174 PLANT STUDIES 
overlies a clay soil, the humus will differ very much in the 
two cases in reference to water. 
The soil cover should also be considered. The common 
soil covers are snow, fallen leaves, and living plants. It 
will be noticed that all these covers tend to diminish the 
loss of heat from the soil, as well as the access of heat to 
the soil. In other words, a good soil cover will very much 
diminish the extremes of temperature. All this tends to 
increase the retention of water. 
127. Light, It is known that light is essential for the 
peculiar work of green plants. However, all green plants 
cannot have an equal amount of light, and some have 
learned to live with a less amount than others. While 
no sharp line can be drawn between green plants which 
use intense light, and those which use less intense light, 
we still recognize in a general way what are called light 
plants and shade plants. We know that certain plants 
are chiefly found in situations where they can be exposed 
freely to light, and that other plants, as a rule, are found 
in shady situations. 
Starting with this idea, we find that plants grow in 
strata. In a forest society, for example, the tall trees rep- 
resent the highest stratum ; below this there may be a 
stratum of shrubs, then tall herbs, then low herbs, then 
forms like mosses and lichens growing close to the ground. 
In any plant society it is important to note the number of 
these strata. It may be that the highest stratum shades 
so densely that many of the other strata are not represented 
at all. An illustration of this can be obtained from a 
dense beech forest. 
128. Wind. It is generally known that wind has a dry- 
ing effect, and, therefore, it increases the transpiration of 
plants and tends to impoverish them in water. This factor 
is especially conspicuous in regions where there are pre- 
vailing winds, such as near the sea-coast, around the great 
lakes, and on the prairies and plains. In all such regions 
