CALAMITES AND LYCOPODS 



379 



with sterile regions intervening. The leaves usually occur 

 in whorls of five, but often they are arranged in spirals. 



At the zone of transition from sterile to fertile regions, 

 imperfectly developed {aborted) sporangia are often formed, 

 and this (with other evidence) has suggested that, in the 



Fig. 274. — Lycopodium Selago. (After Bower.) 



evolution of the sporophyte, the purely vegetative regions have 

 resulted from a sterilization of fertile tissue. The correctness 

 of this interpretation of the origin of the sterile regions is 

 rendered more probable by the fact that the condition 

 found in L. Selago is characteristic of the fossil Lycopods 

 of the coal measures. The possession of a well-developed 



