CHAP, iv.] The Food of Trees 69 



warmth in spring, the energy of the vital process is enhanced 

 as the temperature rises, until it may reach a point at which 

 the plant is unable to carry out the vital operations in a normal 

 manner. In Britain, hot southern slopes with light sandy soil 

 may often prove too warm for woodland trees like Spruce and 

 Silver Fir, whilst cold land with a northern exposure may not 

 supply the amount of warmth requisite for the best develop- 

 ment of Oak. In forestry, no measures can be taken to regulate 

 the atmospheric temperature. But by maintaining a close 

 canopy of foliage, or by the underplanting of standards, much 

 can be done to regulate the soil-temperature, and to retard the 

 commencement of active vegetation. This may even be delayed 

 for about a fortnight or more, and may thus be made to com- 

 mence only at a time when, the atmospheric temperature and 

 the intensity of the light being greater, the process of assimila- 

 tion may be more thorough as well as more energetic. 



Light, which is closely allied to heat, is essential in order that, 

 with the aid of the chlorophyll contained in the leaves, the 

 decomposition of water and of carbonic acid may take place in 

 the elaboration of substances to be used for structural purposes ; 

 whilst at the same time various other processes, such as 

 transpiration, the deposition of matter on the cell-walls, and, 

 in fact, the whole nutrition, are all likewise more or less 

 dependent on the action of light. The intensity of light 

 necessary for the different species of trees, before they can carry 

 out the process of assimilation, varies considerably, as may be 

 seen from the different densities of their foliage ; whilst the 

 amount and intensity of light required by one and the same 

 species of tree also varies according to the other factors of the 

 soil and situation. Thus, for example, under unfavourable 

 circumstances of soil the foliage is less dense, and the leaves 

 are in every way smaller, than if the same species of tree be 

 growing on more favourable localities. Leaves growing in full 

 exposure to light are thicker, contain more chlorophyll in their 

 parenchym, and have a much larger number of stomata than 



