The forms of leaves, flowers and roots of plants are very diverse. Today we will talk about one of the main organs of all green plants. This is a leaf. It is located on the stem, occupies a lateral position on it. The shape of the leaves varies considerably, as do their sizes. For example, in duckweed, an aquatic plant, they are about three millimeters in diameter. Up to a meter can reach a leaf of Victoria Amazon. In some tropical palms, its length is 20-22 m.
General characteristics of plant leaves
The 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 leaves that have fallen for the winter do not grow, although they remain alive. Only after their blooming do they begin to live to the fullest and acquire their characteristic form. The leaf is not an axial organ, however, it is closely related to the stem, which is the axis of the shoot.
Psilophytes, the most ancient land plants, did not have the dismemberment of the body that we are used to. Root, leaf and stem were not distinguished in their structure. It happened somewhat later. In modern plants, the shape of the leaves and their organization are very plastic. These organs differ from stem and root.characteristic features. The leaves of the shoot are its lateral organs. They are formed superficially (exogenously) as tubercles located in the growth cone. However, the leaves themselves do not have a growth cone. They grow on the ground. They do not directly have other leaf or axial organs. Their growth is limited to a certain period of time.
Leaf structure: rules and exceptions
A leaf blade is an expanded part of a leaf. The petiole is its stem-like narrow part. It is with the help of it that the leaf blade is connected to the stem. The base is the part with which the cutting is attached to the stem. There are stipules at the base.
As a rule, the structure of the leaves is dorsal-ventral. Their 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, the leaves of fronds (ferns) grow at the top. As for pine needles, it increases in size over several years. Pine needles grow intercalated at the base.
However, the leaves of Velvichia mirabilis can be considered the most surprising exceptions to these rules. This is a gymnosperm plant that is found in South Africa (Kalahari Desert). The tumbular trunk of Velvichia mirabilis (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-like, they are leathery. These leaves die off at the ends, and at the baseare constantly growing. As a result, their life expectancy can exceed 100 years.
How to classify leaves?
The external diversity of leaves is so great that it is impossible to create a single classification system based on one or more features. There are several classifications, which we will now discuss.
Classification by petiole
There are three ways in which leaves are attached to the stem. There are plants with and without petioles. In the first case, the leaves of such a plant are called petiolate, and in the second - sessile. The base of some plants grows, covering the stem above the node. In this case, the leaf is called vaginal. The stem seems to be embedded in it. If the sessile leaf of the plant descends down the stem, it is called decurrent. A typical example is the thistle. If the leaf of the plant covers the stem, it is called stalked.
Complex and simple leaves
Go to the next classification. Leaf blades can also be very diverse in shape, size, structure and other parameters. There may be one or more. If there is only one blade, the leaves are called simple. The shape of tree leaves in this case can be oval, round, lanceolate, oblong, ovoid, 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 (compound) can be as follows: intermittent pinnate, thrice pinnate, double pinnate,unpaired-pinnate, paired-pinnate, digitate, ternary.
However, simple leaves are not so simple either. Let's consider this using the example of the monstera plant known to many. Its leaf consists of only one leaf blade, therefore it is considered simple. However, its shape is very bizarre. Leaves of this type are called dissected. There are other types as well. If the dissection of the plate does not exceed a quarter of its width, the shape of the leaves of the trees is lobed. If it is cut into a third, it is called separate. It also happens that the cut reaches the main vein of the leaf. In this case, the shape of the plant leaves is dissected.
Number of cuts, shape of leaf blades and margins
Go to the next classification. Plants can also differ in the number of cuts per leaf. If it is divided into 3 parts, it is called trifoliate, if into 5 - palmate, if into more parts - pinnate (dissected, separate, lobed).
Leaf blades are also classified by shape. There are many of their forms: ovoid, round, spear-shaped, lanceolate, linear, oblong, heart-shaped, arrow-shaped, etc. Edges can also be classified on the same basis. The most common form of the leaf edge is whole (whole leaves). However, there are several other types. Jagged, crenate, prickly-toothed (prickly), serrated, winding leaves stand out according to the shape of the edge.
Heterophilia
Do you know this concept? If not, then we note that the leaves on one shoot canhave different shapes, colors and sizes. This phenomenon is called heterophilia. It is characteristic, for example, of the arrowhead, buttercup and many other species.
Plant veins
When examining the leaf blade of a plant, you can see that it has veins. These are conducting vessels. Their location on the sheet can also be different. Venation is the arrangement of leaves. There are several types of it: reticulate (pinnate and palmate), dichotomous, arcuate, parallel. Monocotyledonous plants are characterized by arcuate or parallel venation, while dicotyledonous plants are reticulate.
We propose to consider and compare the leaves of oak and maple, determine their shape.
Oak Leaves
Oak is a plant characteristic of a temperate climate. It can be found in various regions of the Northern Hemisphere. Tropical highlands - the southern limit of its growth. Its leaves are leathery. They stay on the tree in evergreen species for several years, while in other species they fall off annually or remain on the branches, gradually breaking down and drying out. The shape of the oak leaf is lobed. However, sometimes there are whole ones. This form of oak leaf is observed in some evergreen species. In white, for example, the leaves are quite large (up to 25 cm). This type of tree has an oblong-oval leaf shape. In spring, the crown turns bright red, and in summer it changes its color to bright green, while the lower part turns white. Leaf color in autumnvaries. It can be from rich purple to burgundy. The shape of autumn leaves does not change.
Red oak (otherwise called northern) is a tall tree (up to 25 m) with a dense crown. Its leaves are large and have pointed lobes. This tree got its name because of the foliage, which has a reddish color in autumn and spring.
Maple Leaves
Maple is native to Eurasia. This is a deciduous tree with a dense, rounded, wide crown. It reaches a height of 30 meters. A tree can live up to 200 years under favorable conditions. 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 lobes ending in pointed lobes. In this case, the three front blades do not differ from each other, and the two lower ones are somewhat smaller. There are rounded notches between all of them. Leaf petioles are long. As for the color, it also differs depending on the season. In summer, the leaves are dark green above and light green below. In autumn, they turn brown, red, burgundy and brown shades.
So, we have considered the main forms of leaves. In conclusion, let's talk about their role.
Meaning of leaves
The most important function is the formation of organic matter. Large and flat sheet plate captures sunlight. It is in the leaves that the process of photosynthesis takes place. With their help, the plant also evaporates water. It can change the intensity of this process,closing and opening the stomata. In addition, gas exchange occurs with the help of leaves. Carbon dioxide and oxygen enter through the stomata. Oxygen is needed for respiration, and carbon dioxide is needed by the plant for the synthesis of organic substances. During leaf fall, unnecessary substances are removed, the surface of above-ground organs decreases during an unfavorable period. The plant evaporates less water, the crown accumulates less snow, which means it will not break.
