SPHENOPHYLLALES 



[CH. 



more usual type of strobilus ; instead of the dorsal and ventral 

 lobes of the bracts sharing between them the duties of protec- 

 tion and spore-production, the whole of each bract is constructed 

 on the plan of the maximum spore-output, the laminar termina- 

 tions of the sporangiophores serving the purpose of protection. 

 The cone may be described as more specialised than the normal 

 type of strobilus for reproductive purposes^. 



It has been stated, on evidence which is unsatisfactory, that 

 Sphenophyllum possesses two kinds of spores. While regarding 

 the genus as homosporous on the evidence before us, it is 



Pig. 116. Sphenophyllostachys Dawsoni. (After Thoday.) A. Larger spores ; 

 B, abortive spores ; C, mature spores showing the characteristic spines. 



interesting to find that cases occur in which the spores in the 

 same sporangium exhibit a marked difference in size. Attention 

 has been called by Williamson and Scott ^ to variation in the 

 dimensions of spores: a more pronounced difference in size has 

 been recorded by Mr Thoday ^ who gives 120/i as the maximum 

 and 90/i as the minimum diameter of the spores in a cone of 

 Sphenophyllostachys Dawsoni. The presence of several abortive 

 spores in the sporangium (fig. 116) containing the larger 

 spores favours the view that this difference in size may be the 

 first step towards the development of heterospory. 



1 See also Browne, Lady Isabel (09) p. 4. 



^ Williamson and Scott (94) A. p. 911. 3 Thoday (06). 



