584 
MR. F. O. BOWER ON THE COMPARATIVE MORPHOLOGY OF THE 
b. Plumular-leaves. 
The first-formed plumular leaves are arranged in pairs, the first pair decussating 
with the cotyledons. As the plant grows older the arrangement of the subsequentlv- 
formed leaves is spiral. 
The apex of the stem terminates in a flattened cone, in which, as described by 
Strasburger for Cycas revoluta, there is no single apical cell, but the cells at the 
periphery divide frequently by periclinal walls : there is thus no continuous layer of 
dermatogen covering the punctum vegetatioms. The leaf makes its first appearance 
at the periphery of this flattened cone, as a simple, uniformly meristematic, hemi¬ 
spherical swelling, but with the side facing the apex of the stem slightly flattened. 
It is covered externally by a well marked and continuous layer of dermatogen : thus 
the above mentioned periclinal divisions of the peripheral cells are limited to the apex 
of the stem, and are not found in the young leaf. The growth of the leaf is not 
uniform in all directions, but is symmetrica] on either side of the median plane : a 
slight hyponastic curvature of the phyllopodium is all that represents the circinate 
vernation of that part in the Ferns : this is more marked in old than in young plants 
(Plate 38, figs. 22, 23), thus the leaves of older plants overarch the apex more com¬ 
pletely than is the case in younger plants. Growth is at first in a longitudinal direc¬ 
tion, the cells dividing chiefly by transverse walls. The tissues of the young leaf, as 
seen in longitudinal section, are actively meristematic throughout, while the cells 
at the extreme apex show the relatively large nuclei, and thin walls characteristic of 
an apical meristem. Single cells of the dermatogen grow out as unicellular hairs, 
which are particularly numerous on the dorsal surface, and on the median portion of 
the ventral surface. These have to be removed in order to study the further develop¬ 
ment of the leaf, and are not, as a rule, represented in the figures. 
While the whole leaf increases in length, transverse dilatation goes on unequally in 
different directions, and at different parts of its length. It is more marked in a 
direction perpendicular to the median plane than parallel to it, and though it is more 
extensive at the base of the leaf than it is higher up, still it is continuous throughout 
on the same plan. The result of this is that the leaf shows two marginal weals, 
which may be traced as continuous from the base to the apex (Plate 38, fig. 24). 
Starting from the base, it is found that the area of insertion of the leaf is semilunar, 
with somewhat rounded corners ; a transverse section of the leaf a little higher up 
shows a widening out of the corners as thin wings, and this increases upwards to a 
certain point, becoming then again gradually reduced. There is in fact a sheathing 
base to the young leaf of Cycas, which is produced by transverse dilatation through¬ 
out the tissues of the base of the leaf, but especially localised near the two margins 
(compare figs. 24 and 25). Tracing the marginal weals again upwards, they are found 
to extend uniformly as smooth, rounded, parallel ridges to the apex of the young 
leaf. The conformation of the whole phyllopodium from base to apex is fundamentally 
