SPHENOPHYLLS 217 



One is in the structure of the stem, the other in 

 the position of the sporangium-bearing organs. 



In Sphenophylls the primary wood was solid 

 and developed from without inwards; in Cala- 

 mites there was a pith, and the whole of the wood 

 developed from within outwards. The Lower 

 Carboniferous Protocalamites (see above, p. 209) 

 helps to bridge this gap, for there a certain part 

 of the wood developed from without inwards, 

 encroaching on the pith, though not to any 

 great extent. 



In Sphenophylls the sporangium-bearing organs 

 were always parts of the sporophjdl; the 

 bracts, if present, were sterile segments of the 

 same sporophyll. In most Calamites the sporan- 

 gium-bearing organs were independent of the 

 bracts; we might suppose that each sporangio- 

 phore (shield-like scale) represented an entire 

 sporophyll. 



There are, however, some reasons, connected 

 with the course of the vascular strands, for sup- 

 posing that where bracts were present in the 

 Calamites, the sporangiophores had originally 

 been connected with them, i. e. with the whorl 

 of bracts next below, so that in these cases the 

 arrangement may have once been the same as 

 in the Sphenophylls. On the other hand, it is 

 not necessary to press this interpretation in 

 every case. In the Ferns we see that in some 



