ORGANIC EVOLUTION 



151 



its sperm nuclei through its ruptured tip. One of these 

 unites with the egg cell, effecting fertilization. Then the 

 embryo sporophyte of the new generation begins to develop, 

 also in captivity, within the wall of the old macrospore, and 

 at the e pense of the prothallial tissue which now fills it. 

 As with the pteridophytes studied, but one embryo is 

 developed; if several eggs are fertilized, one in developing 

 gains the ascendency and crowds the others out. The form 

 of the embryo is indicated in (fig. g6h). 



I '9 





7/ 



Fig. 96. Diagrams of development of the seed in the pine, ff, 

 a single archegonium, penetrated by the pollen tube (/) from 

 which the two sperm nuclei are liberated, one of them is unit- 

 ing with the egg nucleus (o) ; h, the seed; e, the embryo, 

 which develops from the fertilized egg; s, endosperm (remains 

 of the gametophyte) ; /, the germinating seed; the sporophyte 

 slipping off the old seed coat (5) after having consumed the 

 endosperm that was contained in it. 



The seed. — After developing a little way the embryo enters 

 upon a resting stage, the integument hardens about it into 

 a seed coat, and the whole becomes a seed, and falls away to 

 resume its development when it finds suitable conditions 

 somewhere in the soil. The seed is thus a composite struc- 

 ture consisting of three parts of very different nature. The 

 seed coat and macrosporangium wall represent and are a 

 part of the old sporophyte ; the remains of the prothallial 

 tissue within (known as endosperm) is gametophyte, and 



