PHANEROGAMIA 473 



the macrosporangium of Selaginella, in the macrosporangium (ovule) of 

 the Phanerogams there is usually a single embryo-sac mother-cell 

 which divides into four daughter-cells ; three of these do not develop 

 further, while the fourth becomes the embryo-sac. The formation of 

 the PROTHALLIUM or ENDOSPERM and of the ARCHEGONIA or EGG-CELLS 

 differs in the several classes of Phanerogams. The fertilised ovum 

 grows into the embryo while still enclosed within the macrospore and 

 at the expense of the parent plant. When the embryo has reached a 

 certain stage in its development, which is different and characteristic 

 in different plants, its growth is arrested, and after the separation 

 from the parent plant it undergoes a period of rest. It is still 

 surrounded by the other portions of the macrosporangium, viz. the 

 prothallium or endosperm, the nucellus (if this still persists), and the 

 seed coat formed from the integuments. THE COMPLETE STRUCTURE 



Fio. 433. Part of section through one of the coty- 

 ledons of the Pea, showing cells with reserve FIG. 434. Cell from the endosperm of 

 material, am, Starch grains ;al, aleurone grains; Phytelephas tnacrocarpa, with re.srnv 

 p, protoplasm; n, nucleus, (x 160.) cellulose, (x 225.) 



DERIVED FROM THE OVULE IS TERMED A SEED, AND THE FURTHER 

 DEVELOPMENT OF THE UNOPENED MACROSPORANGIUM TO FORM A SEED 



is CHARACTERISTIC OF ALL PHANEROGAMS. As seed-plants or Sperma- 

 phyta the latter may be contrasted with the seedless Cryptogams. 



The several parts of the ripe seed are thus the EMBRYO, the 

 PROTHALLIUM or ENDOSPERM surrounding it, the remains of the 

 NUCELLUS which is distinguished from the endosperm as PERISPERM, 

 and the SEED-COAT. The cells of the endosperm and the perisperm 

 when present are stored with reserve materials such as starch, proteid 

 substances, or fatty oils (Figs. 433, 434) ; cellulose in the form of 

 greatly thickened cell walls may also be a reserve substance. When 

 the seed after a period of rest commences to germinate, these sub- 

 stances are utilised by the young plant until it becomes capable of 

 independent nutrition. Since the nucellus is as a rule completely 

 obliterated by the enlarging endosperm, few seeds possess a perisperm 

 (Fig. 435). Frequently no endosperm is present in the seed, and the 

 storage of reserve material takes place throughout the embryo (Fig. 

 436) or in its swollen seed-leaves or COTYLEDONS. 



