174 J'LAXT STIUIKS 



overlies a clay soil, the humus will differ very much iu the 

 two cases in reference to water. 



The soil cover should also be considered. ^Plie common 

 soil covers are snow, fallen leaves, and living })lants. It 

 will he 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 temperatui'e. All this tends to 

 increase the retention of water. 



V17. 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 S(_)me have 

 learned to live with a less amount than others. While 

 no shai'p line can lie drawn Ijetwecn green i)lants which 

 use inleiise light, and those whii/h use less intense light, 

 Ave still recognize in a general way what a.re called liglif 

 phtiils arid sliude ji;/rn(/,s. AVc know that certain plants 

 are chielly found iu situations where they ca-n lie exposed 

 freely to light, and that other plants, as a rule, are found 

 in shadv situations. 



Starting with this idea, we tiiul that plants grow in 

 strata. In a forest six'iety. for example, the tall trees rep- 

 resent the highest sti'atum ; below this there may be a 

 sti'atum of shrul)s, then tall hei'lis, then low herbs, then 

 f(n'ms like mosses and lichens growing close to the ground. 

 In auy plant society it is important tn note the number of 

 tlicsc strata. It may lie that the highest stratum shades 

 so densely that many of the other strata are not represented 

 at. all. An illustration of this ca.n be olitained from a 

 dense lieech forest. 



128. Wind. — It is generally known that wind has a dry- 

 ing elt'ect, and, therefore, it increases the transpiration of 

 plants and tends to impoverish tiiem in water. 'I'liis factor 

 is especially conspicuous in regions wdtere thei'e are pre- 

 vailing winds, such as near the sea-coast, arouiul tjie great 

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



