352 MR. К. 5. ADAMSON : AN ECOLOGICAL. STUDY 
Tug GRounp FLORA. 
The floras on the two types of soil are хо markedly different that they will 
be considered separately. 
Ground Flora of the Caleareous Clay. 
The ground flora of this part can be divided into three more or less sharply 
defined societies * depending on various factors. These three societies are 
dominated respectively by Spirea Ulmaria, by Spiræa Ulmaria and Deschampsia 
cwspitosa, and by Mercurialis perennis. Subdivisions and intermediate societies 
occur, but are of secondary importance. The main factors determining the 
boundaries of these societies are soil-moisture and light, but, except perhaps 
in the case of. the Mercurialis perennis Society, which is drier at all times, 
neither factor is sufficient by itself to cause the differentiation. 
Light.—lhe amount of light admitted by the shrubs in different stages of 
their growth is extremely varied and has a very marked effect on the 
vegetation. This, however, is probably not all due to the effect of light 
alone ; in all places where the light is reduced shelter occurs, and the amount 
of this 1s roughly proportional to the reduction of the light. This shelter 
auses a reduced rate of evaporation and consequently of transpiration due 
to the stillness of the air and increased relative humidity. This fact, which 
cannot well be separatedi from the light, certainly has considerable effect ; 
but at present it is impossible to say how much of the observed effect is duc 
to either factor singly. 
Estimations of the light t were made by means of a photographie 
actinometer with sensitive paper. "This method seems the most convenient 
for general use, but has the drawback that it measures mainly the actinic 
rays, while those used most by plants are towards the other end of the 
spectrum, and И gives а momentary measure not а light-sum. No other 
method, however, seemed practicable at the time. А standard test for the 
sensitive paper was arbitrarily fixed and the times of reaching this both in 
the open and in different parts of the wood were noted. No records of. the 
absotute light-intensity f $ were made ; the figures given are relative light- 
intensities in terms of the intensity of the light in the open at the time. 
Great differences occur in the amount of light penetrating into the wood, 
both according to the season and more especially with regard to the growth 
of the shrub layer. The light penetrating the trees varies according to the 
species and to their density. Oak and Aspen cut off comparatively little, 
the relative intensity among them being about 0*4 to 0*3 ; while with shady 
trees, such as Acer campestre, the amount is reduced to 0*3 to 0.2. The trees 
* Clements, 1905, p. 296. T Wiesner, 1907 ; also Blackman, 1907, p. 273. 
t Woodhead, 1906, р. 357. $ Clements, 1905, р. 56. 
