100 THE MORPHOLOGY OF PTERIDOPHYTES 



branches, about sixteen in number, each bearing two re- 

 flexed sporangia. Only detached cones have, so far, been 

 found, but they are presumed to have belonged to some 

 member of the Sphenophyllales, because of the triarch or 

 hexarch arrangement of the primary wood in the axis. 



Sphenophyllostachys { = Bowmanites) Dawsoni, on the 

 other hand, is known to have been borne on stems like those 

 of Sphenophyllum plurifoliatum. The cone was up to 12 cm 

 long and i cm in diameter and bore whorls of bracts, fused 

 into a cup near the base, but with free distal portions. In the 

 axils of these bracts, and fused with them to a certain extent 

 (Fig. 15P), were branched sporangiophores. In one form 

 (forma a) each sporangiophore had three branches arranged 

 in a very characteristic way (Fig. 15Q), each terminating in 

 a single reflexed sporangium. In another form (forma y), 

 there were six branches. 



Sphenophyllostachys { = Bowmanites) Roemeri was similar 

 in its organization to S. Dawsoni, forma a, except that each 

 branch of the sporangiophores bore two reflexed sporangia 

 (Fig. 15R). 



In recent years, a number of relatively simple cones have 

 been described, which are nevertheless beheved to belong to 

 the Sphenophyllales. Thus, in Bowmanites bifurcatus, each 

 sporangiophore forked only once, while in Litostrobus 

 iowensis the sporangia were borne singly on a short un- 

 branched stalk. In the latter species, the sporangia were not 

 reflexed but, despite this very simple organization, an 

 affinity with Bowmanites is presumed, because of the 

 number of bracts and the number of sporangia in a whorl 

 (twelve and six respectively).^ The discovery of these simple 

 cones has led to the suggestion that, within the Spheno- 

 phyllales, evolution has involved progressive simplification. 



While the vast majority of the Sphenophyllales were 

 homosporous, at least one, Bowmanites delectus, was hetero- 

 sporous^ with megaspores about ten times the size of the 

 microspores. 



