Problem of Xeromorphy in Marsh Plants . 837 
year (about March) when the average evaporation is slight and the light- 
intensity low. As the season progresses and the curves of evaporation 
and light rise steeply (Text-fig. 9), and further, as the shoot itself grows up- 
wards into drier and lighter strata, the leaves exhibit continually increasing 
hairiness (PI. LXXXI, and Text-fig. 10). (2) The curve of hairiness of non- 
flowering shoots rises steadily to June or July ; subsequently falling till 
glabrous leaves are once more reached in autumn. This corresponds very 
closely indeed to the curves in Text-fig. 9. Finally, (3) the same general 
relations obtain even in the case of individual partly hairy leaves. Just as 
Text-fig. 10. Diagrammatic section through a group of plants of Spiraea Ulmaria , showing 
relation of glabrous and hairy leaves to strata of varying humidity. Glabrous leaves, black with 
white veins ; hairy leaves, white with black veins. (June.) x about f. 
successive leaflets occupy higher and less humid levels, so they exhibit 
increasing hairiness from below upwards (PI. LX XX 1 1 and LXXXI 1 1 , and 
Text-fig. 10). 
The same relations are broadly true of other hairy marsh plants, 1 and 
also for the structural variations described earlier in this paper. These 
field observations are at once seen to agree with the general experimental 
results quoted above. They suggest considerable responsiveness, on the 
part of X. Ulmaria and other plants, to even slight variations in the 
environment, 
See § 6 ; also Yapp (’08), p. 691. 
