The endosperm is important because it is the main source of food for the embryo. In gymnosperms it is haploid and is formed by the repeated divisions of megaspore nucleus, (i.e., female gametophyte and endosperm terms in gymnosperms are the same). In angiosperms on the other hand it is a new structure formed in most cases as a result of the fusion of the two polar nuclei and one of the male gametes. Since all the three fusing nuclei are usually haploid, the endosperm contains the triploid (3x) number of chromosomes.
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Endosperm formation is supressed in Orchidaceae,Trapaceae and Podostemonaceae. In these forms, triple fusion is completed but the fusion product either degenerates immediately or undergo only 1 or 2 divisions.
Types of Endosperm Formation
1. Nuclear type : The endosperm mother nucleus divides repeatedly without wall formation. In later stages the nuclei may remain free or wall formation follows. This type of endosperm is common in polypetalae, e.g., Capsella.
2. Cellular type : The first and most of the subsequent divisions are accompanied by wall formation so that the embryosac becomes divided into several chambers. This type of endosperm formation is common in gamopetalae, e.g., Villarsia.
3. Helobial type : It is intermediate between nuclear and cellular type and is common in order Helobeales. The first division of endosperm mother nucleus is followed by wall formation forming two unequal cells, a very big micropylar and small chalazal. Micropylar cell divides by free nuclear division later followed by wall formation. In the chalazal 1 or 2 divisions may takes place and it functions as a small haustorial cell e.g., Eremurus.
Histology of Endosperm
The cells of endosperm are usually isodimetric and store large quantities of food materials whose exact nature and properties vary much from one plant to another. As a rule walls are thin, devoid of pits but when hemicellulose is the chief reserve food they are greatly thickened and pitted. Endosperm is well-known for high degree of polyploidization of its cells during development.
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There are no intercellular spaces in cells of endosperm. Endosperm may be starchy (Rice), oily (Castor). In cereals outermost layer of endosperm constitutes aleurane tissue (protein).
Endospermic and nonendospermic seeds (i.e., final fate of endosperm) :
1. Endospermic seeds : In some seeds the endosperm forms a permanent tissue (Ricinus, Phoenix, Triticum) which persists until the germination of seed. Such seeds are called endospermic seeds (= albuminous seeds).
2. Nonendospermic seeds : In other seeds (cucurbits, Pisum, Arachis) it is used up by the growing embryo and is no longer seen in the mature seed. Such seeds are called nonendospermic seeds (ex-albuminous seeds). Of special interest is Symplocarpus in which the embryo devours not only the endosperm but also integuments so that the embryo lies naked inside the ovary wall. An even more extreme case is that of Melocanna bambusiodes in which embryo dissolves even the ovary wall and is completely naked at maturity.
3. Xenia: Focke (1881) : described this phenomenon for the first time. In plants like maize the influence of male gamete is shown in the development of endosperm. If the male parent has a yellow endosperm and female parent a colourless endosperm, after fertilization the endosperm of the new seed becomes yellow. This transference of characters by a male gamete and its influence on endosperm is known as xenia.
4. Metaxenia : The effect of pollen on the character of the seed coat or pericarp is called metaxenia.
5. Mosaic endosperm : Occasionally there is lack of uniformity in the tissue of endosperm. The patches of two different colours are observed in Zea mays forming a sort of irregular pattern. Some portion of endosperm may be starchy and other part is sugary.
6. Ruminate endosperm : Mature endosperm with any degree of irregularity and unevenness in its surface contour is called ruminate endosperm, e.g., Passiflora calarata and Cocoloba.