DEVELOPMENT OF PLANTS 333 



or microspores and larger ones or megaspores. The formation 

 of two kinds of spores, or heterospory, 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- 

 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 

 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 two or four 

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



(b) The Germination of the Spores. — The most important and 

 suggestive feature in the life history of Selaginella appears in 

 the germination of the spores. The spores germinate in the 

 sporangia and not after being shed, as in the other orders. The 

 microspore forms two cells while in the sporangia, a small one 

 often compared to a remnant of the male gametophyte or pro- 

 thallium and a large cell which ultimately forms a single anthe- 

 ridium. At about this stage of germination the microsporan- 

 gium opens by a vertical cleft, permitting the scattering of the 

 spores. The larger cell of the microspore now forms a central 

 mass of gamete mother cells which are surrounded by a single 

 layer of wall cells (Fig. 243, i). The male gametes are of the 

 same character as noted in the Lycopodiiim and are set free by 

 the disintegration of the wall cells. The megaspores begin to 

 germinate even before they have reached their full size. The 

 nucleus of a megaspore divides repeatedly, forming numerous 

 nuclei which become arranged about the walls of the young spore. 



