OF A CAMBRIDGESHIRE WOODLAND, 358 
in the wood, however, never form a close canopy but are more ог less widely 
separated. The areas between appear to the eye to have the full intensity 
of light, but measurements revealed the fact that the intensity was only 
about 0*8 to 0-75, owing to the cutting off of some of the diffuse light. And in 
parts where there are no shrubs and no eover from trees the effect is still 
more noticed, the intensity just above the ground flora being only about 06. 
The shrubs being сиё periodically have а varied effect in cutting off light 
from the ground flora. They take about 6 to 8 vears to form a close canopy, 
and during this period a certain amount of direct sunlight can penetrate to 
the ground flora, though the intermediate parts have much reduced light- 
intensity, about 0*06. At about 9 to 11 years the average light-intensity 
above the ground flora is about 0°07 to 0*03, while when the shrubs are full- 
grown little or no direct sunlight сап penetrate and the diffuse light is very 
much cut ой; the intensity here is as little as ("01 and may descend still 
lower. All intermediate intensities occur in different parts. 
These figures are all based on estimations made during the summer, when 
the full foliage is developed. Earlier in the year in the prevernal and vernal 
aspect * the relative amount of light penetrating is much greater. 
The variations of the ground flora with the intensities of the light will be 
discussed later, but some special cases may be mentioned here. In general, 
when the light-intensity is very much reduced, the plants do not flower ; or 
if they do, in much less quantity and later in the season. For example, 
Spirea Ulmaria in sub-open conditions commences flowering about the end 
of June; but under deep shade it produces much fewer flowers, which do 
not open till the latter half of July. Some species are very intolerant of 
reduced light, notably the larger Graminz. Two other plants occurring in 
the wood may be mentioned here as rather extreme cases of intolerance—— 
Hyperium hirsutum and Campanula Trachelium. Both are abundant in light 
parts, but the former does not flourish when the light is less than 0*2 and hardly 
flowers at all with less than 0°3. Campanula Trachelium will stand greater 
shading, but it never occurs in less than 0°08. 
Evaporation—As mentioned above, the effects of this factor are closely 
united with those of light ; the factors producing a reduction of the one causes 
at the same time a reduction of the other. 
Experiments have been carried out to measure the rate of evaporation in 
different parts of the wood. The instruments used for this purpose were of 
the same pattern as those employed by Professor Yapp in his experiments on 
Wicken Fen +. I desire to take this opportunity of expressing my thanks to 
him for his kindness and trouble in getting the evaporimeters made for me, 
and also for his valuable advice as to methods of experimentation. 
* Clements, 1905, p. 315. 1 Yapp, 1909, р. 285. 
