xxx FORMATION OF THE SEED 377 



of fertilization, or the conversion of the ovum into the 

 oosperm. 



The mode of formation of cells described in the preceding 

 paragraph should be specially noted. Instead of the ordin- 

 ary process of fission hitherto met with, the products of 

 division of a nucleus become surrounded by protoplasm, 

 cells being produced which lie freely in the interior of the 

 mother-cell. This is called free cell-formation. 



The development of the oosperm is a very complicated 

 process, and results in the formation not of a single polyplast 

 but of four, each at the end of a long suspensor (D, spsr\ 

 in the form of a linear aggregate of cells, which by its elonga- 

 tion carries the embryo (emb) down into the tissue of the 

 prothallus. As a rule only one of these embryos comes to 

 maturity : it develops a rudimentary stem, root, and four 

 or more cotyledons, and so becomes a phyllula. 



While these processes are going on the female cone 

 increases greatly in size and becomes woody. The mega- 

 sporangia also become much larger, their integuments (E, /), 

 becoming brown and hard, and the megaspore in each 

 enlarges so much as to displace the nucellus : at the same 

 time the cells of the prothallus filling the megaspore develop 

 large quantities of plastic products, such as fat and albumin- 

 ous substances, to be used in the nutrition of the embryo : 

 the tissue thus formed is the endosperm (end\ The mega- 

 sporangium is now called a seed (see p. 365). 



Under favourable circumstances the seed germinates. 

 By absorption of moisture its contents swell and burst the 

 seed-coat, and the root of the phyllula (r) emerges, followed 

 before long by the stem (st) and cotyledons (ct). The 

 phyllula thus becomes the seedling plant, and by further 

 growth and the successive formation of new parts is converted 

 into the adult. 



