NATURAL HISTORY OF THE FOREST. 147 



While light is usually accompanied by heat and 

 it is difficult to discern how much of the effect of 

 it on plant growth is to be ascribed to the heat 

 which causes transpiration, and how much to the 

 light as such, yet it is now well known that light 

 itself exercises various influences upon vegeta- 

 tion, some of which are still imperfectly or not at 

 all understood. It is light which is indispensable 

 in the formation of chlorophyll — the material 

 which imparts the green color to plants ; it is 

 light, a certain degree of Hght, upon which the 

 assimilation of carbonic acid in the chlorophyll 

 and the formation of starch are dependent; it is 

 light, together with other factors, which influences 

 transpiration by the foHage, which determines the 

 development of the crown and of the whole tree 

 in direction and quantity of growth. 



It has been observed that various plants show 

 need of a greater or smaller amount of light for 

 their development. Some plants always seek the 

 shady places in the woods ; others enjoy the full 

 sunshine of the meadow. The dense spruce forest 

 permits only a moss-cover on the soil, while the 

 open-foliaged oak forest permits a host of shrubs 

 and herbs to subsist. Just so, some trees are found 

 thriving under the shade of others, while these are 

 intolerant of the shade of their neighbors, or can 

 endure it only a short time. So all important and 

 so well known is the influence of light on the de- 

 velopment of a forest crop that on the difference of 



