Which plants have leaf venation. Leaf structure, external and internal

A leaf is the most important organ of a plant; its main function is photosynthesis, i.e. the synthesis of organic substances from inorganic ones. However, the leaves of different plant species differ in their external structure. By the shape of the leaf you can often determine what type of plant it belongs to. The diversity of the external structure of leaves is mainly due to the fact that plants are adapted to different living conditions.

Plant leaves vary in size. The smallest leaves are less than a centimeter in size (woodlouse, duckweed). Huge leaves are characteristic of some tropical plants. So the aquatic plant Victoria has a leaf diameter of more than a meter.

In the external structure of the leaves of most plants there areleaf blade And petiole. The leaf blade contains predominantly photosynthetic tissue, and the petiole serves to connect the leaf blade to the stem. However, some plant species have leaves without petioles. Leaves with petioles characteristic of most trees (maple, linden, birch, etc.). Leaves without petioles characteristic of aloe, wheat, corn, etc.

Upon external examination of the sheet, the so-called veins. They are better visible on the underside of the leaf. The veins are formed by conductive bundles and mechanical fibers. Water and minerals move through the conductive tissue from the roots, and organic substances move in the opposite direction, from the leaves. The mechanical tissue gives the leaves strength and rigidity.

At parallel venation The veins in the leaf blade are parallel to each other and look like straight lines.

At arc venation the arrangement of the veins is similar to parallel, but the further away from the central axis of the leaf blade, the more the vein has the shape of an arc rather than a straight line.

Parallel and arc venation is characteristic of many monocots. Thus, many cereals (wheat, rye) and onions have parallel veins, and lily of the valley has an arc vein.

At reticulate venation The veins in the leaf form a branching network. This veining is characteristic of many dicotyledonous plants.

There are other types of leaf venation.

Simple and compound leaves

Depending on the number of leaf blades on one petiole, leaves are divided into simple and complex.

U simple leaves Only one leaf blade develops on one petiole (birch, aspen, oak).

U compound leaves several or many leaf blades grow from one common petiole; Moreover, each such leaf has its own small petiole, which connects it to the common petiole. Examples of plants with compound leaves are rowan, acacia, and strawberry.

Leaf arrangement

The plant stem has nodes and internodes. Leaves grow from the nodes, and internodes are the sections of the stem between the nodes. The arrangement of leaves on the stem may vary depending on the type of plant.

If the leaves are arranged one at a time at the nodes, while all the leaves together give the appearance of an arrangement as if in a spiral along the stem, then we speak of next arrangement of leaves. This arrangement is typical for sunflower, birch, and rose hips.

At opposite arrangement leaves grow two at each node, opposite each other. Opposite arrangement is found in maple, nettle, etc.

If more than two leaves grow at each node, then they speak of whorled leaf arrangement. It is typical, for example, for elodea.

There is also rosette arrangement of leaves when there are almost no internodes, and all the leaves grow as if from one place in a circle.

The structure of the leaf blade. Shown are the palisade (top, tightly packed cells) and spongy (bottom, loosely packed cells) parts of the mesophyll, located between the upper and lower epidermal layers

Typically, the sheet consists of the following fabrics:

Epidermis- a layer of cells that protect against the harmful effects of the environment and excessive evaporation of water. Often, on top of the epidermis, the leaf is covered with a protective layer of waxy origin (cuticle).
Mesophyll, or parenchyma- internal chlorophyll-bearing tissue that performs the main function - photosynthesis.
Network of veins, formed by conducting bundles consisting of vessels and sieve tubes, for the movement of water, dissolved salts, sugars and mechanical elements.
Stomata- special complexes of cells located mainly on the lower surface of leaves; through them, water evaporation and gas exchange occur.

Epidermis

Plants in temperate and northern latitudes, as well as in seasonally dry climatic zones, can be deciduous, that is, their leaves fall or die with the advent of an unfavorable season. This mechanism is called dropping or falling off. In place of the fallen leaf, a scar forms on the branch - leaf trail. In autumn, the leaves may turn yellow, orange or red, as with decreasing sunlight the plant reduces the production of green chlorophyll, and the leaf becomes colored by accessory pigments such as carotenoids and anthocyanins.

Veins

The leaf veins are vascular tissue and are located in the spongy mesophyll layer. According to the branching pattern, the veins, as a rule, repeat the branching structure of the plant. The veins consist of xylem - tissue that serves to conduct water and minerals dissolved in it, and phloem - tissue that serves to conduct organic substances synthesized by leaves. Typically xylem lies on top of phloem. Together they form the main tissue called leaf core.

Leaf morphology

Canadian spruce needles ( Picea glauca)

Main types of leaves

A leaf-like appendage in certain plant species, such as ferns.
Leaves of coniferous trees that have a needle- or awl-shaped shape (needles).
Leaves of angiosperms (flowering plants): The standard form includes a stipule, a petiole, and a leaf blade.
Lycopods ( Lycopodiophyta) have microphyllous leaves.
Involucre leaves (the type found in most herbs)

Location on the stem

As the stem grows, the leaves are arranged on it in a certain order, which provides optimal access to light. Leaves appear on the stem in a spiral, both clockwise and counterclockwise, at a certain divergence angle. The exact Fibonacci sequence is observed in the divergence angle: 1/2, 2/3, 3/5, 5/8, 8/13, 13/21, 21/34, 34/55, 55/89. This sequence is limited to a full rotation of 360°, 360° x 34/89 = 137.52 or 137° 30" - an angle known in mathematics as the golden angle. In the sequence, the number gives the number of revolutions until the sheet returns to their original position. The example below shows the angles at which the leaves are located on the stem:

The next sheets are located at an angle of 180° (or 1/2)
120° (or 1/3): three sheets per turn
144° (or 2/5): five leaves in two turns
135° (or 3/8): eight leaves in three turns

Typically, leaf arrangement is described using the following terms:

Next(sequential) - leaves are arranged one at a time (in a queue) for each node.
Opposite- leaves are arranged two at each node and usually crosswise in pairs, that is, each subsequent node on the stem is rotated relative to the previous one at an angle of 90°; or in two rows, if not unfolded, but there are several nodes.
Whorled- leaves are arranged in three or more at each node of the stem. Unlike opposite leaves, in whorled leaves, each subsequent curl may or may not be at an angle of 90° from the previous one, rotating at half an angle between the leaves in the curl. However, note that the opposite leaves may appear whorled at the end of the stem.
Rosette- leaves arranged in a rosette (a bunch of leaves arranged in a circle from one common center).

Sheet sides

Any leaf in plant morphology has two sides: abaxial and adaxial.

Abaxial side(from lat. ab- “from” and lat. axis- “axis”) - the side of the lateral organ of a shoot (leaf or sporophyll) of a plant, facing away from the growth cone (apex) of the shoot when planting. Other names - dorsal side, dorsal side.

The opposite side is called adaxial(from lat. ad- “k” and lat. axis- “axis”). Other names - ventral side, ventral side.

In the vast majority of cases, the abaxial side is the surface of the leaf or sporophyll facing the base of the shoot, but occasionally the side that is formed abaxially turns 90° or 180° during development and is located parallel to the longitudinal axis of the shoot or faces its apex. This is typical, for example, for the needles of some spruce species.

The terms “abaxial” and “adaxial” are useful in that they allow us to describe plant structures using the plant itself as a frame of reference and without resorting to ambiguous designations such as “top” or “bottom” side. Thus, for shoots directed vertically upward, the abaxial side of the lateral organs will, as a rule, be lower, and the adaxial side - upper, however, if the orientation of the shoot deviates from the vertical, then the terms “upper” and “lower” side can be misleading.

Separation of leaf blades

Based on the way the leaf blades are divided, two basic leaf shapes can be described.

Simple sheet consists of a single leaf blade and one petiole. Although it may consist of several lobes, the spaces between these lobes do not reach the main vein of the leaf. A simple leaf always falls off entirely.
Complex sheet consists of several leaves, located on a common petiole (called rachis). Leaflets, in addition to their leaf blade, may also have their own petiole (which is called petiole, or secondary petiole). In a complex leaf, each blade falls separately. Since each leaflet of a compound leaf can be considered a separate leaf, locating the petiole is very important when identifying the plant. Compound leaves are characteristic of some higher plants, such as legumes.
- U palmate(or palmate) leaves, all leaf blades diverge radially from the end of the root, like the fingers of a hand. The main leaf petiole is missing. Examples of such leaves include hemp ( Cannabis) and horse chestnut ( Aesculus).
- U feathery leaves, leaf blades are located along the main petiole. In turn, feathery leaves can be odd-pinnate, with an apical leaf blade (example - ash, Fraxinus); And paripirnate, without apical plate (example - mahogany, Swietenia).
- U bipinnate leaves are divided twice: the blades are located along the secondary petioles, which in turn are attached to the main petiole (example - albizia, Albizzia).
- U trifoliate leaves have only three blades (example: clover, Trifolium; bean, Laburnum)
- Finger-notching the leaves resemble pinnate ones, but their plates are not completely separated (for example, some mountain ash, Sorbus).

Characteristics of petioles

Petiolate leaves have a petiole - a stalk to which they are attached. U thyroid The leaf petiole is attached inside from the edge of the blade. sedentary And entwining leaves have no petiole. Sessile leaves are attached directly to the stem; in entwining leaves, the leaf blade completely or partially envelops the stem, so that it seems that the shoot is growing directly from the leaf (example - Claytonia pierced-leaved, Claytonia perfoliata). In some species of acacia, for example the species Acacia koa, the petioles are enlarged and expanded and perform the function of a leaf blade - such petioles are called phyllodes. At the end of the phyllode, a normal leaf may or may not exist.

Stipule characteristics

Stipule, present on the leaves of many dicotyledonous plants, is an appendage on each side of the base of the petiole and resembles a small leaf. Stipules may fall off as the leaf grows, leaving behind a scar; or they may not fall off, remaining together with the leaf (for example, this happens in roses and legumes).

Stipules can be:

free
fused - fused with the base of the petiole
bell-shaped - in the form of a bell (example - rhubarb, Rheum)
girdling the base of the petiole
interpetiolate, between the petioles of two opposite leaves
interpetiolate, between the petiole and the opposite stem

Venation

There are two subclasses of venation: marginal (the main veins reach the ends of the leaves) and arcuate (the main veins extend almost to the ends of the leaf edges, but turn before reaching it).

Types of venation:

Reticulate - local veins diverge from the main veins like a feather and branch into other small veins, thus creating a complex system. This type of venation is typical for dicotyledonous plants. In turn, reticulate venation is divided into:
- Pinnate nerve venation - a leaf usually has one main vein and many smaller ones, branching from the main one and running parallel to each other. Example - apple tree ( Malus).
- Radial - the leaf has three main veins emanating from its base. An example is redroot, or ceanothus ( Ceanothus).
- Palmate - several main veins diverge radially near the base of the petiole. Example - maple ( Acer).
Parallel - the veins run parallel along the entire leaf, from its base to its tip. Typical of monocots such as grasses ( Poaceae).
Dichotomous - there are no dominant veins, the veins are divided into two. Found in ginkgo ( Ginkgo) and some ferns.

Worksheet Terminology

Sheet Description Terminology

Leaves with different shapes. Clockwise from the right corner: triple lobed, oval with finely serrated edge, shield-shaped with palmate venation, pointed imparipinnate (in the center), pinnately dissected, lobed, oval with entire edge

Leaf shape

Needle: thin and sharp
Pointed: wedge-shaped with a long apex
Bipinnate: each leaf is pinnate
Heart-shaped: heart-shaped, the leaf is attached to the stem in the area of the dimple
Wedge-shaped: the leaf is triangular, the leaf is attached to the stem at the apex
Deltoid: triangular leaf, attached to the stem at the base of the triangle
Palmate: leaf divided into finger-like lobes
Oval: oval leaf with a short tip
Crescent: shaped like a sickle
Fan-shaped: semicircular, or fan-shaped
Arrow-shaped: a leaf shaped like an arrowhead, with flared blades at the base
Lanceolate: long leaf, wide in the middle
Linear: the leaf is long and very narrow
Blade: with multiple blades
Obcordate: heart-shaped leaf attached to the stem at the protruding end
Oblanceolate: the upper part is wider than the lower part
Obovate: teardrop-shaped, the leaf is attached to the stem at the protruding end
Round: round shape
Oval: the leaf is oval, ovate, with a pointed end at the base.
Palmate: divided into many lobes
Thyroid: leaf rounded, stem attached from below
Pinnate: two rows of leaves
- Imparipinnate: pinnate leaf with apical leaf
- Piripnate: pinnate leaf without apical leaf
Pinnately dissected: the leaf is dissected, but not to the middle
Reniform: kidney-shaped leaf
Diamond: diamond shaped leaf
Spatulate: spade-shaped leaf
Spear-shaped: sharp, with spines
Subulate: in the form of an awl
Trifoliate: leaf divided into three leaflets
Tripinnate: each leaflet is in turn divided into three
Single-lobed: with one leaf

Leaf edge

The edge of a leaf is often a characteristic of the plant genus and helps identify the species:

Full edge - with a smooth edge, without teeth
Ciliated - with fringe around the edges
Serrated - with teeth, like a chestnut. The tooth pitch can be large or small
- Rounded - with wavy teeth, like beech.
- Fine-toothed - with small teeth
Lobed - rugged, with notches that do not reach the middle, like many oaks
Serrated - with asymmetrical teeth directed forward towards the top of the leaf, like

The shape of the leaf blade can be surprisingly varied and bizarre, while it is an important systematic feature and is determined by the ratio of the length and width of the blade. Leaf blades are rounded(aspen), oval(hazel), oblong, lanceolate(willow), linear(rye), needle-shaped, ovoid(plantain), obovate(elm), etc. (Fig. 37).

When describing a leaf morphologically, attention is paid to the shape of the apex and base of the leaf blade. The tip of the leaf is dull, sharp, pointed etc. (Fig. 38). The basis may be round, wedge-shaped, heart-shaped, arrow-shaped, spear-shaped etc. (see Fig. 38).

The leaves also differ in the nature of the edge of the blade. The edges are rarely even; such leaves are called entire. If the edge of the sheet has notches extending no deeper than 1/4 of the width of the half-plate, the sheet is called solid, and its edge is called cut. According to the shape of the cuttings, they are distinguished jagged, wavy, crenate, serrate, notched edges of leaf blades (see Fig. 38).

Leaves whose edges are cut deeper than 1 / 4 half plates are called dismembered. Depending on the depth of the incisions, there are bladed, separate And dissected leaves (Fig. 39).

Simple and compound leaves. Simple the leaf has one leaf blade and falls off entirely when the leaves fall. A leaf consisting of several leaf blades, each of which has its own petiole, is called complex. As a rule, the leaf blades of such a leaf fall off independently of each other. The axis on which the blades of a complex leaf sit is called rachis(from the Greek rhachis - spinal ridge). The shape of the compound leaf is very diverse. Depending on the location of the leaf blades on the rachis, there are ternate, palmate And pinnately leaves (see Fig. 39). Pinnately compound leaves with an odd upper blade are called odd-pinnate, and with an even number of leaf blades - paripirnate.

Sometimes on one plant you can find leaves of different shapes (Fig. 40). This phenomenon is called heterophily(diversity of leaves). For example, fig leaves that are higher up on the tree are more lobed, allowing sunlight to penetrate into the lower part of the canopy.

Leaf size and lifespan. Leaf sizes are most often in the range of 3-10 cm, but there are plants with much larger or smaller leaves. The largest leaves are found on some tropical plants, for example, on the raffia palm they reach a length of 20 m. The leaves of the Victoria regia water lily growing in the Amazon reach enormous sizes (Fig. 41). With a diameter of about 2 m, they are capable of keeping weights of up to 40 kg afloat. Bananas, corn, and hogweed have very large leaves. On the other hand, many herbaceous plants have very small leaves, sometimes even difficult to distinguish with the naked eye. Small leaves are also found on perennial plants; for example, the leaves of the evergreen shrub heather are only a few millimeters in size.

The lifespan of one leaf varies from several months (for deciduous plants) to several years (evergreens). Among the leaves there are also long-living record holders. Velvichia is amazing, a kind of dwarf tree, it has only two leathery, belt-like leaves that grow throughout the life of the plant, gradually dying off at its top and growing at the base (Fig. 42). It should be noted that Welwitschia live for several centuries, and the age of some specimens reaches 2000 years or more.

Leaf arrangement The leaves are arranged on the stem in a certain order. This pattern, called leaf arrangement, was first described in the first half of the 19th century. There are three main types of leaf arrangement: alternate, opposite and whorled (Fig. 43).

Regular (spiral) location is the most common. In this case, the leaves move away from each node one by one, forming a spiral around the stem (apple tree).

Opposite the arrangement is characterized by the presence of two leaves at each node, one opposite the other (lilac, carnation).

At whorled arrangement on one node several independent leaves are placed at once (lily, raven's eye)

Typically, the leaves are arranged on the plant so as to minimally shade each other. , forming the so-called sheet mosaic. This is achieved by different sizes of leaf blades and petioles (maple).

Leaf modifications. During the course of evolution, many plants, along with true leaves, developed various modifications (Fig. 44).

Thorns. The most common leaf modification is spines. Barberry has sharp 3-7-parted spines - these are former leaves in which mesophyll does not develop, and white acacia spines are formed from stipules. The spines of cacti are also of leaf origin. The spines play a protective role, protecting plants from being eaten by animals, and reduce evaporation by reducing the surface area of the leaves.

Mustache. The transformation of leaves into tendrils is typical for plants of the legume family. In many species of peas, vetch, and china, the tendrils are formed from the central vein of the upper leaf (simple tendrils) or several leaves (branched tendrils). In the stipule, the entire leaf is transformed into a tendril, and the functions of the leaf are performed by large stipules. Tendrils can also be formed from stipules or from leaf petioles.

Scales. In many plants, the leaves change into scales. Thick juicy scales of the bulb store nutrients. The scales covering the buds perform a protective function, and the leaves-scales of saxaul help reduce transpiration.

Leaves of insectivorous (carnivorous) plants. About 450 species of mainly tropical plants are known, whose leaves have turned into special trapping devices. When there is a lack of nitrogen and minerals in the soil, insects are a good additional food for these amazing plants.

Phyllodes. In plants of arid regions, for example Australian acacias, leaf blades are reduced during development, and leaf petioles turn into phyllodes - flattened blades. Phyllodes are the main organ of photosynthesis in such plants.

Parts of a flower. Speaking about leaf metamorphoses, we should not forget that the main parts of the flower - the plant's reproductive organ - are also modified leaves. But we'll talk about this later.

Typically, the sheet consists of the following fabrics:

Epidermis- a layer of cells that protect against the harmful effects of the environment and excessive evaporation of water. Often, on top of the epidermis, the leaf is covered with a protective layer of waxy origin (cuticle).
Mesophyll, or parenchyma- internal chlorophyll-bearing tissue that performs the main function - photosynthesis.
Network of veins, formed by conducting bundles consisting of vessels and sieve tubes, for the movement of water, dissolved salts, sugars and mechanical elements.
Stomata- special complexes of cells located mainly on the lower surface of leaves; through them, water evaporation and gas exchange occur.

Epidermis

Veins

Leaf morphology

Canadian spruce needles ( Picea glauca)

Main types of leaves

A leaf-like appendage in certain plant species, such as ferns.
Leaves of coniferous trees that have a needle- or awl-shaped shape (needles).
Leaves of angiosperms (flowering plants): The standard form includes a stipule, a petiole, and a leaf blade.
Lycopods ( Lycopodiophyta) have microphyllous leaves.
Involucre leaves (the type found in most herbs)

Location on the stem

The next sheets are located at an angle of 180° (or 1/2)
120° (or 1/3): three sheets per turn
144° (or 2/5): five leaves in two turns
135° (or 3/8): eight leaves in three turns

Typically, leaf arrangement is described using the following terms:

Next(sequential) - leaves are arranged one at a time (in a queue) for each node.
Opposite- leaves are arranged two at each node and usually crosswise in pairs, that is, each subsequent node on the stem is rotated relative to the previous one at an angle of 90°; or in two rows, if not unfolded, but there are several nodes.
Whorled- leaves are arranged in three or more at each node of the stem. Unlike opposite leaves, in whorled leaves, each subsequent curl may or may not be at an angle of 90° from the previous one, rotating at half an angle between the leaves in the curl. However, note that the opposite leaves may appear whorled at the end of the stem.
Rosette- leaves arranged in a rosette (a bunch of leaves arranged in a circle from one common center).

Sheet sides

Any leaf in plant morphology has two sides: abaxial and adaxial.

The opposite side is called adaxial(from lat. ad- “k” and lat. axis- “axis”). Other names - ventral side, ventral side.

Separation of leaf blades

Based on the way the leaf blades are divided, two basic leaf shapes can be described.

Simple sheet consists of a single leaf blade and one petiole. Although it may consist of several lobes, the spaces between these lobes do not reach the main vein of the leaf. A simple leaf always falls off entirely.
Complex sheet consists of several leaves, located on a common petiole (called rachis). Leaflets, in addition to their leaf blade, may also have their own petiole (which is called petiole, or secondary petiole). In a complex leaf, each blade falls separately. Since each leaflet of a compound leaf can be considered a separate leaf, locating the petiole is very important when identifying the plant. Compound leaves are characteristic of some higher plants, such as legumes.
- U palmate(or palmate) leaves, all leaf blades diverge radially from the end of the root, like the fingers of a hand. The main leaf petiole is missing. Examples of such leaves include hemp ( Cannabis) and horse chestnut ( Aesculus).
- U feathery leaves, leaf blades are located along the main petiole. In turn, feathery leaves can be odd-pinnate, with an apical leaf blade (example - ash, Fraxinus); And paripirnate, without apical plate (example - mahogany, Swietenia).
- U bipinnate leaves are divided twice: the blades are located along the secondary petioles, which in turn are attached to the main petiole (example - albizia, Albizzia).
- U trifoliate leaves have only three blades (example: clover, Trifolium; bean, Laburnum)
- Finger-notching the leaves resemble pinnate ones, but their plates are not completely separated (for example, some mountain ash, Sorbus).

Characteristics of petioles

Stipule characteristics

Stipules can be:

free
fused - fused with the base of the petiole
bell-shaped - in the form of a bell (example - rhubarb, Rheum)
girdling the base of the petiole
interpetiolate, between the petioles of two opposite leaves
interpetiolate, between the petiole and the opposite stem

Venation

There are two subclasses of venation: marginal (the main veins reach the ends of the leaves) and arcuate (the main veins extend almost to the ends of the leaf edges, but turn before reaching it).

Types of venation:

Reticulate - local veins diverge from the main veins like a feather and branch into other small veins, thus creating a complex system. This type of venation is typical for dicotyledonous plants. In turn, reticulate venation is divided into:
- Pinnate nerve venation - a leaf usually has one main vein and many smaller ones, branching from the main one and running parallel to each other. Example - apple tree ( Malus).
- Radial - the leaf has three main veins emanating from its base. An example is redroot, or ceanothus ( Ceanothus).
- Palmate - several main veins diverge radially near the base of the petiole. Example - maple ( Acer).
Parallel - the veins run parallel along the entire leaf, from its base to its tip. Typical of monocots such as grasses ( Poaceae).
Dichotomous - there are no dominant veins, the veins are divided into two. Found in ginkgo ( Ginkgo) and some ferns.

Worksheet Terminology

Sheet Description Terminology

Leaf shape

Needle: thin and sharp
Pointed: wedge-shaped with a long apex
Bipinnate: each leaf is pinnate
Heart-shaped: heart-shaped, the leaf is attached to the stem in the area of the dimple
Wedge-shaped: the leaf is triangular, the leaf is attached to the stem at the apex
Deltoid: triangular leaf, attached to the stem at the base of the triangle
Palmate: leaf divided into finger-like lobes
Oval: oval leaf with a short tip
Crescent: shaped like a sickle
Fan-shaped: semicircular, or fan-shaped
Arrow-shaped: a leaf shaped like an arrowhead, with flared blades at the base
Lanceolate: long leaf, wide in the middle
Linear: the leaf is long and very narrow
Blade: with multiple blades
Obcordate: heart-shaped leaf attached to the stem at the protruding end
Oblanceolate: the upper part is wider than the lower part
Obovate: teardrop-shaped, the leaf is attached to the stem at the protruding end
Round: round shape
Oval: the leaf is oval, ovate, with a pointed end at the base.
Palmate: divided into many lobes
Thyroid: leaf rounded, stem attached from below
Pinnate: two rows of leaves
- Imparipinnate: pinnate leaf with apical leaf
- Piripnate: pinnate leaf without apical leaf
Pinnately dissected: the leaf is dissected, but not to the middle
Reniform: kidney-shaped leaf
Diamond: diamond shaped leaf
Spatulate: spade-shaped leaf
Spear-shaped: sharp, with spines
Subulate: in the form of an awl
Trifoliate: leaf divided into three leaflets
Tripinnate: each leaflet is in turn divided into three
Single-lobed: with one leaf

Leaf edge

The edge of a leaf is often a characteristic of the plant genus and helps identify the species:

Full edge - with a smooth edge, without teeth
Ciliated - with fringe around the edges
Serrated - with teeth, like a chestnut. The tooth pitch can be large or small
- Round-toothed - with wavy teeth, like

Leaves can be complete or incomplete, simple or complex, and have a different structure from each other. The structure of the complete sheet is as follows:

External leaf structure

A normal complete leaf consists of a leaf blade, a petiole, as well as a base and adjacent stipule. Photosynthesis and other physiological processes occur in the leaf blade. The petiole attaches the plate to the shoot; the place of attachment is called the base of the leaf; paired stipules, sometimes shaped like spines, are adjacent to it.

Some plants have incomplete leaves, the structure of which is missing one of the elements: petiole, stipules or blade. Sometimes, even on one plant, there may be leaves with a petiole (they are called petiolate) or without it (the so-called “sessile” leaves). In some plants (for example, umbellifers or cereals), the base grows and covers the stem.

The structure of a simple and complex leaf also differs. Simple leaves have one petiole on one base, to which one blade is attached. Compound leaves have several petioles and blades, with each blade connected to a petiole.

The plates of simple ones are together with the petiole, and in complex ones - in turn. The figure shows the types of structure of compound leaves: 1 – pinnately compound, 2 – trifoliate, 3 – palmately compound leaf.

Internal structure of the leaf

The internal structure of the leaf is the same in plants of various species. Each leaf blade has a layer of skin on both sides called the epidermis.

The main leaf tissue, called mesophyll, is divided into palisade tissue and the underlying spongy tissue. The elongated cells of the palisade tissue contain chlorophyll, which is involved in the process of photosynthesis, and the rounded cells of the spongy tissue are filled with air. There are significant intercellular spaces between the cells of spongy tissue.

Another feature of the internal structure of the leaf is that most leaves are penetrated by veins. The veins play the role of peculiar vessels through which the leaf is supplied with minerals and water, and gets rid of organic substances produced during photosynthesis. In addition, mechanical tissue is formed around the large veins, which gives the sheet strength.

A special word should be said about the special openings in the epidermis of the leaf - stomata, through which excess moisture is released from the plant (a process called transpiration). Stomata consist of two cells that close and open the stomatal fissure; with their help, the volume of water vapor removed from the plant is regulated. Thanks to this feature of the internal structure of the leaf, the plant is protected from overheating and withering.

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