14 
DR R. 0. DAVIE ON 
unconnected with the anatomy of their leaves, the genera to which these species 
belong appear to be more or less related to one another and to the Cyatheaceae on the 
one hand and the Pterideae on the other (Bower, ’08,. pp. 615, 616). It is tempting to 
conclude that the appearance of the combination type of pinna-trace in these genera 
and in Lonchitis is dependent on a strongly-impressed hereditary tendency (probably 
originating in large-leaved forms of the Cyatheaceae), and is not directly related to 
the size of the pinnae in individual leaves. But this by no means invalidates the 
conclusions already reached regarding the factors which control the form of the 
Fern leaf-trace and the method of separation from it of the pinna-trace. The general 
principles seem clearly established from the examination of the members of the genus 
Polypodium , and from the comparison of the species of Aspidium, Polystichum, and 
Dryopteris. The leaf-traces and pinna-traces of Balantium culcita and Leptochilus 
guianensis are difficult to explain on these general principles. But that the two 
latter species (both of which have leaves relatively small in comparison with those of 
nearly related species) show modifications of the normal types of pinna-supply seems 
to suggest that their divergences may be explained from hereditary tendencies, 
acting in a circle of affinity where heredity has a powerful influence. That there 
has been, more than once in the course of evolution, a change from the extramarginal 
to the marginal type of pinna-trace is shown by a glance at the tabular scheme of 
the methods of pinna-supply among the Ferns (Davie, ’14, p. 354). The change 
has occurred in the Cyatheaceae, where we usually have the extramarginal type 
but find the marginal in Balantium culcita. It has occurred in the Davallieae, 
where Dennstsedtia, Microlepia • (except M. hirsuta ) and Leptolepia have the 
extramarginal type, and Nephrolepis, Lindsaya, and Davallia have the marginal 
type. It appears again in the Aspidieae, Dryopteris, Didymochlsena, and Polystichum 
having extramarginal pinna-traces, while Aspidium has the marginal type. And 
in the genera mentioned above, related on the one hand to the Cyatheaceae and 
their derivatives, and on the other to the Pteridese, while the combination type 
of pinna-trace is prevalent, it is sometimes a combination of a “reinforcement” with 
an extram arginally -derived strand ( Gyathea , Alsophila, etc.), sometimes of a “ re- 
inforcement” with a strand derived marginally ( Hypolepis , Histiopteris, Lonchitis). 
Thus sometimes in a natural circle of affinity we find the extramarginal derivation 
retained and the “reinforcement” discarded {e.g. in the Cyatheaceae, in the species of 
the genus Cibotium, and in Dicksonia antarctica, Lab.) ; sometimes the “ reinforce- 
ment ” is retained and the extramarginal derivation exchanged for the marginal {e.g. 
in Hypolepis — and its derivatives — in the circle of the Dennstaediinae) ; sometimes 
both reinforcement and extramarginally derived strand are lost and a modified type 
of marginal pinna-trace replaces them {e.g. in Balantium culcita in the Cyatheaceae). 
It is therefore evident that in drawing general conclusions regarding the types of 
pinna-trace among the Ferns, and in comparing the methods of branching found 
among their leaf-traces, we must not neglect the possible influences of heredity, though 
