TREE GROWTH. 15 



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. 

 Our 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 development. 

 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 ex- 

 citement. During the rest period the plants undergo very few 

 changes and yet there is undoubtedly 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. 



The Amount of Water Lost by Trees in Winter. After many 

 careful experiments A. L. Knisely, M. S., concludes that 

 a Soft Maple standing 30 or 35 feet high with a trunk 

 15 to 18 inches in diameter near the ground, exposing from 

 750 to 800 square feet of bark surface, may lose daily by evap- 

 oration from 6 to 7 pounds of water when dormant. An apple 

 tree 30 years old and 15 inches in diameter at the base, expos- 

 ing from 800 to 1000 square feet of bark surface may lose 

 daily while dormant from 10 to 13 pounds of water. These 

 figures are from results obtained during winter weather in 

 New York where the relative humidity of the air is higher than 

 in Minnesota which would lessen evaporation. It is probable 

 that during our winters here the evaporation from trees will 

 greatly exceed that in New York and that greater evaporation 

 is nearly always responsible for some trees being tender here 

 and hardy in New York and other places with similar con- 

 ditions. 



