THE LIFE HISTORY OF A GYMNOSPERM 375 



Meanwhile the integument becomes firmer and finally forms 

 the hard, protective seed coat, or testa (Fig. 299, F, t). Adjacent 

 tissue of the cone scale above the ovule develops a membranous 

 wing (Fig. 299, C, w), which separates from the scale of the cone 

 with the ovule as a part of the seed. These changes take place 

 during the summer, and the cone is not fully mature (Fig. 299, 

 Z>) and the seeds ripe until the following autumn, which is there- 

 fore more than a year after the cones are pollinated. Then the 

 scales of the cone, now quite woody in texture, separate, and the 

 seeds are shaken out and, since they are winged (Fig. 299, JE), 

 may be carried for a considerable distance by the wind. 



356. The life history of a gymnosperm. The life history of 

 a gymnosperm, beginning with the sporophyte (for the gameto- 

 phyte phases are now so inconspicuous that they only appear 

 during the process of seed formation), may be formulated as 

 follows : 



pollen grain Male Gametophyte sperm nucleus v 



\ 



> 



' 







/ (""crospore) (protoplasmic contents 



of pollen tube) 



JL j o poen e 



P yw \ einforyQ sac _ Female Gametophyte egg ' 



(megaepore) ( protoplasmic contents 



of embryo sac) 



This in abbreviated form becomes 



ff--s 

 es _ FeG _ e >-*>> etc. 



This formula should be compared with that of some hetero- 

 sporous pteridophyte, as Selaginella (Sec. 326), to make clear the 

 relationships. When carefully studied it will be found to be 

 merely an elaborated form of the simple formula of alternation 

 of generations. 



G< S e >-S-sp-G< S e >-S-sp-G, etc. 



The peculiarities of the life history of a gymnosperm are due 

 to heterospory (and this is true of all seed plants), by means of 

 which two sexual plants, male and female, have been differen- 

 tiated, and the fact that both gametophytes live wholly or 

 almost wholly as parasites upon the sporophyte. 



