316 SPORES OF SELAGINELLA 



Ly cop odium, in that two kinds of spores are formed, small spores 

 or microspores and larger ones or megaspores. The formation 

 of two kinds of spores, or heterosrjory, is not confined to the 

 Lycopodiales. It occurs among certain genera of the Filicales 

 and, as has been stated, characterizes some of the fossil Equise- 

 tales. The megaspores are generally formed at the base of the 

 strobilus and the microspores occupy the upper sporangia (Fig. 

 242). The sporangia are called microsporangia and megaspor- 



FIG. 242. Sporophylls and spores of Selaginella: <2, strobilus with lower 

 sporophylls separated for spore dissemination. 3, megasporophyll with 

 sporangium containing four megaspores. 4, microsporophyll. 5, mega- 

 spore enlarged. 6, microscope equally magnified. 6A, more enlarged view 

 of the microspore. The triangular surfaces of the spores show that these 

 spores have been formed in tetrads from a spore mother cell as in pre- 

 vious groups. 



angia accordingly as they contain small or large spores and for 

 the same reason the leaves may be designated as micro- and 

 mega-sporophylls. The two kinds of spores originate in the 

 same manner as previously noted and the difference in size is 

 due to the amount of food which they receive. In the case of 

 the microspores, the numerous mother cells of the sporangia form 

 four spores each in the usual manner; but in the megasporangia 

 only .one of the mother cells divides in this manner, and in some 

 cases only two spores are formed. The other mother cells do 



