240 Davie.—Comparative List of Fern Pinna-traces , with 
supplied with their pinna-systems on the marginal plan. But wherever the 
extra-marginal type of pinna-supply has been found in connexion with any 
of the pinnae of a leaf, it has also been found that the basal pinnae are 
supplied extra-marginally. In a paper recently published 1 from the notes 
of the late Professor Gwynne-Vaughan, it is remarked that c it must be 
remembered that in some species the lowest branches of the rachis and of 
the primary branches are themselves reduced in size, being markedly 
smaller than some of those higher up. This reduction affects the method 
of branching so that it may present features of a more or less primitive 
type.’ In the species which I have examined, several plants have had 
basal pinnae smaller than those higher up the leaf, but, though there 
sometimes is a marked reduction in the size of the pinna-trace for these 
pinnae, compared with that of the pinna-traces of the higher branches, no 
actual divergence from the type prevalent throughout the lower pinnae has 
been found. Dr. J. M. Thompson tells me, however, that he has found an 
irregular occurrence of the extra-marginal type of pinna-supply in leaves of 
Trismeria trifoliata (L.), Diels—sometimes the extra-marginal type is found, 
sometimes the marginal. And the one may succeed the other and in turn 
be succeeded by it as one passes up the leaf from pinna to pinna. In the 
leaves of Trismeria trifoliata 'which I have examined, the extra-marginal 
type certainly occurs regularly throughout the length of the leaf. 
We find, too, that in Ferns which have the extra-marginal type of 
pinna-supply in the fully mature leaves, the marginal type is regularly 
found in the earliest leaves. 2 In order, therefore, to make use of the 
criterion of the type of pinna-supply, we must examine the lower pinnae of 
the older leaves of the Ferns we wish to compare. 
The results of the investigations detailed in the three earlier papers and 
in this may now be summarized. 
The amount of xylem in the adaxial portion of the leaf-trace is 
dependent on the situation in which the Fern grows; 3 the abaxial compli¬ 
cations depend on the length of the leaf; 4 the reinforcement of the adaxial 
strands, on the close crowding together of the pinnae ; 5 the reinforcement 
of the portion of the pinna-trace derived from the adaxial side of the leaf- 
trace, on the size and complexity of the pinnae. 6 
The presence of adaxial hooks in the leaf-trace seems in great measure 
to depend on a factor connected with heredity; the hooks may have 
appeared as Fern-leaves increased in size. 7 
The form of the pinna-trace depends (1) on the presence or absence of 
hooks in the leaf-trace (an inherited feature—useful, therefore, in phylo- 
1 Ann. of Bot., vol. xxx, p. 491. 2 Trans. Roy. Soc. Edin., vol. 1 , pp. 360, 361. 
3 Ibid., vol. lii, p. 4. 4 Ibid., pp. 5, 6, 8, 12. 
5 Ibid., pp. 6, 7. 6 Ibid., pp. 7, 9, 10; 11, 13, 14; above, p. 236. 
7 Cf. A. G. Tansley, Ev. of Fil. Vase. System, p. 117. 
