522 General Botany 



The embryo consists of a hypocotyl, a plumule, and on-e 

 (monocots) or two (dicots) cotyledons. This is the new sporo- 

 phyte generation. Since the sperm and the egg each contain n 

 chromosomes, the cells of the sporophyte contain the double 

 number (2 n) of chromosomes. 



The xeniophyte. The germination and growth of the endo- 

 sperm nucleus leads to the formation of a tissue surrounding the 

 embryo, called the endosperm. In some seeds (e.g., castor bean 

 and corn), this tissue persists and becomes very large, and 

 accumulates starch and other nutritive materials. In other 

 seeds (e.g., common bean) the endosperm is a temporary tissue, 

 that is consumed by the growing embryo. 



The endosperm of the angiosperms is formed by successive 

 divisions of a nucleus that resulted from the union of three 

 nuclei. Its cells, therefore, contain three sets (3 n) of chromo- 

 somes. To distinguish this kind of an endosperm from the very 

 different one of the gymnosperms (female gametophy te) , it is 

 sometimes called the xeniophyte. 



The seed. In the seed of an angiosperm, therefore, there are 

 (or for a time were) three distinct generations represented : 

 (i) the integuments, a part of the first sporophyte producing the 

 seeds; (2) the endosperm, or xeniophyte; and (3) the embryo, 

 or second sporophyte. 



The seed coats. Strictly speaking, the seed coats are the 

 mature integuments of the ovule. In many plants having 

 simple pistils that contain a single ovule, the seed coat at maturity 

 includes the pistil wall. This is the case in grasses and in the 

 buttercup, rose, carrot, and sunflower families. Such a seed is 

 termed an akene, or, in the grasses, a grain. 



Divisions of the angiosperms. As noted earlier in the book, 

 the angiosperms consist of two great classes of plants, the dicoty- 

 ledons and the monocotyledons. The former seem to be the 

 older and to have given rise to the monocots in comparatively 

 recent geological time. 



