836 Yapp . — Spiraea Ulmaria , Z., and its Bearing on the 
leaves. As a rule, these occupy in nature the more shady and humid, and 
the sunnier and drier positions respectively. This is true as regards both 
time and space ; i. e. whether we consider the leaves in relation to the 
annual march of the seasons, or the vertical distribution of humidity and 
light in the vegetation. The curves in Text-fig. 9 represent the mean 
evaporation and light intensity for several years. Both curves rise to 
a maximum in July and then fall steadily towards autumn. Further, I have 
shown that in marshes there is a rapid rise in the average evaporation as the 
successive strata ot the vegetation are ascended. 1 Lately, I have also 
determined by experiment that the same is true of light-intensity. 
If the degree of hairiness could be conveniently represented by means 
of curves, it would be found that the curves of actual hairiness in Spiraea 
Ulmaria follow very closely the mean curves of evaporation and light- 
intensity, both in space and time. Thus, (1) in the erect flowering shoots, 
the glabrous spring leaves are formed near the ground, at a time of 
1 Yapp (’09), p. 287 et seq. Dachnowski (’ll), p. 146, has recently confirmed my evaporation 
results, in the case of the bog vegetation of Cranberry Island. 
