FORMATION OF THE SEED 373 



Protoplasm collects round each of the daughter nuclei, con- 

 verting them into cells, one of which remains undivided, 

 while the other divides, and its substance, in some way not 

 understood, passes from the pollen-tube into the ovum, where 

 it forms a cell-like body, to which the name of male pronucleus 

 (see p. 259) has been applied. This conjugates with the 

 nucleus of the ovum, or female pronucleus, and thus effects 

 the process 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 and 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. 361). 



