow of organic matter from the leaf to the stem and root. It is observed in the fall (sometimes, in dry years, in the summer). Leaf fall is a plant adaptation to protect against excessive water loss. Together with the leaves are removed various harmful metabolic products that are deposited in them (for example, calcium oxalate crystals).
Подготовка к листопаду начинается еще до наступления неблагоприятного периода. Lower air temperature leads to the destruction of chlorophyll. Other pigments become visible (carotenes, xanthophylls), so the leaves change color.
Stem cells near the stem begin to divide hard and form across it separating layer of parenchyma, which is easily stratified. They become rounded, smooth. Between them, there are large intercellular spaces that allow cells to easily separate. The leaf remains attached to the stem only due to the vascular fibrous bundles. On the surface of the future leaf scar pre-formed protective layer cork fabric.
Monocotyledonous plants and herbaceous dicotyledons do not form a separating layer. The leaf dies off, gradually collapses, remaining on the stem.
Fallen leaves are decomposed by soil microorganisms, fungi, animals.
General characteristics of the leaf of a plant
The leaves of most plants are green, most often - flat, usually two-sidedly symmetric. Sizes from a few millimeters (duckweed) to 10-15m (in palm trees).
The leaf is formed from the cells of the educational tissue of the growing cone of the stem. Leaf bud differentiates into:
- Leaf plate,
- the stem with which the leaf is attached to the stalk,
Some plants do not have petioles, such leaves, unlike petioles, are called sedentary. Stipules also do not occur in all plants. They are different sizes of paired appendages at the base of the leaf stem. Their form is diverse (films, scales, small leaves, spines), the function is protective.
Simple and complex leaves distinguished by the number of leaf blades. A simple sheet has one plate and disappears entirely. The complex on the scape is a few plates. They are attached to the main petiole with their small petioles and are called leaflets. When a complex leaf dies off, the leaves first fall off, and then the main stem.
Examples of simple and complex leaf types
Types of sheet plates
Leaf blades are diverse in shape: linear (cereals), oval (acacias), lanceolate (willow), ovate (pear), arrow-shaped (arrowhead), etc.
Leaf plates in different directions are pierced with veins, which are vascular-fibrous bundles and give strength to the sheet. In leaves of dicotyledonous plants, most often there is a reticular or pinnate veining, and in leaves of monocotyledons - parallel or arc.
The edges of the leaf blades can be solid, such a sheet is called entire (lilac) or with grooves. Depending on the shape of the notch, there are toothed, serrate, crenate leaves along the edge of the leaf blade. The teeth of the toothed leaves have more or less equal sides (beech, filbert), in the serrated leaves one side of the tooth is longer than the other (pear), crested have sharp recesses and blunt bulges (sage, budur). All these leaves are called whole, since the grooves in them are shallow, do not reach the width of the plate.
Types of sheet plates
In the presence of deeper grooves, the leaves are lobed, when the depth of the notch is half the width of the plate (oak), separate - more than half (poppy). In dissected leaves, the notches reach the midrib or the base of the leaf (burdock).
Under optimal growth conditions, the lower and upper leaves of the shoots are not the same. Distinguish grassroots, middle and upper leaves. Such differentiation is determined in the kidney.
The lower, or the first, leaves of the shoot are buds of the buds, outer dry scales of the bulbs, cotyledon leaves. Bottom leaves during the development of escape usually fall off. Leaves and basal rosettes belong to the grassroots. Median or stem leaves are typical for plants of all kinds. Upper leaves usually have smaller sizes, are located near flowers or inflorescences, are colored in different colors, or are colorless (covering leaves of flowers, inflorescences, bracts).
Types of sheet layout
There are three main types of leaf location:
- Regular or spiral,
At the next arrangement, single leaves are attached to the stem nodes in a spiral (apple, ficus). At the opposite - two leaves in a knot are located one against the other (lilac, maple). Turbid leaf location - three or more leaves in the node cover the stem with a ring (elodea, oleander).
Any leaf arrangement allows the plants to capture the maximum amount of light, since the leaves form a mosaic of leaves and do not obscure each other.
Types of location
Mouth - plant respiratory organs
In the skins are stomata - the gap formed by two closing, or stomatal, cells. Trailing cells have a semilunar form and contain the cytoplasm, nucleus, chloroplasts and the central vacuole. The shells of these cells are thickened unevenly: the inner, facing the gap, thicker than the opposite.
Stomatal slit leaf
Changes in the turgor of the guard cells change their shape, due to which the stomatal gap is open, narrowed or completely closed depending on environmental conditions. So, during the day, the stomata are open, and at night and in hot, dry weather, they are closed. The role of stomata is to regulate the evaporation of water by the plant and gas exchange with the environment.
The stomata are usually located on the lower surface of the leaf, but there are also on the upper, sometimes they are distributed more or less evenly on both sides (maize); in aquatic floating plants, the stomata are located only on the upper side of the leaf. The number of stomata per unit of leaf area depends on the type of plant, growth conditions. On average, they are 100-300 per 1 mm 2 surfaces, but it can be much more.
The plant uses an average of 0.2% of the leaked water to build its body, the rest is spent on thermoregulation and transport of mineral substances.
Transpiration creates sucking power in the leaf and root cells, thereby maintaining a constant movement of water through the plant. In this regard, the leaves are called the upper water pump in contrast to the root system - the lower water pump, which pumps water into the plant.
Evaporation protects the leaves from overheating, which is of great importance for all the life processes of the plant, especially photosynthesis.
Plants in dry places, as well as in dry weather, evaporate more water than in conditions of high humidity. Regulated by the evaporation of water except the stomata protective formations on the skin of the sheet. These formations: cuticle, wax coating, hairiness from various hairs, etc. In succulent plants, the leaf turns into spines (cacti), and its function is performed by the stem. Plants in wet habitats have large leafy plates; there are no protective formations on the skin.
Transpiration - the mechanism of water evaporation by the leaves of a plant
With difficult evaporation of plants observed guttation - release of water through the stomata in the drip-liquid state. This phenomenon occurs in nature, usually in the morning, when the air approaches saturation with water vapor, or before rain. Under laboratory conditions, guttation can be observed by covering young wheat seedlings with glass caps. After a short time, droplets of liquid appear on the tips of their leaves.
Discharge system - leaf fall (leaf fall)
Biological adaptation of plants to protection against evaporation is leaf fall - the mass abscission of leaves in the cold or hot season. In temperate zones, trees shed their leaves for the winter, when the roots cannot feed water from frozen soil, and the frost dries the plant. In the tropics, leaf fall is observed in the dry season.
Preparation for dropping leaves begins with a weakening of the intensity of life processes in late summer - early autumn. First of all, the destruction of chlorophyll occurs, other pigments (carotene and xanthophyll) last longer and give the leaves autumnal color. Then, at the base of the leaf stem, the parenchymal cells begin to divide and form a separating layer. After that, the leaf comes off, and a trace remains on the stem - a leaf scar. By the time the leaves fall, the leaves age, they accumulate unnecessary metabolic products that are removed from the plant along with fallen leaves.
All plants (usually trees and shrubs, less often - grass) are divided into deciduous and evergreen. At deciduous leaves develop during one growing season. Every year with the onset of adverse conditions they fall. The leaves of evergreen plants live from 1 to 15 years. Dying off parts of the old and the emergence of new leaves occurs constantly, the tree seems evergreen (coniferous, citrus).
The leaf is a lateral vegetative organ growing from the stem, having bilateral symmetry and a growth zone at the base. The leaf usually consists of a leaf blade, petiole (with the exception of sessile leaves), for some families the stipules are characteristic. The leaves are simple, having one leaf plate, and complex - with several leaf plates (leaves).
Leaf blade - expanded, usually flat part of the leaf, performing the functions of photosynthesis, gas exchange, transpiration and in some species - vegetative reproduction.
Sheet base (sheet pillow) - the part of the leaf connecting it with the stem. Here is an educational tissue that gives rise to leaf blades and petiole.
Stipules - paired leaf-shaped formations at the base of the leaf. They may fall off when you expand the sheet or persist. Protect axillary lateral buds and intercalated educational leaf tissue.
Scape - constricted part of the leaf, connecting with its base the leaf plate with the stem. It performs the most important functions: it orients the sheet in relation to the light; it is the location of the intercalary educational tissue, due to which the leaf grows. In addition, it has a mechanical meaning for attenuating impacts on the sheet from rain, hail, wind, etc.
Simple and complex leaves
A sheet can have one (simple), several or many leaf plates. If the latter are equipped with joints, then this sheet is called complex. Thanks to the joints on the common leaf stalk, leaves of complex leaves fall one by one. However, in some plants, complex leaves may fall off completely.
The shape of whole leaves, distinguished as lobed, separate and dissected.
Lobed I call a sheet, in which the cuts at the edges of the plate reach one quarter of its width, and with a larger recess, if the cuts reach more than a quarter of the width of the plate, the sheet is called separate. The blades of a separate sheet are called lobes.
Dissected they call a leaf, whose cuts along the edges of the plate reach almost the midrib, forming plate segments. Separate and dissected leaves can be palmate and pinnate, double palmar and double pinnate, etc. accordingly, a paldent-separated leaf, a pinnorassy sheet, and an unpaired pinnorassy sheet in potatoes are distinguished. It consists of a finite lobe, several pairs of lateral lobes, between which are even smaller lobules.
If the plate is elongated and the lobes or segments are triangular, the sheet is called strugoid (dandelion), if the lateral lobes are unequal, decrease to the base, and the final share is large and rounded, it turns out to be a lyre leaf (radish).
As for complex leaves, among them distinguish trifoliate, palmatological and peristosyllable leaves. If a complex leaf consists of three leaflets, it is called triylose or trifoliate (maple). If the stalk leaflets are attached to the main stalk as if at one point, and the leaflets diverge radially, the leaf is called finger-composite (lupine). If on the main petiole side leaflets are located on both sides along the length of the petiole, the leaf is called peristosis.
If such a leaf ends up at the top with an unpaired single leaflet, it turns out an odd-pinnate leaf. If there is no finite, the leaf is called paired-feather.
If each leaf of the peristosyllabic leaf, in turn, is complex, then a double peristosyllabic leaf is obtained.
Forms of solid leaf blades
Complicated leaf is called the one on the stem of which there are several leaf plates. They are attached to the main petiole with their own petioles, often independently, singly, fall down, and are called leaflets.
The forms of leaf plates of various plants differ in shape, degree of dissection, the shape of the base and top. Outlines can be oval, round, elliptical, triangular and others. Blade is elongated. Its free end can be sharp, blunt, pointed, pointed. Its base is narrowed and drawn to the stem, it can be rounded, heart-shaped.
Attaching leaves to the stalk
The leaves are attached to the shoot long, short stalks or are sedentary.
In some plants, the base of the sessile leaf coalesces over a large distance with the shoot (decaying leaf) or the shoot penetrates the leaf plate through (pierced leaf).
Leaf blade shape
Leaf blades are distinguished according to the degree of dissection: shallow cuts - jagged or palmate edges of the sheet, deep cuts - lobed, separate and dissected edges.
If the edges of the leaf blade do not have any notches, the sheet is called full edge. If the notches on the edge of the sheet are shallow, the sheet is called solid.
Bladed a sheet is a sheet whose plate is dissected into lobes up to 1/3 of the width of a semi-sheet.
Separated sheet - a sheet with a plate dissected to ½ the width of a half sheet.
Dissected leaf - leaf, the plate of which is dissected to the main vein or to the base of the leaf.
The edge of the leaf blade - serrata (acute angles).
The edge of the leaf blade - crenate (rounded protrusions).
The edge of the leaf blade - notched (rounded notches).
Leaf skin structure
The upper skin (epidermis) is a covering tissue on the reverse side of the leaf, often covered with hairs, cuticle, wax. Outside, the sheet has a skin (coating tissue), which protects it from the adverse effects of the external environment: from drying, from mechanical damage, from penetration of pathogens to the internal tissues. Peel cells are live, in size and shape they are different. Some of them are larger, colorless, transparent and tight to each other, which increases the protective qualities of the coating fabric. Cell transparency allows sunlight to penetrate inside the leaf.
Other cells are smaller, they contain chloroplasts, giving them a green color. These cells are arranged in pairs and have the ability to change their shape. At the same time, the cells either move away from each other, and a gap appears between them, or they approach each other and the gap disappears. These cells were called closing, and the gap between them appeared stomatal. The mouth opens when the guard cells are saturated with water. With outflow of water from the locking cells, the stomata closes.
Through the stomatal gaps, air enters the inner cells of the leaf, through them the same gaseous substances, including water vapor, leave the leaf to the outside. With insufficient water supply of the plant (which can happen in dry and hot weather), the stomata close. With this, the plants protect themselves from draining, since water vapor with closed stomatal crevices does not go outside and is stored in the intercellular spaces of the leaf. Thus, plants retain water in the dry season.
The structure of leaf veins
In addition to bast, wood is also a part of the conducting beam. In the leaf vessels, as well as at the root, water moves with mineral substances dissolved in it. The plant absorbs water and minerals from the soil by its roots. Then from the roots through the vessels of the wood these substances enter the elevated organs, including the leaf cells.
The composition of the numerous veins include fibers. These are long cells with pointed ends and thick lignified shells. Large leaf veins are often surrounded by mechanical tissue, which consists entirely of thick-walled cells - fibers.
Thus, along the veins there is a transfer of a solution of sugar (organic matter) from the leaf to other plant organs, and from the root - water and mineral substances to the leaves. From the sheet, the solutions move through sieve tubes, and to the sheet - through the vessels of wood.
The lower peel is the integumentary tissue on the underside of the leaf, usually the stomata.
Vital activity sheet
Green leaves are organs of air nutrition. The green leaf has an important function in the life of plants - organic substances are formed here. The structure of the leaf fits well with this function: it has a flat leaf plate, and the leaf pulp contains a huge amount of chloroplasts with green chlorophyll.
Substances necessary for the formation of starch in chloroplasts
Purpose: find out what substances are necessary for the formation of starch?
What we do: put two small indoor plants in a dark place. After two or three days, we put the first plant on a piece of glass, and next we place a glass with a solution of caustic alkali (it will absorb all carbon dioxide from the air), and we will cover all this with a glass cap. In order to prevent air from reaching the plant from the environment, lubricate the edges of the cap with Vaseline.
Второе растение также поставим под колпак, но только рядом с растением поместим стакан с содой (или кусочком мрамора), смоченными раствором соляной кислоты. В результате взаимодействия соды (или мрамора) с кислотой выделяется углекислый газ. В воздухе под колпаком второго растения образуется много углекислого газа.
Both plants will put in the same conditions (in the light).
The next day, take a sheet of each plant and treat it first with hot alcohol, wash and act with iodine solution.
What we observe: in the first case, the color of the sheet has not changed. The dark blue was the leaf of the plant, which was under the hood, where there was carbon dioxide.
Conclusion: this proves that carbon dioxide is necessary for the plant to form organic matter (starch). This gas is part of the atmospheric air. The air enters the leaf through the stomatal gaps and fills the spaces between the cells. Carbon dioxide penetrates into all cells from the intercellular spaces.
Organic matter in leaves
Purpose: find out which cells in the green leaf form organic matter (starch, sugar).
What we do: A potted geranium plant will be placed for three days in a dark cupboard (so that nutrients can flow out of the leaves). After three days, remove the plant from the cabinet. Attach an envelope of black paper with the cut out word "light" onto one of the leaves and place the plant in the light or under a light bulb. After 8-10 hours, cut the sheet. Remove the paper. Dip a sheet in boiling water, and then for a few minutes in hot alcohol (chlorophyll dissolves well in it). When the alcohol turns green and the leaf becomes discolored, rinse it with water and place in a weak iodine solution.
What we observe: blue letters appear on the bleached sheet (starch turns blue from iodine). The letters appear on the part of the sheet on which the light fell. So, in the lighted part of the sheet formed starch. It should be noted that the white strip on the edge of the sheet is not stained. This explains the fact that there is no chlorophyll in the plastids of the cells of the white strip of the leaf of the geranium bordered. Therefore, starch is not detected.
Conclusion: Thus, organic substances (starch, sugar) are formed only in cells with chloroplasts, and light is necessary for their formation.
Special studies by scientists have shown that sugar forms in the light in chloroplasts. Then, as a result of transformations, starch is formed from sugar in chloroplasts. Starch is an organic substance that does not dissolve in water.
The light and dark phases of photosynthesis are distinguished.
During the light phase of photosynthesis, light is absorbed by pigments, the formation of excited (active) molecules with an excess of energy, photochemical reactions take place, in which excited pigment molecules take part. Light reactions take place on the chloroplast membranes, where chlorophyll is located. Chlorophyll is a highly active substance that absorbs light, primary storage of energy and its further conversion into chemical energy. Yellow carotenoid pigments also take part in photosynthesis.
The process of photosynthesis can be represented as a summary equation:
Thus, the essence of light reactions is that light energy turns into chemical energy.
Dark photosynthesis reactions take place in the chloroplast matrix (stroma) with the participation of enzymes and light reaction products and lead to the synthesis of organic substances from carbon dioxide and water. For dark reactions do not need direct light.
The result of dark reactions is the formation of organic compounds.
The process of photosynthesis is carried out in chloroplasts, in two stages. In granahs (thylakoids), reactions caused by light occur — light, and in stroma, reactions not associated with light — dark, or carbon fixation reactions.
1. Light, falling on chlorophyll molecules, which are located in the membranes of grana thylakoids, leads them to an excited state. As a result, the ē electrons leave their orbits and are transported by carriers beyond the thylakoid membrane, where they accumulate, creating a negatively charged electric field.
2. The place of released electrons in chlorophyll molecules is occupied by electrons of water ē, since water under the action of light undergoes photodegradation (photolysis):
Hydroxyls ОН‾, becoming radicals ОН, unite: 4OH → 2Н2O + O2↑, forming water and free oxygen that is released into the atmosphere.
3. The H + protons do not penetrate the thylakoid membrane and accumulate inside using a positively charged electric field, which leads to an increase in the potential difference on both sides of the membrane.
4. When the critical potential difference (200 mV) is reached, the H + protons rush along the proton channel in the ATP synthetase enzyme, which is integrated into the thylakoid membrane, to the outside. At the exit from the proton channel, a high level of energy is generated, which goes to the synthesis of ATP (ADP + F → ATP). The formed ATP molecules pass into the stroma, where they participate in carbon fixation reactions.
5. The protons H +, which emerged on the surface of the thylakoid membrane, are connected to the electrons ē, forming atomic hydrogen H, which is used to restore the NADP +: 2ē + 2Н + = NADF + → NADF ∙ Н2 (carrier with hydrogen attached, reduced carrier).
In this way, a light-activated chlorophyll electron is used to add hydrogen to the carrier. NADPH2 enters the chloroplast stroma, where it participates in carbon fixation reactions.
Carbon fixation reactions (dark reactions)
It is carried out in the stroma of the chloroplast, where ATP, NADP ∙ H are delivered.2 from thylakoids gran and CO2 from the air. In addition, there are constantly located five-carbon compounds - pentoses C5which are formed in the cycle of Calvin (cycle fixation WITH2) Simply this cycle can be represented as follows:
1. To pentose C5 joins CO2, resulting in an unstable hexagonal compound C6which splits into two three carbon groups 2C3 - triozy.
2. Each triosis 2C3 takes one phosphate group from two ATP, which enriches molecules with energy.
3. Each triosis 2C3 adds one hydrogen atom from two NADP ∙ H2.
4. After that, some trioses combine to form 2C carbohydrates.3 → C6 → C6H12ABOUT6 (glucose).
5. Other trioses combine to form pentoses 5C3→ 3C5, and are again included in the cycle of fixation WITH2.
The total reaction of photosynthesis:
In addition to carbon dioxide, water is involved in the formation of starch. Her plant gets out of the soil. The roots absorb water, which rises through the vessels of the conducting bundles into the stem and further into the leaves. And already in the cells of the green leaf, in chloroplasts, organic matter is formed from carbon dioxide and water in the presence of light.
What happens to organic matter formed in chloroplasts?
Starch formed in chloroplasts under the influence of special substances turns into soluble sugar, which is supplied to the tissues of all plant organs. In the cells of some tissues, sugar can turn back into starch. Reserve starch accumulates in colorless plastids.
From sugars formed during photosynthesis, as well as mineral salts absorbed by the roots of the soil, the plant creates substances that it needs: proteins, fats, and many other proteins, fats, and many others.
Part of the organic substances synthesized in the leaves is spent on the growth and nutrition of the plant. The other part is deposited in stock. In annual plants, spare substances are deposited in the seeds and fruits. In the biennial in the first year of life, they accumulate in the vegetative organs. In perennial grasses, substances are stored in underground organs, and in trees and shrubs, in the core, the main bark and wood tissue. In addition, organic matter begins to accumulate in fruits and seeds in a given year of life.
Types of plant nutrition (mineral, air)
In living cells of the plant is constantly metabolism and energy. Some substances are absorbed and used by the plant, others are released into the environment. From simple substances are formed complex. Complex organic substances are split into simple ones. Plants accumulate energy, and in the process of photosynthesis and release it during breathing, using this energy to carry out various life processes.
Thanks to the stomata, the leaves also perform such an important function as gas exchange between the plant and the atmosphere. Through the stomata sheet with atmospheric air enters carbon dioxide and oxygen. Oxygen is used when breathing, carbon dioxide is necessary for the plant for the formation of organic substances. Through the stomata into the air oxygen is released, which was formed during photosynthesis. Removed and carbon dioxide, which appeared at the plant in the process of breathing. Photosynthesis is carried out only in the light, and breathing in the light and in the dark, i.e. constantly. Breathing in all living cells of plant organs occurs continuously. Like animals, plants die with cessation of breathing.
In nature, there is a metabolism between the living organism and the environment. Absorption by a plant of some substances from the external environment is accompanied by release of others. Elodea, being an aquatic plant, uses carbon dioxide dissolved in water to feed it.
Purpose: find out what substance elodie releases into the environment during photosynthesis?
What we do: we cut the stems of the twigs under water (boiled water) at the base and cover with a glass funnel. A test tube, filled to the brim with water, is placed on the funnel tube. This is done in two versions. To put one capacity in a dark place, and another - to expose on a bright solar or artificial light.
Add carbon dioxide to the third and fourth containers (add a small amount of baking soda or you can breathe into the straw) and also put one into the dark in sunlight.
What we observe: after some time, in the fourth version (a vessel standing in bright sunlight) bubbles begin to stand out. This gas displaces water from the tube, its level in the tube is displaced.
What we do: when the water is completely displaced by gas, it is necessary to carefully remove the tube from the funnel. Tightly close the hole with the thumb of the left hand, and with the right quickly bring the smoldering splash into the tube.
What we observe: the torch lights up in a bright flame. Looking at the plants that were placed in the darkness, we see that gas bubbles from the elodea are not released, and the test tube is left filled with water. The same with test tubes in the first and second version.
Conclusion: it follows that the gas that the elodie allocated - oxygen. Thus, the plant produces oxygen only when there are all conditions for photosynthesis - water, carbon dioxide, light.
The relationship of respiration and photosynthesis
The whole process of respiration takes place in the cells of the plant organism. It consists of two stages, during which organic substances are split into carbon dioxide and water. At the first stage, with the participation of special proteins (enzymes), glucose molecules break down into simpler organic compounds and some energy is released. This stage of the respiratory process occurs in the cytoplasm of cells.
At the second stage, simple organic substances formed at the first stage, under the action of oxygen, decompose into carbon dioxide and water. This releases a lot of energy. The second stage of the respiratory process occurs only with the participation of oxygen and in special cells of the cell.
The absorbed substances in the process of transformation in cells and tissues become the substances from which the plant builds its body. All transformations of substances that occur in the body, always accompanied by energy consumption. A green plant, as an autotrophic organism, absorbing the light energy of the Sun, accumulates it in organic compounds. In the process of respiration during the splitting of organic substances, this energy is released and used by the plant for the vital processes that occur in the cells.
Both processes - photosynthesis and respiration - follow the path of numerous chemical reactions in which some substances are converted into others.
Thus, in the process of photosynthesis, sugars are formed from carbon dioxide and water obtained by a plant from the environment, which then turn into starch, fiber or proteins, fats and vitamins - substances necessary for the plant to nourish and store energy. In the process of respiration, on the contrary, the organic substances created during the process of photosynthesis are split into inorganic compounds - carbon dioxide and water. At the same time the plant receives the released energy. These transformations of substances in the body are called metabolism. Metabolism is one of the most important signs of life: with the cessation of metabolism, plant life ceases.
The influence of environmental factors on the structure of the sheet
The leaves of plants in wet places are usually large with a large number of stomata. A lot of moisture evaporates from the surface of these leaves.
The leaves of plants in arid areas are small in size and have adaptations that reduce evaporation. This is a dense pubescence, wax coating, a relatively small number of stomata, etc. In some plants, the leaves are soft and juicy. Water is stored in them.
The leaves of shade-tolerant plants have only two or three layers of rounded, loosely adjacent cells. Large chloroplasts are located in them so that they do not obscure each other. Shadow leaves are usually thinner and have a darker green color, as they contain more chlorophyll.
In open space plants, the leaf pulp has several layers, closely adjacent columnar cells. They contain less chlorophyll, so the light leaves have a lighter color. Those and other leaves can sometimes be found in the crown of the same tree.
The outer wall of each cell of the skin of the leaf is not only thickened, but also protected by the cuticle, which does not let water in well. The protective properties of the skin are greatly enhanced by the formation of hairs that reflect the sun's rays. Due to this, the heating of the sheet is reduced. All this limits the possibility of evaporation of water from the surface of the sheet. With a lack of water, the stomatal gap closes and the steam does not come out, accumulating in the intercellular spaces, which leads to the cessation of evaporation from the surface of the sheet. Plants in hot and dry habitats have a small plate. The smaller the surface of the sheet, the less danger of excessive water loss.
In the process of adaptation to environmental conditions, the leaves of some plants have changed because they began to play a role not typical of typical leaves. In the barberry part of the leaves have changed into spines.
Aging leaves and leaf fall
Leaf fall is preceded by leaf aging. This means that in all cells the intensity of life processes decreases - photosynthesis, respiration. The content of substances already existing in the cells that are important for the plant decreases and the supply of new substances, including water, is reduced. The disintegration of substances prevails over their formation. Unwanted and even harmful products accumulate in cells, they are called end products of metabolism. These substances are removed from the plant when the leaves are dropped. The most valuable compounds through conductive tissues flow from the leaves to other plant organs, where they are deposited in the cells of the accumulating tissues or immediately used by the body for nourishment.
Most of the trees and shrubs in the aging period leaves change color and become yellow or purple. This is because chlorophyll is destroyed. But besides it in the plastids (chloroplasts) there are substances of yellow and orange color. In the summer they were as if disguised by chlorophyll and the plastids were green. In addition, in the vacuoles accumulate other dyes yellow or red-crimson. Together with plastid pigments, they determine the color of autumn leaves. In some plants, the leaves retain their green color until they die.
Even before the leaf falls from the shoot, a cork layer is formed at its base on the border with the stem. Outward from it forms a separating layer. Over time, the cells of this layer become clotted from each other, as the intercellular substance that connected them and sometimes the cell walls become more and more delicate. The leaf is separated from the stem. However, for some time it is still preserved on escape due to the conducting beams between the leaf and the stem. But there comes a moment of violation and this connection. The scar in place of the separated leaf is covered with a protective cloth, cork.
As soon as the leaves reach their limiting sizes, aging begins, leading eventually to the death of the leaf - its yellowing or redness associated with the destruction of chlorophyll, the accumulation of carotenoids and anthocyanins. As the leaf ages, the intensity of photosynthesis and respiration also decreases, chloroplasts degrade, some salts accumulate (calcium oxalate crystals), plastic substances flow from the leaf (carbohydrates, amino acids).
In the process of leaf aging near the dicotyledon woody plants, a so-called separation layer is formed, which consists of easily exfoliating parenchyma. On this layer, the leaf is separated from the stem, and on the surface of the future leaf scar a protective layer of cork fabric is formed in advance.
На листовом рубце заметны в виде точек поперечные сечения листового следа. Скульптура листового рубца различна и является характерным признаком для систематики лепидофитов.
У однодольных и травянистых двудольных отделительный слой, как правило, не образуется, лист отмирает и разрушается постепенно, оставаясь на стебле.
In deciduous plants, the fall of the leaves for the winter has an adaptive meaning: by dropping the leaves, the plants drastically reduce the evaporating surface, they are protected from possible breakages under the weight of snow. In evergreen plants, massive leaf fall is usually timed to the beginning of the growth of new shoots from the buds and therefore occurs not in autumn, but in spring.
Autumn leaf fall in the forest has an important biological significance. Fallen leaves are a good organic and mineral fertilizer. Every year, in their deciduous forests, fallen leaves serve as material for mineralization produced by soil bacteria and fungi. In addition, the fallen foliage stratifies seeds that have fallen to fall, protects the roots from freezing, prevents the development of moss cover, etc. Some species of trees shed not only foliage, but also one-year-old shoots.
Crimson or Cercis european, Judah tree (Cercis siliquastrum)
Bilberry ordinary, myrtolist (Vaccinium myrtillus)
The main part of a regular sheet is its plate. Leaf blade - This is an extended flat formation that performs the functions of photosynthesis, gas and water exchange. In addition to the plate leaves often have scape - an elongated cylindrical stem-like part, with the help of which the plate is attached to the stem. If there is a petiole, the leaf is called petiolate, and in its absence - sessile. The bottom of the sheet is his base - can grow and cover the stem in the form of a tube. This formation is called the leaf sheath. Quite often, at the base of the leaf at the petiole there are special processes - stipules. Stipules are paired, of various shapes and sizes, green or colorless, loose or accrete with a petiole. Stipules may fall off as the leaf grows or not fall off.
Simple are the leaves that have one leaf plate on the scape, and in a complex leaf, several plates are attached to one scape, called leaflets.
A simple sheet. The leaf plate of a simple sheet may be integral or, on the contrary, dissected, i.e. more or less rugged, consisting of the protruding parts of the plate and grooves. To determine the nature of the dissection, the degree and shape of the broken platelets and the correct naming of such leaves, first of all, it is necessary to take into account how the protruding parts of the plate - the blades, lobes, segments - are distributed with respect to the petiole and the main vein of the leaf. If the protruding parts are symmetrical to the main vein, then such leaves are called feathery. If the protruding parts come out as if from one point, the leaves are called finger-like. The depth of the cut-out leaf blades distinguishes the leaves: lobed, if the notches (depth of cuts) do not reach half the width of the half-plate (the protruding parts are called blades), separate, with the depth of the cut-outs reaching deeper than half the width of the half-plate (the protruding parts are the lobes) cut the depth of the cuts reaching the main vein or almost touching it (the protruding parts are segments).
Complicated sheet. Complicated leaves, by analogy with simple ones, are called feathery and palmate with the addition of the word “complex”. For example, peristosyllabic, palmatosyllabic, ternary, etc. If a complex leaf ends in one leaf, the leaf is called unparticularly complex. If it ends in a pair of leaflets, it is called a pair-perish-complex.
The dismemberment of a plate of a simple sheet, as well as the branching of parts of a complex sheet, can be multiple. In these cases, taking into account the order of branching or dismemberment, they speak of twice-, thrice-, fourfold or palmate, simple or complex leaves.
1 - entire, 2 - notched, 3 - wavy, 4 - prickly, 5 - toothed, 6 - biconoidal, 7 - serrated, 8 - crenate
Top shapes The shapes of the tops, bases and edges of leaf plates are also signs used in the description and definition of plants.
What are the leaves: external structure
The green plate in all cases is located on the side of the shoot, in the node of the stems. The vast majority of plants have flat foliage that distinguishes this part of the plant from others. This type of sheet is not without reason, since due to the flat form, it ensures maximum contact with air and light. This plant organ is bounded by leaf blade, petiole, stipule and base. In nature, there are also species of plants that have no stipules and petioles.
According to the shape of the plate
There are the following sheet plates that are in shape:
- broadly ovate,
- obversely lamellar,
- obliquely ovoid,
The edges of the plant can be:
- notched out
Plate tops can be:
- notched out
The bases of the green plates can be of the following forms:
- roundly wedge-shaped,
- wedge shaped
- drawn by
Types of venation
When the study of the appearance of the considered part of the plant takes place, the veins, which are small bunches, are clearly visible. Thanks to the veins, the plate is fed with water and mineral salts, as well as the elimination of organic matter accumulated in the plant.
The main types of venation are: arcuate, parallel, reticular or pinnate, palmate. As the arc venation of leaves, one can cite examples of such plants: lilies of the valley, plantain, which have a large venation, presented in the form of one central flat vein, around which all other veins are arranged in an arcuate manner. As a parallel venation, we can consider examples of maize and wheat plants.
As examples of mesh venation are sheets of aspen, oak, birch. They have a main vein, which is surrounded by many small, creating a kind of grid.
As an example of the finger-like venation, a platanovo maple, caustic buttercup, presented in the form of large veins, which diverge in a fan-shaped manner, has many smaller fan branches.
By leaf position
The leaf layout is presented in the form of whorled, alternate, rosette and opposite.
As an example of a whorl leaf layout, you can consider the horsetail of the forest, the next leaf position - vanilla leaves, the rosette leaf position - plantain leaves, the opposite leaf position - Rostok's eyebright.
Leaf morphological parts
The leaf, as a rule, is a flat dorsiventral organ, the shape and dimensions of which contribute to the creation of the maximum photosynthesizing surface at optimal values of transpiration. The number of leaves on a plant is very different. It is believed, for example, that one oak tree carries up to 250,000 leaves. The flat shape makes the leaf bifacial, i.e. two-way. Therefore, we can speak of the upper and lower sides of the leaf, bearing in mind the orientation of these sides in relation to the top of the shoot. The upper side can also be called the ventral, or adaxial, and the lower side - the dorsal, or abaxial. This is due to the position of the leaf bud in the bud. The upper and lower sides often differ significantly among themselves in their anatomical structure, the nature of venation and color. Leaf sizes most often range from 3 to 10 cm, however, giant leaves of some palm trees up to 15 m long are known. The largest leaves of the famous Amazonian water lily Victoria regia (Victoria regia) reach 2 m in diameter. The size, shape and degree of dissection of the leaves, although they are hereditary traits of a particular species, are very variable and also depend on the living conditions of its individuals. An adult leaf is usually dissected into a plate or several plates (in complex leaves) and the petiole is a narrow stem-shaped part connecting the plate and the shoot node. The lowest part of the leaf articulated with the stem is called the base of the leaf. Often, at the base of the leaf, a pair of lateral outgrowths - stipules (Fig.1) are noticeable in different sizes and shapes. Plate - the main part of the sheet, as a rule, performing its main functions. The plate is reduced extremely rarely, and then its functions are taken either by an expanded leaf-like petiole - phyllody (in Australian acacias), or large leaf-shaped stipules (in some types of rank).
Fig.1. A - petiolate, B - sedentary, B - with a cushion at the base of the petiole, G and D - vaginal, with stipules: free - Е, accreting to the petiole - F, axillary intergrowths - V. 1 - plate of leaf, 2 - base of petiole, 3 - vagina, 4 - stipules, 5 - petiole, 6 - axillary kidney
The scape usually rounded or oblate in cross section. In addition to the supporting and conductive functions, it can retain its ability to insert growth for a long time and can adjust the position of the plate, bending towards the light. Quite often the scape does not develop, and then the leaf is called sessile. A leaf with a petiole is called petiolate.
The base of the sheet takes a different shape. Very often it is narrowed or has the appearance of a small thickening (leaf pad). However, not infrequently, especially in grasses and umbrellas, it expands and forms a closed or open tube, called a leaf sheath. The leaf sheath protects the axillary buds, contributes to the long-term preservation of the intercalary stem meristem and often serves as a means of additional support for the shoot.
In the leaf bosom, a kidney can form (which in this case is called the axillary kidney
In the process of leaf formation, the stipules grow before the plate and play a protective role, making up part of the kidney covers. After deployment, the stipules often fall off or dry up. Occasionally, they have dimensions comparable to those of the leaf blade (especially in complex leaves, in particular, in pea leaves), and function as photosynthetic organs. In the buckwheat family, the stipules form a so-called bell as a result of accretion, which encompasses the stem above the node in the form of a short filmy tube.
Not all plants have all of the above-mentioned parts of the leaves, in some species, paired stipules are not clearly expressed or absent, the petiole may be missing, and the leaf structure may not be lamellar. The huge variety of structure and location of the leaves are listed below.
External sheet characteristics, such as shape, edges, hairiness, etc., are very important for identifying a plant species, and botanists have created rich terminology for describing these characteristics. Unlike other plant organs, leaves are the determining factor, as they grow, form a certain pattern and shape, and then fall off, while the stems and roots continue to grow and change throughout the life of the plant and for this reason are not the determining factor.