PTERIDOPHYTA 



233 



SelagineUa is heterosporous, each strobihis usually bearing two kinds of 

 sporangia — microsporangia and megasporangia (Fig. 1895, C). The 

 microsporangia, which are often reddish, generally occur in the upper part 

 of the strobilus, while the megasporangia, which are commonly yellowish, 

 are borne below. The megasporangia are usually sUghtly larger than the 

 microsporangia and are generally four-lobed. 



B 



Fig. 189. SelagineUa willdenovii. A, branch with leaves and .strobili, twice natural size; 

 B, a microsporophyll with a microsporangiiun containing numerous microspores, X25; C, 

 a megasporophyll with a megasporangium containing four megaspores, X25; several 

 microspores and a megaspore drawn to the same scale are also shown. 



Both kinds of sporangia develop alike as far as the stage in which the 

 sporogenous tissue is differentiated. In the microsporangium practically 

 all the mother cells divide to produce tetrads, and consequently many 

 small spores are formed (Fig. 192A). These are the microspores. In the 

 megasporangium, on the other hand, all the mother cells degenerate but 

 one, which greatly enlarges and forms a tetrad of thick-walled spores that 

 eventually fill the sporangium (Figs. 191 and 192B). These are the mega- 

 spores. The sporophylls that produce the microsporangia are micro- 

 sporophylls, while those bearing megasporangia are megasporophylls. 

 Usually the sporophylls themselves, however, are of approximately the 

 same size and form. Like the foliage leaves, each sporophyll bears a 

 ligule. It is situated just beyond the sporangium (Figs. 190 and 192). 



Upon germination, the microspores give rise to male gametophytes and 

 the megaspores to female gametophytes. Thus heterospory involves not 

 only a differentiation of spores but also a differentiation of gametophytes. 



