HOW TREES GROW 17 



rapidly, forming a complex root system extending through the 

 soil, continually advancing into new spaces between the soil 

 grains. A crown of leaves raised on a stem above the earth's 

 surface is now exposed freely to light and air. Through numer- 

 ous channels in the roots, stem, and branches and leaf veins 

 the water and salts from the soil move upward and supply all 

 living parts. The sugars and other substances formed in the 

 leaves move downward. 



Because trees give ofT water by evaporation at a very rapid 

 rate, the supply must be kept up or the leaves wilt and die. 

 Water is constantly supplied to these leaves, taking the place 

 of the water that is lost by the air and used by the plant. Just 

 what forces operate to secure this rise of water to the tops of 

 the trees have not yet been clearly worked out. The water is in 

 general pulled up from above, and its rise depends fundament- 

 ally on two well known properties of water, cohesion and 

 adhesion. Some trees are different in their microscopic struc- 

 ture and are able to raise water higher than others and it is 

 possible that the height of any tree species is limited by this 

 ability to raise soil water to the leaves. That may be why the 

 dogwood can grow only to a scant twenty feet high, while the 

 sequoia rears its head three hundred feet and more above the 

 ground. 



Drawn up by these still obscure forces, the water rises into 

 the leaves, the tree's laboratories for making sugar, and these 

 newly made sugars move downward from the leaves building 

 up the many growing parts of the plant. From the sugars other 

 necessary foods, such as fats and proteins are made and go to 

 form the tissues and structures of the living tree. By this patient 

 addition year after year of millions of microscopically small 



