10 PRINCIPLES OF AMERICAN FORESTRY. 



cient quantity, even if a portion of the leaf mould is carried 

 away. If, however, this removal is continued annually 

 for a long period, any but exceedingly fertile soils are 

 likely to become exhausted, just as land on which field 

 crops are grown cannot produce crops forever without 

 manuring. 



The Yearly Round of Life in a Tree. In the spring the 

 tree starts into growth and feeds on the plant-food stored 

 up the preceding year; the leaves unfold and commence 

 furnishing plant-food. These two sources of food push the 

 growth along very rapidly in the spring and early summer. 

 By the first of July the food stored up the previous season is 

 exhausted in many trees, and growth is entirely dependent 

 upon the food furnished by the leaves. The growth at this 

 time is generally much slower than in the spring, and as 

 the capacity of the tree for building up plant-food increases, 

 it commences to store up starch, sugar, and other foods in its 

 cells with which to start growth the following spring, and 

 the cell- walls become thicker and firmer. This maturing 

 of the tree is termed the ripening of the wood, and when 

 completed the tree is ready for winter. The hardiest 

 trees generally ripen their wood early in the autumn 

 and then cease growing, although probably some food is 

 being stored up so long as the leaves remain green on the 

 trees. 



Rest Period of Plants. With very few exceptions all 

 plants require an occasional rest period for their best devel- 

 opment. Some species get it naturally by being dried and 

 others by being frozen. And even when plants are kept 

 under growing conditions the year round they have periods 

 of rest and of excitement. During the rest period the 

 plants undergo very few changes, and yet there is undoubt- 

 edly some growth during mild weather in winter, and, as 

 evaporation must be going on most of the time from twigs 

 and buds, water must be supplied from the roots. 



