The endosperm is the storage tissue of the seeds of flowering and gymnosperms, necessary for the initial nutrition of the embryo. Its structure and origin in these two departments is different and closely related to the characteristics of the reproductive system. The development and role of the endosperm in angiosperms depends on the type of structure of the seed.
What is endosperm
The endosperm of flowering plants is a nutrient-rich hybrid triploid tissue resulting from double fertilization. Such a structure does not have homology with the female growth, in contrast to a similar formation in gymnosperms.
The main reserve substances that endosperm has include starch, fats, proteins, hemicellulose and aleuron grains. In the storage tissue of the seeds of cereal and Asteraceae, the Aleuron protein forms a continuous surface layer.
Features of the formation of endosperm in angiosperms
A complete understanding of what endosperm is is impossible without knowledge of the characteristics of the reproductive biology of angiosperms. Triploid storage tissue develops from the diploid central cell of the embryo sac after it is fertilized by one of the sperm.
There are 2 types of endosperm formation:
- nuclear (nuclear) - at the beginning there is a large fission of nuclei, and then cytokinesis;
- cellulary (cellular) - each doubling of genetic material is accompanied by cytokinesis.
In the nuclear endosperm, the formation of cell walls occurs in the direction from the periphery to the center.
The structure and function of the endosperm
Endosperm cells are large and contain many nutrients. Cell membranes can be either thin or very thick (horny). The surface of the endosperm in most cases is smooth, but in a number of families it becomes wrinkled in the process of seed maturation (lastonovye, palm, walnut). This type of stocking fabric is called ruminated. It has been established that in such an endosperm, the metabolic rate with other parts of the seed is higher than in a smooth one. This is due to an increase in the area of contact with surrounding tissues.
In the ripening seed, the endosperm performs the function of processing and transmitting nutrients from the mother's body to the fetus. This period is characterized by high metabolic activity, it does not last very long, after which the endosperm begins to accumulate nutrients, turning into storage tissue.
The ratio of the size of the endosperm and the germ
In the process of seed maturation, endosperm can develop to a greater or lesser extent, which depends on the family to which the plant belongs. For example, in cereals, liliaceae, and magnolias, storage tissue occupies a large part of the volume, while in apples, on the contrary, it is so crowded out by the embryo that it remains only as a thin layer under the seed coat.
The stocking tissue of some plants by the time the seed ripens completely. This can be determined by some morphological features of its structure. For example, the meatiness of the cotyledons suggests that such an endosperm is quickly consumed, giving nutrients to the buds of future leaves of the plant. In this case, it is the cotyledons that serve as the main nutrient reservoir for the development of the embryo. This is most clearly seen in legumes.
Nevertheless, the complete absence of endosperm in the mature seed is a rather rare phenomenon that occurs only in 15% of flowering plant families. In the remaining angiosperms (both dicotyledonous and monocotyledonous) this tissue is necessarily present at least in a small amount.
Some seeds contain another type of storage tissue - the diploid perisperm, which can completely replace the endosperm, or be present along with it. Several types of seeds are distinguished by the content of storage tissues.
The endosperm in the gymnosperms
The stocking tissue of the seeds of plants of the Pynophyta division is very different from what the endosperm of angiosperms is. First of all, this is due to another mechanism of reproduction. In gymnosperms, endosperm is essentially a female growth that develops from a megaspore that has matured in nucellus. The cells of this endosperm have a single set of chromosomes.
Stocking tissue is formed as a result of multiple mitotic fission of the nucleus of the haploid megaspore, which result in the formation of intercellular septa. First, the peripheral part develops, and then the central part. After this, archegonies begin to form in the endosperm, which, undergoing fertilization, turn into a weakly differentiated embryo.
The main reserve substance of the gymnosperm endosperm is starch, fats are present to a lesser extent. Nutrient components enter the tissue from nucellus and integument.