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. It takes another full year 

 for these changes to take place, and the cone is not fully mature 

 (Fig. 299, D) and the seeds ripe until somewhat more than two 

 years after pollination. 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,^) they 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 , 



SpOrO- ("licro^ore) (i,roto f ,la*,nic contents 



J pollen tube) 



L f \ poen ue) / 



V J ( \ embryo sac Female Gametophyte egg 



(megaspore) (protoplasmic contents 



of embryo sac) 



This in abbreviated form becomes 



M G s 



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. 



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. 



