LYCOPSIDA 89 



Species, however, retain more than the one functional mega- 

 spore mother cell, so that up to twelve or even more mega- 

 spores may result. Yet other species are pecuhar in that, out 

 of the single tetrad of megaspores, one, two or three may be 

 abortive, so that in the extreme condition the megasporan- 

 gium may contain only one functional megaspore. S. rupestris 

 usually has two megaspores in each sporangium and some- 

 times only one, while S. sulcata regularly has only one. 

 S. rupestris is further remarkable in that the megaspores are 

 not shed, but are retained within the dehisced megasporan- 

 gium and fertilization takes place while it is still in situ. 

 Thus, it happens that young sporophytes may be seen grow- 

 ing from the cone of the parent sporophyte. Few seed-plants 

 have achieved this degree of vivipary, yet in Selaginella it 

 occurs in a species belonging to the allegedly primitive 

 Homoeophyllum group. 



In those species whose cones contain both megasporo- 

 phylls and microsporophylls, it is usual for the former to be 

 near the base of the cone and the latter near the apex. This 

 further emphasizes the point, already made, that the arrange- 

 ment in lycopods is the inverse of that observed both in the 

 gymnospermous Bennettitales and in hermaphrodite flowers 

 of angiosperms. 



Fig. 14D illustrates the appearance of a megasporophyll 

 in longitudinal section, with the hgule (i) and the differential 

 thickening in the sporangium wall, while Figs. 14B and 14C 

 illustrate a dehiscing microsporangium and megasporangium 

 respectively. Contractions of the thick-walled cells of the 

 megasporangium cause the megaspores to be ejected for a 

 distance of several centimetres, but dispersal of the micro- 

 spores is mainly by wind currents. 



Long before the spores are shed, nuclear divisions have 

 started to take place, so that the prothallus is well advanced 

 when dehiscence occurs. The stages in the formation of the 

 male prothallus are very similar to those figured for Isoetes 

 and, at the moment of Uberation, the male prothallus 



