CHAPTER XXIX. 
SUMMARY FOR SPHENOPHYLLALES AND 
FOR SPORANGIOPHORIC PTERIDOPHYTES GENERALLY. 
Ir 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.2 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.* There is also 
the difference of branching, which is terminal and dichotomous in the 
1 Studies, p. 499. * Proc. Roy. Soc., vol. lxix., p. 343+ 
®Compare Scott, Progressius, i., p. 166. 
