378 MR. В. 8. ADAMSON : AN ECOLOGICAL STUDY 
Clay in the wood gives rise to two very distinct soil-ty pes—the one a heavy 
calcareous clay, the other a non-ealeareous loam. 
The vegetation of these two soil-types is quite different. The calcareous 
soil supports an Ash-Oak Wood Association, while the non-caleareous loam 
gives rise to an Oak Wood Association. The Ash-Ouk Wood is the type 
largely developed on the calcareous clays and marls of Cambridgeshire and 
the South of England generally. Тһе ground flora of each Association is 
divided into plant societies, which are controlled mainly by the soil-moisture, 
the intensity of light, and the rate of evaporation. 
The chief societies of the Ash Oak Association are dominated by Spirwa 
Ulmavia, either pure or mixed with Deschampsia evspitosa, and by Mercurialis 
perennis. Each society shows several local variations, depending largely on 
light and changes in soil-moisture. 
“The societies of the Oak Wood Association are dominated by Нови; mollis 
and Pteris aquilina, and by Holcus mollis. 
The main features of the societies are as follows :— 
A. Societies of the Ash Oak Association. 
I. Spirwa Ulmaria Society has a range of light-values of *005—1 and a 
high water-content, the summer average being 36-43 per cent. 
II. Spirea Ulnaria-Deschampsia eespitosa Society has a water-content 
as IL., but light ‘03-6. 
ПТ. Mercurialis perennis Society has a varying quantity of light and a 
summer water-content of 22-26 per cent. 
IV. Fragaria vesca Society is intermediate in conditions between II. and 
TIL., summer water-content about 3 per cent. 
B. Societies of the Oak Wood Association. 
I. Pteris aquilina-IHolcus mollis Society has a summer water-content of 
19-22 per cent. Soil a heavy loam. 
II. Holcus mollis Society has a summer water-content of 17-19 per cent. 
Soil rather more sandy. 
The different Societies of the two Associations pass gradually into one 
another. 
The structure of individual plants is in some cases considerably modified by 
varying conditions of life. As a rule those that are most modified can 
withstand а great range of conditions. 
In conclusion, I desire to express my very sincere thanks to Dr. €. E, Moss 
(who originally suggested this investigation to me) and to Mr. A. С. Tansley 
for their kind help and suggestions during the progress of this study, I lave 
also to thank Professor Seward for obtaining permission for me in the first 
case and for providing the instruments used in this work, 
