356 MR. В. х, ADAMSON! AN ECOLOGICAL STUDY 
relative humidity very closely, though the changes in evaporation are often 
much greater for a small humidity change than would be expected ; the 
maxima and minima often slightly lag behind those of the relative humidity. 
Apart from the ehanges mentioned above, due to the shrub-growth, the 
rates of evaporation are different in the different societies. The ground rate 
is greatest in the Mercurialis Society and least in the Spirwa-Ulmaria Society 
The Spirra-Deschampsta Society has a rather higher rate than the Spirra 
Society, but less than the Mercurialis Society. Figures for these societies 
obtained in positions of practically equal shrub-growth, work out in the 
proportion of:— 
Mercurialis Кое. : Spirwa-Deschampsia Soc. : Spirwa Soe. 
/ 7 į 
= 100 : `7 : 84:5 
This difference is due, at any rate partially, to the much less dense and 
deep character of the ground vegetation in the Mercurialis Society, Tn the 
early part of the vear the difference is much less, and owing to the persistent 
tufts that of the роне Deschampsia Society may be least. 
With regard to the absolute rates of evaporation a direct. comparison 
cannot be made with the figures of Yapp* or Livingston f, as none of my 
instruments was free from the shelter of the trees. It was not found practicable 
to raise an instrument above the tree level. However, some idea of this 
protection can be obtained by comparing the evaporation rates of E,, which 
had only the protection of the trees, with Yapp’s instrument, which was 
free from the vegetation. Thus the average rate for 24 hours of E, was: 
during May, 2277 c.c.: June, 22:9 e.c. ; August, 20:4 с.с.; or, in linear 
evaporation: May, 0:110 em. (07043 ins.) ; June, O-111 em. (0:044 ins.) ; August 
0:099 ст. (0'039 ins). Yapp's A *, which was clear of vegetation, during 
August 1908 gave an average of 532 ec. for 24 hours, The relative 
evaporation would be: A: E, = 100: 42-6 : 43°04 : 32-7 for May, June, 
and August respectively, This would show a protective action of the trees of 
60 per cent. of the evaporation, and as the relative humidity is probably 
greater in the Fen than here the actual reduction may be still greater. 
The readings for ground evaporation may be compared with those given by 
Transeau f for forest associations in Michigan. Considerable similarity 
apparently exists. Transeau’s Station 6, а deciduous forest with a decided 
shrub-layer, gave a daily average of 848 c.e., while 7 с.е. was obtained in 
this wood in an open part of the ground flora ; Transeau's © Swamp Forest” 
gave 2:28 e.c. as compared with 2:58 e.c. for the Spirwa-Dese ‘hampsia Society, 
The figures for the Spiræa Society under deep shade, 1:75 с.с., are smaller 
than any given by Transeau for forests. However, as Transeau does not 
* Yapp, 1909, p. 295. T. Livingston, 1906, р. 29, | Transeau, 1908, p. 224, 
