What are cotyledons? It turns out that this is not just an anatomical feature of plants, but an important systematic feature. This concept was introduced into science by the Italian Marcello Malpigi. Following him, the Englishman John Ray found that some plants have one cotyledon, while others have two.
General Description of Angiosperms
The group of plants that we consider in our article occupies a dominant position in the system of the organic world. Systematics number about 250 thousand of their species. This is the Angiosperms, or Flowering division . The names speak eloquently about the characteristics of plants. All of them form a flower - a generative organ in which sexual reproduction occurs. Thanks to its presence, angiosperms form a seed protected by the walls of the pericarp.
Classes of flowering plants
Flowering plants are combined into systematic units of the next rank. What are cotyledons? This is the number of embryonic leaves, the main feature underlying this classification.
Depending on their number, single and dicotyledonous plants are distinguished. They also come together in families. One cotyledon has an embryo of cereal, lily and onion. Dicotyledons include nightshade, cabbage, bean, rosaceae, and other plant families.
What are cotyledons?
Let us consider the structure of the embryonic leaves with specific examples. The embryo, which is formed in the flowering as a result of double fertilization, consists of a root, stem, kidney and leaflet. The cotyledon is part of the latter. During germination, they appear first.
One cotyledon has a germ of a seed of wheat. As many leaflets can be seen at the seedling. In monocotyledons, they develop from the educational tissue - the meristem, and the cotyledon itself remains in the seed. It consists of a shield - the nutrient tissue necessary for the development of the seedling, and a protective cap - keoptil.
Bean seed cotyledons consist of two lobes. Many sprouted this plant on their own. As a result of germination, two embryonic leaves are formed that perform the function of photosynthesis. Further, the plant goes into the postembryonic phase. Sprouted cotyledons are replaced by real leaves that develop from the apical meristem. This is an educational tissue located at the top of the shoot and the tip of the root.
Comparative characteristics
Two or one cotyledon that a seed germ has and the mechanism of their development are not the only systematic features used by taxonomy to determine the class of a plant. For monocotyledons, a fibrous root system is characteristic , which develops in the form of a bundle directly on the shoot. Such plants do not have cambium - lateral educational tissue. Therefore, among them there are no woody forms. Monocotyledonous leaves are simple, with arc or parallel venation. The taxonomy refers to them approximately 50 thousand species.
The root system is dicotyledonous rod. It penetrates deeper into the soil, providing plant nutrition in the driest places. On the cross section of the stalk of the dicotyledon, cambium is clearly visible. Therefore, among them there are not only herbs and shrubs, but also trees. The leaves of these plants can be both simple and complex. Venation mesh or cirrus. The majority of flowering plants belong to dicotyledons - about 200 thousand species.
Invisible Cotyledons
In most cases, germinal leaves develop on the surface of the soil. They provide germination of the seed, dropping its shell. The embryonic leaves of dicotyledons provide the plant with organic substances, since they carry out photosynthesis.
But in nature there are also cotyledons that develop underground. Plants with this type of embryonic leaf have several advantages. If you cut off their aboveground part, the body will not die, but after a while it will recover again. This will occur due to the educational tissue of the embryo, which is underground.
Underground cotyledons do not carry out photosynthesis. Their functions are to protect and ensure the viability of the embryo over a long period of dormancy. Examples of such plants are acorns, chestnuts, nuts.
So, in our article we figured out what cotyledons are. These are the parts of the germ of the plant that germinate first. Depending on their numbers, angiosperms are combined in two classes: monocotyledonous and dicotyledonous.