152 BOTANY AND PHARMACOGNOSY. 



around the wall of the embryo-sac, divide and re-divide ; the large 

 vacuole in the center becomes filled with a watery or milky fluid, 

 and later the nuclei, with portions of the cytoplasm, may be 

 enclosed by a cellulose wall and become permanent cells, in which 

 the embryo is embedded. Likewise in the nucellus, changes are 

 also taking place; the cells are found to be dividing, and storing 

 starch, oil, aleurone and other food materials, like the cells of the 

 embryo-sac. The cells in which these materials are stored are 

 known as reserve cells and in the nucellus they constitute the 

 PERISPERM, while those formed in the embryo-sac make up the 

 ENDOSPERM. Usually the endosperm of seeds is prominently 

 developed while the perisperm occurs as a thin layer; in carda- 

 mom, however, the endosperm and perisperm are both well devel- 

 oped (Fig. 253). In some instances the embryo may not fill the 

 embryo-sac, as in cocoanut, and sometimes, as in the almond, both 

 of the reserve layers are consumed in the development of the 

 embryo when the seed is said to be without endosperm (Fig. 187). 

 The perisperm and endosperm are sometimes spoken of to- 

 gether as the albumen of the seed, but as the cells comprised in 

 these layers contain not only protoplasmic contents and aleurone 

 grains, but starches, oils and other substances, the term is mis- 

 leading. On this basis, seeds containing either endosperm or 

 perisperm, or both, have been designated as albuminous, but on 

 account of these layers containing larger proportions of other 

 substances than proteids it would be better to speak of them as 



RESERVE LAYERS (FigS. 121, 122). 



While these changes in the nucellus and embryo-sac have been 

 going on there have been equally great changes in- the coats of 

 the ovules, which later constitute the seed-coats. In the seed 

 the two coats are generally readily distinguishable. The inner, 

 as in Ricinus, Pepo, etc., is thin, light in color, of a delicate 

 structure, and is known as the tegmen ; the outer is more or less 

 thickened, of a darker color and firmer in structure, and is known 

 as the TESTA. In some instances the perisperm, or both perisperm 

 and endosperm, may be reduced to a thin layer when it is consid- 

 ered to form a part of the seed-coat, as in mustard. In other cases 

 the two coats are so closely vmited that they are not easily distin- 

 guished, as in stramonium. 



