MORPHOLOGY OF HIGHER TLAXTS. 425 



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

 known as reserve cells, and in the niicellus they constitute tlie 

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

 ENDOSPERM. Usually the endosperm of seeds is prominently de- 

 veloped, wdiile the perisperm occurs as a thin layer ; in some seeds, 

 however, the endosperm and perisperm are both well developed. 

 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. 248). 



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 proteins it' would be better to speak of them as 



RESERVE LAYERS (FigS. 247, 250). 



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 develop into 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 considered 

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

 two coats are so closely united that they are not easily distin- 

 guished, as in stramonium. 



The terms used in describing the kinds of ovules (atropous, 

 anatropous, campylotropous, etc.) are retained in tlie description 

 of the seeds ; and in describing the dififerent parts of the seed some 

 of the terms which were applied to the ovule are also retained, as 

 chalaza and raphe ; the seed when ripe usually becomes detached 

 from its stalk, and the resulting scar is called the hilum ; that 

 part of the seed corresponding to the foramen of the ovule i.N 

 more or less closed and is known as the micropyle ; the embryo 



