Shape of plant leaves

The forms of leaves, flowers and plant roots are very diverse. Today we will talk about one of the main organs of all green plants. This is a sheet. It is located on the stem, occupies a lateral position on it. The shape of the leaves varies considerably, as does their size. For example, a duckweed, an aquatic plant, they are about three millimeters in diameter. Up to a meter can reach the list of the Amazonian Victoria. In some tropical palm trees its length is 20-22 m.

General characteristics of plant leaves

A leafless tree is a broom of various sizes. It is often difficult to determine its appearance in winter, when the crown is bare. Trees with foliage, which fell for the winter, do not grow, although they remain alive. Only after their dissolution they begin to live fully and acquire their characteristic form. The leaf is not an axial organ, but it is closely connected with the stem, which is the axis of the shoot.

The psilophytes, the most ancient terrestrial plants, did not have the dismemberment of the body familiar to us. In their structure, the root, the leaf and the stem did not stand out. It happened a little later. In modern plants, the shape of the leaves and their organization are very plastic. From the stem and root these organs differ in their characteristic features. The leaves of the shoot are its lateral organs. They are formed superficially (exogenously) as bumps located in the cone of growth. However, the leaves of the cone do not increase . They grow in base. There are no other leaf or axial organs directly on them. Their growth is limited to a certain length of time.

Leaf structure: rules and exceptions

The leaf blade is the widened part of the leaf. The petiole is its stem-shaped narrow part. It is with the help of it that a leaf blade connects to the stem. The base is the part to which the stem is attached to the stem. At the base are stipules.

As a rule, the structure of leaves is dorsal-abdominal (dorsiventral). The plane of symmetry is one, and it divides them into 2 halves, symmetrical to each other. However, there are many exceptions to these rules. For example, leaves of vayi (ferns) grow in the tip. As for pine needles, it increases in size for several years. In the pine, the needles grow with an insertion growth at the base.

However, the most surprising exceptions to these rules are the leaves of Velvichia mirabilis. This is a gymnosperm plant that occurs in South Africa (the Kalahari desert). The Velvichia mirabilisk cobble-stone (40 cm in height and 1 meter in diameter) forms only 2 leaves. Their length reaches three meters. The shape of the leaves is belt-shaped, they are leathery. These leaves die from the ends, and at the base are constantly growing. As a result, their lifespan may exceed 100 years.

How can I classify the leaves?

The external variety of leaves is so great that it is impossible to create a single classification system based on one or several characteristics. There are several classifications about which we will now talk.

Classification by petiole

There are three ways by which leaves are attached to the stem. Plants with and without petioles stand out. In the first case, the leaves of such a plant are called petiolate, and in the second - sedentary. The base of some plants grows, embracing the stem above the node. In this case, the sheet is called vaginal. The stem seems to be embedded in it. If the plant's sessile sheet descends down the stem, it is called run-down. A typical example is a thistle. If the leaf of the plant covers the stalk, it is called a staple.

Complex and simple leaves

We proceed to the following classification. Sheet plates can also be very diverse in form, size, structure and other parameters. They can be one or more. If there is only one plate, the leaves are called simple. The shape of the leaves of the trees in this case can be oval, round, lanceolate, oblong, ovate, linear, obovate. When there are several plates on one petiole, we are talking about complex species. The arrangement of leaf blades can also be different. The shape of the leaves (complex) can be as follows: intermittent saphenous, tridneeperistosyllabic, doubly pylorigous, paranulipiform, paranoperistosyllous, palchatosyllous, trochosyllabic.

However, simple leaves are not so simple. Consider this in the example of a monster known to many plants. Its sheet consists of only one leaf plate, so it is considered simple. However, its shape is very bizarre. Leaves of this type are called dissected. There are other types. If the dissection of the plate does not exceed a quarter of its width, the shape of the leaves of the trees is lobate. In case it is cut by a third, it is called separate. It also happens that the incision reaches the main vein of the leaf. In this case, the shape of the leaves of the plants is dissected.

Number of cuts, shape of leaf blades and edges

We proceed to the following classification. Plants can also vary in the number of cuts on the leaf. If it is divided into 3 parts, it is called triple, if it is 5-palchate, if for more parts - pinnately (dissected, separate, lobate).

Sheet plates are also classified by shape. There are many of their forms: ovoid, rounded, spear-shaped, lanceolate, linear, elongated, cordate, arrow-shaped, etc. On the same basis, it is also possible to classify edges. The most common form of the edge of the leaf is a one-piece (entire leaves). However, there are several other types. Dentate, crenate, spiny-toothed (spiny), serrate, tortuous leaves are distinguished according to the shape of the margin.

Heterophilia

Is this concept familiar to you? If not, we note that the leaves on one shoot can have different shapes, colors and sizes. It is this phenomenon that has become known as heterophilia. It is typical, for example, for the shooter, buttercup and many other species.

Plant veins

When examining a leaf blade of a plant, it can be seen that there are veins on it. They are conductive vessels. Their location on the sheet can also be different. Venation is a way of arranging the leaves. There are several types: mesh (pinnate and fingered), dichotomous, arc, parallel. For monocotyledonous plants, arc or parallel venation is characteristic , and for dicotyledons it is reticulate.

We suggest to consider and compare the leaves of oak and maple, determine their shape.

Oak leaves

Oak is a plant characteristic of temperate climate. It can be found in various regions of the Northern Hemisphere. The tropical highlands are the southern limit of its growth. Its leaves are leathery. They keep on the tree for evergreen species for several years, while other species fall annually or remain on the branches, gradually breaking down and drying out. The shape of the oak leaf is lobed. However, sometimes there are also whole. This form of oak leaf is observed in some evergreen species. In white, for example, the leaves are rather large (up to 25 cm). This species has oblong-oval leaf shape. In spring, the crown becomes bright red, and in the summer changes its color to bright green, while the lower part becomes white. The color of the leaves varies in autumn. It can be from rich magenta to burgundy. The forms of autumn leaves do not change at the same time.

Red oak (otherwise it is called northern) is a tall tree (up to 25 m), which has a dense crown. Its leaves are large, with sharpened lobes. This tree got its name because of the foliage, which has a reddish color in autumn and spring.

Maple Leaves

The homeland of maple is Eurasia. This deciduous tree, which has a dense, round, wide crown. In height it reaches 30 meters. A tree can live under favorable conditions up to 200 years. Its leaves are large, their diameter reaches 18 cm. They have pronounced veins. The shape of the maple leaf is as follows: it has 5 blades, terminating with pointed points. In this case, the three front blades are not different from each other, and the two lower ones are slightly smaller. Rounded notches are present between all of them. Leaves are long. As for color, it also differs depending on the time of year. In summer, the leaves are dark green on top and light green from below. In autumn they acquire brown, red, burgundy and brown shades.

So, we looked at the basic forms of leaves. In conclusion, let's talk about their role.

The importance of leaves

The most important function is the formation of organic substances. A large and flat sheet of plate collects sunlight. It is in the leaves that the process of photosynthesis proceeds . With their help, the plant also evaporates water. It can change the intensity of this process by closing and opening the stomata. In addition, gas exchange takes place with the help of leaves. Carbon dioxide and oxygen come through the stomata. Oxygen is needed for breathing, and carbon dioxide is needed for the plant to synthesize organic substances. During the leaf fall, unnecessary substances are removed, the surface of the aerial organs is reduced in an unfavorable period. The plant evaporates less water, the crown accumulates less snow, which means it will not break down.