OF A CAMBRIDGESHIRE WOODLAND. 351 
[4 
mention whether his figures are corrected, or simply the total evaporimeter 
loss, which would be about 1:5 times as great *, this comparison cannot be 
Ы 
pushed гоо far. Yapp's € (on the ground) averaged 5°85 c.c. per day during 
August, 1908, which is considerably more than the rate in the wood when the 
eround vegetation 1s at all dense. 
The greatest amount of evaporation recorded in the wood for a period of 
24 hours was 32:06 e.c., or in linear evaporation 0156 em, (0°06 in.), given 
by E, on May 10-11, 1910, under conditions suitable to high evaporation : 
sunshine, with a moderate wind and rather low relative humidity, 55-59. 
The greatest reading for 1 hour took place from E, on April 27, 1910, when 
6:20 e.c. (073 em, VOL 1n.) was evaporated between 12 noon and 1 o'elock Р.М. 
This was before the appearance of the leaves, at a temperature of 16? C. and 
relative humidity 51. This relative humidity is extremely low for this part ; 
during the summer readings rarely fall below 55, and average over 60. 
Soil-moisture.—This is the other important factor influencing the distribu- 
tion of the plant societies. 
In general, when the evaporation rate at the surface is low and the humidity 
high, the soil during a period of drought loses more water than with quick 
evaporation, and dries evenly to considerable depths T. On the other hand, 
where the evaporation is very rapid an air-dry layer is formed on the surface, 
corresponding to the “dust mulch”? of agriculturists t $. This dry layer 
prevents loss of water to the same extent in the deeper layers +. This may 
partly explain the reasons for the drop in the soil-moisture curves in the 
summer, when the precipitation Is greater. As mentioned above ||, the increase 
in precipitation is to xome extent counterbalanced by its being more concen- 
trated and by the much lower humidity. 
All the calcareous clay soils are very wet in winter, never falling much below 
40 per cent. The soil of the Mercurialis perennis Society is always drier than 
the others. It falls steadily from a winter average of 44 per cent. till May, 
when it averages 27 per cent. A slight rise occurs in the end of May and the 
beginning of June, after which there isa steady falling to the average minimum, 
in July, of about 24 per cent.: locally it may get much drier, falling to about 
18 per cent. The reason for the lower water-content of this part is mainly 
drainage. This society is developed in the Х.Е, end of the wood, where the 
ground slopes off, and in raised parts elsewhere where ditches have been cut. 
The other two societies show more varied water-contents. The Spirwa 
Ulmaria Society has a marked rise in June to about £3 per cent., and this falls 
off to about 32 per cent., occasionally rising to as much as 50 per cent, in 
* Уарр, 1909, p. 295. T Livingston, 1906, p. 34. 
і Hall, 1908, р. 102. $ Lyon & Fippin, 1909, p. 195. 
|| Cf. under * Rainfall.” 
