348 



SPOROPHYLLS OF SELAGIXELLA 



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- 

 angia accordingly as they contain small or large spores and for 

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



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, microspore 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 previous 

 groups. 



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 one spore is formed. The other mother cells do not 

 develop and are ultimately consumed in nourishing the mega- 

 spores. As a result of the large amount of food transferred to 

 the megasporangium, both it and especially the one to four 

 spores become much larger than the others (Fig. 242, 3-6). 

 The physiological differentiation of the spores noted in Equisetum 



