TREE GROWTH. 19 
nished by the leaves. The growth at this time is gencrally 
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 Lost by Trees in Winter. Aiter 
many careful experiments, A. L. Knisely, M. 5., 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 
