xxxm FORMATION OF THE SEED 459 



pronudeus then conjugates with the nucleus of the ovum, 

 or female pronuckus, and thus effects the process of fertilisa- 

 tion, or the conversion of the ovum into the oosperm. 



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\ 

 formed 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 in- 

 creases greatly in size and becomes woody. The mega- 

 sporangia, now called seeds, also become much larger, their 

 integuments (E, /), becoming brown and hard and constitut- 

 ing the seed-coat or testa, which in the pine is produced into 

 a flattened expansion or wing. The megaspore in each seed 

 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). 



As the cone dries the placental scales separate and expose 

 the seeds, which drop out and may be carried considerable 

 distances by the wind acting upon their wings, before falling 

 to the ground. 



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 con- 

 verted into the adult. 



