468 SPERMATOPHYTES (SEED PLANTS) 



sac develop like buds and form embryos, in which case, of course, 

 there is no fertilization. Polyembryony may also be a source of 

 annoyance to plant breeders, for if plants that are used in cross- 

 ing develop polyembryonous seeds, the offspring arising from 

 these seeds may develop from embryos that were formed by the 

 budding of the nucellus, in which case the embryos have only the 

 characteristics of the mother plant. For example, in crossing 

 different strains of Tobacco, in some cases the plants arising 

 from the seeds obtained by crossing are not hybrids but like the 

 mother plant. Some think this may be due to parthenogenesis 

 and others attribute it to polyembryony. 



Endosperm. — While the embryo is developing, the endosperm 

 nucleus is dividing and its accompanying cytoplasm is increasing. 

 The free nuclei at first form in a chain around the wall and then 

 multiply towards the center. Cell walls are finally formed and in 

 these cells food is stored. In some Angiosperms the endosperm 

 is taken up by the embryo almost as rapidly as formed and stored 

 in the cotyledons, while in other Angiosperms most of the en- 

 dosperm remains outside of the embryo until the seed germi- 

 nates. 



Since the endosperm nucleus contains the contents of a sperm, 

 the character of the endosperm of a seed is often determined by 

 the sperm. Thus, as in case of Corn where the endosperm re- 

 mains outside of the embryo, the color and other characteristics 

 of the endosperm are often like the pollen parent and not at 

 all like those of the mother parent. This feature called xenia 

 has already been referred to. In some seeds, in addition to the 

 formation of endosperm, the portion of the nucellus remaining 

 becomes stored with food and forms what is known as peri- 

 sperm. 



Seed Coat. — ■ As the embryo and endosperm develop, the 

 ovule enlarges rapidly, and at the same time the embryo sac de- 

 stroys much or all of the nucellus and frequently a part or all 

 of the inner integument. Consequently the seed coat consists 

 chiefly of the outer integument, which is usually very much mod- 

 ified for protection. 



It is obvious that the seeds of Angiosperms differ considerably 

 from the seeds of Gymnosperms, for the female gametophyte of 

 Angiosperms is soon destroyed after fertilization by the develop- 

 ing embryo and endosperm, and consequently there is no gamete- 



