THE TEEE. 11 



the summer when the growth was slow. It is termed 

 summer wood. (See page 161.) 



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. 



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 

 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 the winter evapo- 

 ration is much more rapid in Minnesota and the Dakotas 

 than it is in New York, which will partly, at least, account 

 for so much loss in those sections by winter killing. 



We know that after a prolonged period of severely cold 

 weather the twigs of Soft Maple, Apple, and some other 

 trees, have a decidedly shrivelled appearance which disap- 

 pears after a few days of mild weather. Soft-Maple trees 

 standing on dry land will sometimes in the spring appear to 

 have been dried out and to have become partly or entirely 

 dead. It is probable that during the coldest weather very 



