TREES PRODUCE GREAT NUMBERS OF SEEDS 137 



Tree Growths. We have already learned something of 

 growth (Chapter X) and know the part played by the cam- 

 bium. When we think of the growing tree, we must think of 

 the thin cambium in not only the trunk but in every branch and 

 twig, and in every root and rootlet. (Chapter VIII.) When an 

 annular ring is once formed, its position in the tree is never 

 changed, but new annular rings will be formed, each outside 

 of the preceding one. In most trees the inner wood becomes 

 darker and harder with age and is known as the heart-wood, 

 while the outer which is softer and lighter in color is known 

 as the sap-wood. The passage of water through the heart-wood 

 is checked, but it continues to pass through the sap-wood during 

 the life of the tree. As the tree grows, the amount of heart- 

 wood increases by continual additions from the cambium. Tis- 

 sues of different woods have peculiar characters by which 

 students can readily recognize them. 



Conditions of Growth. Different species of trees require 

 different conditions for their growth. Some grow in cold 

 climates, others in warm climates ; some in dry soils, and others 

 in swamps ; some require certain kinds of soil and light exposure. 

 Natural forests are seldom of a single species but of several 

 species which live and grow under the same or similar condi- 

 tions. Under natural conditions there is a continual struggle 

 between the trees for food, water, and light, and many trees 

 die because other trees near them are more vigorous. 



Trees produce great numbers of seeds, but very few of these 

 seeds produce trees. Many of them fall in places where the 

 soil, water or light requirements are unsuited to their existence, 

 or are destroyed by man or some of the lower animals. How- 

 ever, some of them grow where they fall and some of them are 

 carried for long distances, grow and may be the beginning of a 

 new forest area. Some young trees are more vigorous than 

 others of the same kind, grow faster, overtop them and shut 



