TREE GROWTH. 19 



nished 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 condi- 

 tions the year round they have periods of rest and of excite- 

 ment. 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 I,ost 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 evaporation from 

 6 to 7 pounds of water when dormant. An apple tree 30 years 

 old and 15 inches in diameter at the base, exposing from 800 

 to 1,000 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 conditions. 



We know that in this section after a prolonged period of 

 severely cold weather, the twigs of Soft Maple, Apple and some 



