104 R- H. Compton. 
It seems, then, that emphasis should be laid on the small 
power of the Conifers to vary the character of their leaves. The 
predominant types— viz., the acicular leaves of Pinus, Abies , Larix, 
etc , the linear-lanceolate leaves of Taxus , Cunninghamia, etc., the 
scale-like appressed and “ concrescent ” leaves of Cupressus, 
Libocedrus, etc.—are all closely related to one another. The 
development is in all cases small compared with that of the stem: 
i.e., the Conifers are in fact fixedly microphyllous, whatever they 
may he according to phylogenetic theory. 
This prevalence of the narrow acicular, linear-lanceolate, and 
cupressoid types of leaf seems to be connected with the absence of 
lateral pinnation of the foliar vascular system. In the majority of 
cases a single or double median vascular strand runs from end to 
end of the leaf without branching : and even in the § Nageia of 
Podocarpus and certain Araucarieas where the leaf is broader and 
many-nerved, there is not found that copious lateral branching and 
anastomosing of the veins with which the free pinnation of the 
leaves of so many Dicotyledons is associated. 
The failure of the bundles to branch is compensated to some 
extent by the development of transfusion tissue; as in the case of 
certain fossil Lycopods, in which group the same limitation of the 
power of branching, microphylly, and sometimes xerophilous 
structure, seems to obtain. 
Thus it appears that the Conifers are rigidly microphyllous 
forms, and that the power of freely adapting themselves to 
ecological conditions is strictly limited by the lack of plasticity in 
leaf structure. It is suggested that the lack of ability of the foliar 
vascular system to branch with ease is one of the causes which 
have contributed to keeping the leaf small: and that this failure 
may possibly be associated with the presence of transfusion tissue, 
which, though valuable in itself under the circumstances, may 
conceivably tend to prevent improvement in other directions, such 
as that of free pinnation of the vascular strands. 
Given this cramped hereditary type of structure, ecological 
adaptation appears to have been the result of two processes going 
on simultaneously : (1) the development of enormous numbers of 
the rigidly constructed leaves with a view to increased assimilation 
and growth, and (2) the production of xerophily in the individual 
leaves as a compensation for the resulting increase of surface. 
It thus seems clear that the hereditary factor is of great 
importance in the ecological relationships of the Coniferae. It is 
