114 STUDIES OF TREES 



sufficiently aerated and well supplied with water, will 

 support almost any tree. Too much of any one con- 

 stituent will make a soil unfit for the production of 

 trees. If too much clay is present the soil becomes 

 " stiff." If too much vegetable matter is present, the 

 soil becomes " sour." The physical character of the soil 

 is also important. By physical character is meant the 

 porosity which results from breaking up the soil. This 

 is accomplished by ploughing or cultivation. In nature, 

 worms help to do this for the soil, but on streets an 

 occasional digging up of the soil about the base of the tree 

 is essential. 



Humus or the organic matter in the soil is composed of 

 litter, leaves and animal ingredients that have decayed 

 under the influence of bacteria. The more vegetable 

 matter in the humus, the darker the soil; and therefore a 

 good soil such as one finds on the upper surface of a well- 

 tilled farm has quite a dark color. When, however, a soil 

 contains an unusual quantity of humus, it is known as 

 " muck," and when there is still more humus present we 

 find peat. Neither of these two soils is suitable for proper 

 tree growth. 



(3) Influence of light: Light is required by the leaves 

 in the process of assimilation. Cutting off some of the light 

 from a tree affects its form. This is why trees grown in 

 the open have wide-spreading crowns with branches starting 

 near the ground as in Fig. 90, while the same species 

 growing in the forest produces tall, lanky trees, free from 

 branches to but a few feet from the top as in Fig. 91. Some 

 trees can endure more shade than others, but all will grow 

 in full light. This explains why trees like the beech, hem- 

 lock, sugar maple, spruce, holly and dogwood can grow 

 in the shade, while the poplar, birch and willow require 



