392 APPLIED SCIENCE 



conduct water from the roots to the crown, some store away 

 digested food, while others merely strengthen the structure 

 of the wood and hold it together. 



Through these tubular cells, the light and heat of the sun 

 draw up water, which, when converted into sap, keeps the 

 tree alive and enables it to grow. The process by which 

 water is absorbed by a tree and carried to the ends of its 

 leaves is called osmosis a complicated process by which 

 water and certain substances pass through the membranes 

 of the cells. The carbon from the carbon dioxide of the 

 air aids in the formation of new cells, and the water is given 

 off in the form of moisture. Medullary rays aid in carrying 

 the sap from the bark to the wood and vice versa. These 

 rays are narrow strips of cells, sometimes scarcely visible 

 to the naked eye, running from the center to the bark of 

 the tree. 



The cells continuously divide themselves and it is by this 

 process, made possible by the means just described, that the 

 actual growth of a tree takes place. 



439. Annual Rings. The cells form and the tree grows 

 most rapidly in the springtime/fecause there is at that time, 

 an ample supply of water to meet the demands of the tree. 

 New cells form also during the summer, but as the supply 

 of water diminishes in hot weather, the cells formed at this 

 time are smaller and less numerous than those of the spring. 

 During the winter, growth practically ceases. Since a new 

 ring of cells forms every year and the spring cells are larger 

 than the summer cells, each set of cells appears in a cross- 

 section view as a distinct circle or ring. These circles are 

 known as annual rings and by counting them the age of the 

 tree may be ascertained, 



