50 M. C. Slopes. 
work done on water transmission in stems, yet illustrates the fact 
that physiologically the wood of the Gymnosperms is not by any 
means so effective as that of the average Angiosperm. In this 
fact we may now see the fundamental explanation of the figures 
illustrating the amounts transpired by different trees. 
The following are taken from Schimper’s quotation of Von 
Hohnel’s work, as the original was not available. The relative 
amount of water transpired from June 1st to November 30th per 
100 grammes dry weight of leaf:— 
Birch 
67-9 
Oak ... 
... 28-3 
Lime 
61-5 
Red Spruce... 
5-8 
Beech 
56-6 
White Pine... 
5-8 
Maple 
46-2 
Silver Fir ... 
4-4 
Elm 
40-7 
Austrian Pine 
3-2 
This is immediately correlated with the size of the leaves, and 
their protected form in the Gymnosperms, but it appears that their 
leaf-structure is the result of the necessity for economy in trans¬ 
piration enforced by the structure (not the quantity, for the woody 
stems of Conifers are relatively thick for the amount of foliage¬ 
bearing branches) of their wood, which is a family character 
uninfluenced by environment. 
As we know from the study of the evolution of plants, the 
potentialities of adaptation, and generic, even specific, change and 
evolution, do not remain indefinitely in any given group. The 
Gymnosperms, being so ancient a group, did not as such retain their 
adaptability till they had completed the efficiency of their wood, 
but stopped, and still stop short at tracheides with bordered pits, 
just as in their fructifications they have stopped short of evolving 
closed carpels. Corresponding to their lower systematic position is 
their less efficient woody stem, and as a necessary result of this 
the “xerophilous” character of their leaves, even when the plants 
are growing with a good water-supply. 
This would explain the almost universal occurrence of xero- 
phytic structures in the higher Gymnosperms from the Palaeozoic 
upwards. 
It appears then that the xerophytic characters of the Coniferales 
in very many cases are not adaptations to xerophytic conditions in 
their own lines, nor are they “inherited” from the remote past as 
vestigial characters no longer in touch with present day necessities, 
but are the result of physiological limitations of the type of 
wood in this ancient and incompletely evolved group. In other 
words their “ xerophytism ” is not ecological, but phylogenetic. 
The University, Manchester. 
M. C. STOPES. 
