566 DR. M. T. MASTERS: GENERAL VIEW 
projects inwards in thick wedge-shaped masses (Pl. 22. fig. 7). 
Where the tree is much exposed to wind, the hypoderm-cells are 
likely to become thicker, if not always more numerous than in the 
same species growing under less exposed conditions. In the same 
way they are less highly developed in the cotyledons than in the 
adult leaf (Pl. 22. fig. 8). But this, again, is a point which 
requires more attention than I have been able to bestow on it. 
The occasional presence or absence of a layer of thin-walled 
cells, presumably containing water, just beneath the epiderm 
should also be noted, as well as the position and number of the 
rows of stomata. 
An instructive comparison may, as has already been remarked, 
be made between the structure and conformation of the cotyledons 
and of the primordial leaves with those of the adult foliage. 
Without going into details, unnecessary for the present purpose, 
it may be stated that the histological structure of the temporary 
foliar members is of a less highly developed character than 
in the more permanent foliage. This is, of course, what might 
have been expected. The cotyledons are sometimes, as in 
P. Pinea, triangular and with a circular meristele (Pl. 22. fig. 8) 
and unbranched bundle, whilst in the adult leaves of the same 
species the stele is oblong in section and the fibro-vascular 
bundle divided. From a genealogical point of view, it is therefore 
important to note that certain characters that are common to all 
the species in their juvenile state may sometimes continue their 
juvenile characteristics in adult life with comparatively little 
modification. Thus the unbranched bundle, which is a common 
characteristic of the cotyledons and primordial leaves (P1.23. fig. 9) 
in some species, remains in the fully developed adult foliage ; 
whilst in others the bundles, simple in the juvenile state, become 
branched later on. It is quite probable, therefore, that the 
species with branched bundles are the modified derivatives from 
those of simpler form. This assumption is supported by other 
characteristics more readily seen, such as the nature of the bud- 
scales and the peculiarities of the cone-scales. 
The walls of the cell constituting the cortex or mesophyll of 
the leaf are in some species plain (Pl. 23. fig. 10) ; in other species 
thrown into folds, “ plicate ” or sinuous, indicating an enhanced 
physiological function (Pl. 22. fig.7). There is usually no distinct 
palisade-layer in the leaves of Pinus; so that the structure is 
distinct in that respect from that of the leaves of Abies and other 
