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 greatly, as does their size. For example, in duckweed, an aquatic plant, they are about three millimeters in diameter. Up to a meter can reach a sheet of Victoria Amazonian. 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 naked. Trees with foliage that fell in the winter do not grow, although they remain alive. Only after their dissolution do they begin to live to the fullest and acquire their characteristic shape. The leaf is not an axial organ, but it is closely related to the stem, which is the axis of the shoot.
Psilophytes, the most ancient terrestrial plants, did not have a dismemberment of the body, familiar to us. In their structure, the root, leaf and stem did not stand out. It happened a little later. In modern plants, the shape of the leaves and their organization are very plastic. These organs differ from the stem and root by 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 of the cone do not have growth . They grow base. They do not have directly other leaf or axial organs. Their growth is limited to a certain period of time.
Leaf structure: rules and exceptions
The leaf blade is an expanded part of the sheet. 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 by which the stem is attached to the stem. Stipules are at the base.
As a rule, the structure of the leaves is dorso-abdominal (dorsiventral). There is one plane of symmetry, and it divides them into 2 halves, symmetrical to each other. However, there are many exceptions to these rules. For example, the leaves of vayya (ferns) grow in the apex. As for pine needles, it increases in size over several years. In pine needles grows with intercalated growth at the base.
However, the most surprising exceptions to these rules can be considered the leaves of Velvichia mirabilis. This gymnospermous plant is found in South Africa (Kalahari Desert). The curb-shaped trunk of Velvichia mirabilis (40 cm in height and 1 meter in diameter) forms only 2 sheets. Their length reaches three meters. The shape of the leaves is belt-shaped, they are leathery. These leaves die off at the ends, and constantly grow at the base. As a result, their life expectancy may exceed 100 years.
How to classify leaves?
The external variety of leaves is so great that it is impossible to create a unified classification system based on one or more characteristics. There are several classifications that we will talk about now.
Petiole classification
There are three ways in 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 - sessile. The base of some plants grows, covering the stem over the node. In this case, the sheet is called vaginal. The stem seems to be embedded in it. If the sessile leaf of a plant descends down the stem, it is called downward. A typical example is the thistle. If the leaf of the plant covers the stem, it is called the stem-stalking.
Complex and simple leaves
We pass to the following 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 plate, the leaves are called simple. The shape of the leaves of the trees 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 may also be different. The shape of the leaves (complex) can be as follows: intermittently pinnate, three-pinnate, double-pinnate, unpaired, pinnate, palmate, ternate.
However, even simple leaves are not so simple. Consider this with the example of many famous monstera plants. 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. If the section 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 incision reaches the main vein of the leaf. In this case, the shape of the leaves of the plants is dissected.
Number of sections, shape of leaf blades and edges
We pass to the following classification. Plants can also vary in the number of cuts per leaf. If it is divided into 3 parts, it is called ternary, if 5 - palmate, if for a larger number of parts - pinnate (dissected, separate, lobed).
Leaf blades are also classified by shape. Many of their forms are distinguished: ovoid, round, spear-shaped, lanceolate, linear, oblong, heart-shaped, arrow-shaped, etc. On the same basis, edges can also be classified. The most common shape of the leaf edge is solid (whole leaves). However, there are several other species. Toothed, crab, prickly (prickly), serrate, sinuous leaves in the shape of the edge are distinguished.
Heterophilia
Do you know this concept? If not, then note that the leaves on one shoot can have a different shape, color and size. This phenomenon is called heterophilia. It is characteristic, for example, for an arrowhead, buttercup, and many other species.
Veins of plants
When considering the leaf blade of a plant, you can see that there are veins on it. These are conductive vessels. Their location on the sheet may also be different. Venation is a way of arranging leaves. There are several types of it: reticular (cirrus and palmate), dichotomous, arched, parallel. For monocotyledonous plants, arched or parallel venation is characteristic , and for dicotyledonous plants , reticulate.
We offer to consider and compare oak and maple leaves, 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 for evergreen species for several years, while for other species they fall annually or remain on the branches, gradually collapsing 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 rather large (up to 25 cm). This type of tree has an oblong-oval leaf shape. In spring, the crown acquires a bright red color, and in the summer changes its color to bright green, while the lower part turns white. The shade of leaves in autumn varies. It can be from deep purple to burgundy. The shape of the autumn leaves does not change.
Red oak (otherwise it is called northern) is a tall tree (up to 25 m) with a dense crown. Its leaves are large, have pointed lobes. This tree got its name because of foliage, which has a reddish color in autumn and spring.
Maple leaves
The birthplace of maple is Eurasia. This deciduous tree has a dense, rounded, wide crown. It reaches a height of 30 meters. A tree can live under favorable conditions for 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 lobes ending with spiky lobes. In this case, the three front lobes do not differ from each other, and the two lower lobes are somewhat smaller. Rounded notches are between all of them. Petioles of leaves are long. As for color, it also varies depending on the time of year. In summer, the leaves are dark green above and light green below. In the fall they get brown, red, burgundy and brown shades.
So, we examined the main forms of leaves. In conclusion, we will talk about their role.
Leaf value
The most important function is the formation of organic substances. A large and flat sheet plate captures sunlight. It is in the leaves that 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, with the help of leaves there is gas exchange. Carbon dioxide and oxygen flow through the stomata. Oxygen is needed for respiration, and carbon dioxide is needed for plants to synthesize organic substances. During leaf fall, unnecessary substances are removed, the surface of the aboveground organs decreases in an unfavorable period. The plant evaporates less water, the crown accumulates less snow, which means it will not break.