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 
