310 Yapp. ~ 0 n Stratification in the Vegetation of a 
ments referred to in the paper). The general reduction in the size of the 
transpiring organs in the upper layers of the vegetation is also suggestive, 
though it is merely the effect of this on transpiration, and not the causes of 
the diminution in leaf-size, which is dealt with in this paper. 
ii. Thus the mutual protection, both from excessive transpiration and 
also from the mechanical effects of wind, derived from this very general 
‘ association of shoots ’, is probably beneficial, even apart from the more 
obvious cases where the climate is exceptionally rigorous. The structural 
peculiarities of vegetation then, no less than morphological or anatomical 
modifications of leaves and other transpiring organs, may be effectual 
in securing immunity from the dangers of excessive transpiration. 
is. It is shown that the different species vary as regards the depth of 
their root-systems ; the height of their shoots ; the relative positions of their 
transpiring organs with respect to other plants ; the length of their vegeta- 
tive period, &c. In fact, few of the species of plants forming the vegetation 
of a swamp-moor have to face precisely the same set of physiological con- 
ditions. Thus the arguments of authors, who insist that the so-called 
xerophytic structures of marsh plants can have no reference to present-day 
conditions, because both xerophytic and non-xerophytic species often grow 
side by side in nature, are entirely inconclusive. 
In addition to the acknowledgements made in the course of this 
paper, I wish to express my thanks to Dr. H. R. Mill for information 
respecting his evaporation records at Camden Square, which he has placed 
at my disposal ; to Dr. G. A. Schott and Mr. A. A. Robb for criticisms 
and assistance with respect to the physical portions of this paper ; and 
finally to Professor A. C. Seward for permission to use the Cambridge 
Botanical Laboratory as my head quarters during the investigations in the 
neighbouring Fenland. I have also to acknowledge grants to defray the 
cost of instruments, &c. } made by the British Association for the Advance- 
ment of Science. 
