102 R. H. Compton. 
that Coniferae nowadays do not for the most part grow under dry 
conditions of life.” 
There is thus considerable divergence of opinion on the subject, 
which seems to rest upon the lack of adequate evidence as to the 
exact habitat (in the narrow sense) of particular species, and as to 
their physiological powers. The difficulty of fixing criteria in 
matters of structure is also very acute here, as in other cases of 
adaptation. 
The recent work of Professor Groom (loc. cit.), and his 
criticism and extension of von HohnePs results in the estimation 
of the velocity of transpiration, have thrown much light on the 
subject. The most important result of this work is to shew that it 
is fallacious to judge of the xerophily of a plant from leaf-structure 
alone, and that a factor of fundamental importance is the total 
leaf-area. That this should have been so much neglected in the 
past is remarkable, but may perhaps be due to the prevailing 
tendency to base conclusions as to xerophily upon the examination 
of isolated leaves in the laboratory, without reference to the habit 
of the living plant. Clearly, to compare in this way, say, Pinus 
syl'vestris with Welwitscliia mtrabilis, plants whose leaves shew 
quite similar xerophilous structure, can throw little light on the 
water-relations of the plants in nature. 
Professor Groom considers that “ the northern evergreen 
Conilerte are ‘ architectural xeropliytes ,’ in which the extensive 
surface exposed by the evergreen leaves as a whole renders it 
necessary for the individual leaves to be xeromorphic in form and 
xerophytic in structure.” He thinks it possible “that concurrent 
increases in the assimilatory surface and in the xerophytic devices 
generally increase assimilation in relation to transpiration” (an idea 
to some extent supported by the work of Renner 1 on the effects of 
sinking and narrowing of stomata); but that in the absence of 
statistics bearing on the subject “ it is impossible to explain why the 
Conifers should have adopted the device of having a large aggregate 
surface with a greater degree of xerophytism.” 
Now the implication of the above quotations seems to be that 
the plant is extremely “ plastic,” and that in matters of adaptation 
it will always come to adopt that habit and structure which give 
the greatest degree of efficiency under the given environmental 
circumstances. In fact it is tacitly assumed that, had it been more 
advantageous to the Conifers to adopt the habit, say, of an oak-tree, 
1 “ Beitrage zur Physik der Transpiration.” Flora, vol. C., 
p. 451, 1910. 
