The cotyledons or seminal leaves are the first embryonic “leaves” of a developing plant. They are found in plants with seeds and their main function is to nourish the embryo during germination.
Angiosperms, which are the most abundant seed plants in nature, reproduce sexually thanks to the fusion of the nuclei of an egg cell and a pollen grain, which takes place through a process known as “pollination”.
The cell resulting from this union is called a zygote and it subsequently divides to form the embryo that will be protected inside the seed. Gymnosperms, which are the other group of seed plants, although they have “naked” seeds, also contain an embryo inside, produced in the same way.
The seeds not only fulfill functions in the multiplication of the species, but also in their dispersion. In both types of plants, the embryos are anatomically organized into different primordial “organs”, which will later give rise to the root and stem of the mature plant.
These organs are the cotyledons (primordial leaves), the radicle (embryonic root), the plumule (embryonic shoot that gives rise to the epicotyl, the portion of the stem located above the cotyledons) and the hypocotyl (the portion of the stem below the cotyledons).
Cotyledons represent the largest part of a plant embryo. An embryo can have one or more of these embryonic leaves, which is usually used by botanists as a taxonomic character to differentiate seed plants, especially Angiosperms.
Based on the number of cotyledons, angiosperms have been classified into monocots and dicots, if they have one or two cotyledons, respectively. Gymnosperm plant embryos also have cotyledons, and species with two or many more can be found.
As they are the first vegetative leaves of a plant, the cotyledons are rather structures with a “simple” morphology, which differentiates them from the rest of the “true” leaves that form on the stem and branches from the meristems. .
Depending on the species, cotyledons can vary in shape and size, but they are almost always more “fleshy” leaves than true leaves, since they contain a large amount of reserve substances to support the life of the embryo during germination and, in some cases, from seedlings during the early stages of plant development.
The fleshyness of the cotyledons of some plants is due to the fact that they absorb most of the reserve tissues of the seed (endosperm) before it enters a dormant state.
Seeds that are rich in endosperm, on the other hand, produce thinner and membranous cotyledons, which nourish the embryo by absorbing the digestion products of the endosperm and transporting them to it.
Some cotyledons can have a relatively long life in the body of a plant, while others are rather short-lived, as the true leaves develop rapidly. In addition, some cotyledons may acquire green coloration due to the presence of photosynthetic pigments.
Cotyledons of grasses
Grasses are monocotyledonous plants. The seeds of these plants, when fully mature, have a single solid cotyledon called the scutellum, which is closely associated with the endosperm.
In these plants and other monocots, the cotyledon is so large that it represents the dominant structure of the seed.
Epigeal and hypogeal germination
According to the location of the cotyledons relative to the soil during germination, botanists have proposed the existence of two defined patterns of germination: the epigeal and the hypogeal.
When the seed germinates and the cotyledons emerge from the soil surface, germination is called epigeal. In contrast, when the seed germinates and the cotyledons remain below the surface and what emerges is the plumule, germination is known as hypogeal.
Although the functions of cotyledons are quite general, there are some differences between monocots and dicots.
The cotyledons of dicotyledonous plants normally function in the nutrition of the seedling (the embryo during and immediately after germination), that is, they store nutritious substances during embryonic development, which will then serve to promote cell multiplication, growth and the development of the new plant.
The ability of a cotyledon to nourish an embryo has to do with the production of enzymes proteases, amylases and phosphatases, whose expression increases during germination, in order to “digest” the nutritive substances inside and transport them to the rest of the body vegetative in development.
The cotyledons of monocotyledonous plants, on the other hand, do not store reserve substances during embryonic development, but rather absorb them from what results from the digestion of the endosperm, which is the true reserve substance.
The endosperm, mainly composed of complex carbohydrates, is enzymatically degraded in response to different hormonal stimuli and the products of this degradation are those absorbed by the cotyledon to nourish the embryo and / or the seedling.
In many cases, plants that have epigeal germination have photosynthetic cotyledons, which function in the maintenance of metabolic activities during the early stages of plant development.
Examples of cotyledons
Classic examples of cotyledons that cover a large amount of the seed surface lacking endosperm are peas and beans.
In these plants, germination is evident with the protrusion of a small radicle that supports two large and fleshy-looking cotyledons, since all the reserve material necessary to feed the seedling during the first days of germination is found stored there.
This is also true for some cucurbits such as zucchini, squash, cucumber and others, in which two long persistent cotyledons are observed at the base of the stem. In these plants the cotyledons are also fleshy and contain a lot of fat and carbohydrates.
In grasses, cotyledons are not so easy to see, but they are commonly the first leaf to emerge from the seed and are seen emerging from the soil surface.
In this video you can see a cotyledon:
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