PLANT ASSOCIATIONS: ECOLOGICAL FACTORS. 167 



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 cover? 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. 



124. 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 association, for example, the tall trees 

 represent 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 association it is important to note the num- 

 ber 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. 



125. 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 fac- 

 tor is especially conspicuous in regions where there are pre- 

 vailing winds, such as near the seacoast, around the great 

 lakes, and on the prairies and plains. In all such regions 



