PTEEIDOPHYTES 



317 



Heterospory. In the fertile shoots of Selaginella the small spores, 

 called microstores, are borne in microsporangia at the apex of the 

 reproductive strobili, while the large spores, called megaspores, 

 are produced in large megasporangia at the base of the strobili 

 (Fig. 182). The microspores, like the spores of the equiseta and 

 thehomosporous lycopods, are produced 

 in large numbers in each microsporan- 

 gium, but the large megaspores are 

 reduced to from two to four spores in 

 each of the megasporangia (Fig. 183). 

 This discrepancy in size is closely re- 

 lated to the function of the two kinds 

 of spores in the role which each plays 

 in reproduction. " The diameter of the 

 megaspore is usually about ten times 

 that of the microspores, which is equiv- 

 alent to a proportion of 1000:1 in bulk." 

 The larger megaspores produce female 

 gametophytes and on account of their 

 size and weight are not easily scat- 

 tered by the wind when shed from 

 the sporangium. The small micro- 

 spores, on the contrary, are light and easily scattered, like the 

 pollen grains which we have already studied in the higher 

 plants. Like the pollen, therefore, the microspores are the 

 mobile spores of Selaginella, which can be borne to the heavy 

 megaspores at the proper time for reproductive purposes. 

 Since, however, this association of megaspores and micro- 

 spores is necessarily a matter of chance, it follows that the 

 microspores must be produced in great numbers to insure fer- 

 tilization. Although the differentiation of spores in Selaginella 

 and in other heterosporous PteridopTiyta must be regarded as 

 the first step toward the evolution of seeds, it is, nevertheless, 

 in some respects a wasteful arrangement and not without its 

 difficulties in insuring a meeting of the male and female gametes. 

 In the higher seed plants this difficulty is overcome in part by the 

 production of the stigma as a receptive apparatus for the pollen. 



Root 



FIG. 182. Habit of Selaginella, 

 with strobili 



