THE CLUB-MOSSES 173 



little plant are very generally associated with one 

 of the large Lycopods, a Lepidodendron, which 

 was probably a tree. It has been suggested that 

 the slender and fragile Miadesmia may have 

 grown as an epiphyte on the branches of its robust 

 neighbour, just as so many Selaginellas are 

 epiphytic on stronger plants at the present day. 

 If this was so, Miadesmia, growing on trees, 

 would have been in much the same difficulty with 

 regard to its reproduction as the tree-Lycopods 

 themselves. It has already been pointed out 

 (p. 123) that the heterosporous method has the 

 disadvantage that the chances are against the 

 microspores and megaspores coming to rest at the 

 same spot, a condition necessary for fertilisation, 

 so that the production of a vast number of 

 microspores is demanded to ensure an occasional 

 meeting. This difficulty became aggravated when 

 the spores were discharged from the branches of 

 tall trees, whether from the cones of the trees 

 themselves, or from those of an epiphyte. If the 

 necessary meeting could be arranged to take place 

 on the parent plant, the matter would no longer 

 be left solely to chance. This advantage was 

 secured by the adoption of the seed-habit, involv- 

 ing the retention of the megaspore within its 

 sporangium. This may have been one of the 

 circumstances favourable to the development of a 

 seed-like organ in heterosporous trees, and also 



