144 



FUNDAMENTALS OF CYTOLOGY 



cotyledons, and has digested away some of the endosperm; instill others 

 it fills the entire seed, having digested away all the endosperm and even 

 some of the surrounding ovular tissue. All the mitoses during embry- 

 ogeny are equational, so that every nucleus contains two genomes, one 

 from each parent. 



Even before cell division begins in the fertilized egg, the development 

 of endosperm has usually commenced with the division of the primary 

 endosperm nucleus. In manj^ angiosperms there are several of these 

 divisions before cytokinesis occurs, so that the endosperm has an early 



coenocytic stage. Cytokinesis later converts 

 this into a cellular tissue. In other species 

 the endosperm is cellular from the beginning. 

 The endosperm enlarges along with the whole 

 ovule and ma,v become stored with various 

 nutritive materials. In wheat and other 

 grains the outermost layer contains many 

 aleurone granules. Storage material in some 

 plants is deposited in the form of thick 

 cellulose walls, the endosperm being hard, like 

 wood or even ivory. Such stored material is 

 utilized as the seed germinates and the embryo 

 develops into a young plant. 



The endosperm is commonly triploid in 

 nuclear constitution, although it often devel- 

 ops with other numbers of genomes depending 

 upon the number of nuclei involved in the 

 previous fusion. Sometimes unfused as well 

 as fused nuclei take part in endosperm devel- 

 opment, portions of the tissue being mono- 

 ploid. Evidently it is neither the derivation 

 of the nuclei nor their fusion, but rather the 

 conditions under which they develop, that de- 

 termine the formation of endosperm. From the standpoint of compar- 

 ative morphology it is most logical to interpret the endosperm as 

 gametophytic tissue which, unlike that of gymnosperms, is arrested 

 in development until after the pollen tube enters the sac; it then 

 resumes development with or sometimes without fusions and the 

 incorporation of a male nucleus. A male nucleus may, of course, affect 

 particular characters of the endosperm if it comes from a plant of different 

 genetic constitution. 



In most angiosperms the other cells present in the embryo sac before 

 syngamy disintegrate early. The entering pollen tube often destroys 

 one synergid, and the other disappears soon after the egg is fertilized. 



Fig. 105. — Embryo sac or 

 Crepis capillaris, showing two- 

 celled embryo with nuclei in 

 mitosis. The endosperm has 

 begun to develop. (From a 

 preparation by K. Koos.) 



