LEAF IN THE VASCULAR CRYPTOGAMS AND GYMNOSPERMS. 
Gil 
simple cases where there is growth in a given direction, but no branching occurs, the 
characters of the simple leaf are not unlike those of the phyllopodium as described 
for the more complicated forms. Further, it has been shown that as we rise in the 
scale of the Gy mno sperms, gradual modifications of the characters become apparent; 
for instance : (1) there is a gradual differentiation of the main axis of the leaf (the 
phyllopodium), as a supporting organ, from the members of higher order (pinnae) 
which it bears ; (2) peculiar conformations appear at the base of the phyllopodium, 
such as sheaths, and stipules ; (3) there is an earlier arrest of the continued apical 
growth, and increased prominence of intercalary growth ; (4) there is an increased 
robustness of the several parts, which may be recognised clearly on their first 
appearance. All these characters lead on gradually towards the leaf as it is seen 
in the Angiosperms, and more especially in the Dicotyledons. On these grounds I 
conclude that the leaf in the latter plants should also be treated consistently as a 
simple branch or branch-system, throughout its whole length, however greatly the 
prevalence of intercalary growths in various directions, and at various points, may 
displace and distort the original arrangement of the parts. It has also been shown 
that the modifications at the base of the leaf do not exist, or are hardly to be traced 
in the Leptosporangiate Ferns, and that where they are found in higher forms they 
may be referred to modifications of contour of the fundamental winged structure ; on 
this ground it is concluded that such modifications, however prominent they appear in 
the higher plants, are not of such a fundamental nature as to justify the division of 
the leaf in the first instance into two parts (foliar base and upper leaf), which are, as 
I have above pointed out, not morphologically co-ordinate where a branching of the 
leaf occurs. 
It will be noted that the treatment of the leaf as a simple podium or as a branch- 
system implies a distinction between the wings of a phyllopodium and the pinnce, and 
it may be argued that such a distinction cannot be clearly defined : for instance, the 
wavy margin of the leaf of Ginkgo and of the pinme in species of Zamia : are the pro¬ 
jections in these cases to be regarded as members of higher order ? I can only reply 
to this that difficulties of this nature beset every morphological generalisation, and 
that in my opinion the number of cases in which such difficulties come prominently 
forward is not sufficient to justify the rejection of the system which I have proposed. 
I cannot close this paper without acknowledging how much I am indebted to the 
Director and Assistant Director of the Royal Gardens at Kew, not only for the use of 
the Jodrell Laboratory, but also for the unstinted supply of the rare material which 
alone could make this investigation possible. 
4 i 2 
