CHAPTER XXIX. 



SUMMARY FOR SPHENOPHYLLALES AND 

 FOR SPORANGIOPHORIC PTERIDOPHYTES GENERALLY. 



IT has now been seen that the two living genera of Psilotaceae, though 

 differing in the number of sporangia on each sporangiophore, as well as 

 in the size of the appendages, correspond nevertheless in the essential 

 characters of form ; they are alike in the rootless and leafless rhizome, 

 in the irregular alternate arrangement of the appendages, and in the 

 relation of the sporangiophore to the forked sporophyll. The anatomical 

 characters also correspond, though with differences open to biological 

 explanation. No one will therefore doubt the natural affinity of these two 

 genera. 



The relation of the Psilotaceae to the Sphenophylleae has been only 

 lately recognised. Previously they were placed with the Lycopodiaceae, 

 and in the above pages points of similarity to these plants have been 

 repeatedly noted ; such as the dichotomous branching of the primitive 

 monostelic axis, the imperfect differentiation of the vegetative and fertile 

 regions, and the relation of the sporangiophore in the one and of the 

 sporangium in the other to the sporophyll. It was Dr. Scott who first 

 indicated the closer relation between the Psilotaceae and the extinct 

 Sphenophylleae, on the ground of anatomical resemblance, as well as the 

 similarity of the spore-producing parts ; 1 this view was further developed 

 by Thomas, on the basis of observation of many specimens in their 

 native habitat- The chief difference seems to lie in the fact that the 

 appendages of the former are irregularly alternate and distinct, while in 

 the latter they are in regular whorls, and webbed at the base. But the 

 genus Lycopodium, which includes species with whorled and others with 

 irregularly alternate leaves, shows that too much weight must not be 

 attached to such a distinction relating to kindred forms. 3 There is also 

 the difference of branching, which is terminal and dichotomous in the 



1 Studies, p. 499. 2 Proc. Roy. Soc., vol. Ixix., p. 343. 



3 Compare Scott, Progressus, i., p. 166. 



