466 Introduction to the Study of Science 



leaves of tropical and subtropical plants with those of plants native 

 to the temperate and colder regions. Do large leaves transpire more 

 than small leaves? Are small leaves better adapted to a hot, dry 

 climate than large leaves? In what ways does the size of the leaf 

 affect transpiration? 



Compare leaves of typical common plants and seek to ascertain 

 what relation, if any, there may be between the size of the leaves and 

 the external conditions. Examine pine, spruce, tamarack, hemlock, 

 maple, willow, locust, oak, poplar, eucalyptus, rubber plant, grasses, 

 and weeds. 



In studying the relation of size of leaves to external conditions, 

 consider also the relation of size and arrangement of leaves on twigs 

 and branches to sunlight. Examine from different standpoints the 

 exposure of leaves of a tree to light, as looking upward into the tree, 

 and conversely looking upon a tree from some higher position. Note 

 the comparatively small amount of sunlight that escapes through the 

 tree to the ground. Examine the size and arrangement of leaves on 

 such plants as the hickory, maple, horse-chestnut, willow, and oak ; 

 and then on vines such as the grape, ivy, and melon or cucumber. 

 The significance of this will appear more fully later; at present it 

 may be considered incidentally while dealing with other features. 



224. Behavior of water in the earth. After it has fallen 

 upon the soil, water tends to find its way to the lowest permeable 

 layers of the earth's crust. Many soils are superposed upon 

 what is called " hard pan," an almost impenetrable kind of 

 material. Water sinking to this level rests or flows slowly in 

 the direction of least resistance, or downhill. All water which 

 flows away in this manner and is in excess of what the soil can 

 absorb and retain, is called free water. 



Soil water, however, behaves in a very different way. It 

 does not move actively in the soil, but, film-like, temporarily 

 envelops the. soil particles. As the evaporation at the surface 

 increases, the soil water moves upward slowly to the surface. 

 This upward movement of soil water is important for plants, 

 and its control is often of the greatest advantage to the agri- 

 culturist. 



Capillary action. This peculiar behavior of water in the 

 soil may be illustrated by many facts of common observation. 



