GROWTH AND MOVEMENT 483 
Unwise pruning of trees in our cities, much more the heedless hacking 
at the hands of linemen stringing telegraph and telephone wires, and the 
gnawing by horses carelessly hitched to the trees, frequently open the 
way for infection by some deadly parasite. Ice storms, hail, winds, and 
lightning all contribute to serious mechanical injuries at times, whose 
direct effects are less to be feared than the indirect. 
Heat and cold. High temperature is a fruitful cause of local death, 
for this is often associated with a deficiency in the water supply. There 
has been recognized a falling of the leaves, especially of trees, in mid- 
summer, which is due to the heat, and may amount to a large per cent of 
the total foliage. The older leaves, and those least favorably situated 
for receiving sufficient water (the latter are at the same time most ex- 
posed to the direct rays of the sun) are the ones that suffer most. Low 
temperatures kill tender plants by direct injury to the protoplasts, even 
before the freezing point is reached. Others are killed only by the freez- 
ing itself, probably because this withdraws water from the protoplast 
and vacuoles, thus concentrating the solutions, perhaps to a point where 
certain solutes may become poisonous. There are many plants, how- 
ever, which are able to withstand freezing, and on gradual thawing 
the water is taken back into the protoplast again. All the trees and 
shrubs and the persistent parts of herbaceous perennials are liable to be 
solidly frozen, often more than once, in the winters of the northern 
states and Canada, but they usually bear this unharmed, though the trees 
then have almost a maximum water content. The most serious danger 
in the northern winters, especially to the evergreens, is that during a 
warm period the evaporation will surpass the income from the shaded and 
frozen soil. 
Temperature and water. In general the proportion of water present 
determines the resistance to injury by low and high temperatures, other 
things being equal. Thus air-dry seeds withstand the lowest tempera- 
ture yet tried, that of liquid hydrogen ( 250 C.), 1 and germinate freely 
when planted ; while the same seeds, if soaked in water until swollen, will 
be killed at a very much higher temperature. In like manner tempera- 
tures short of absolute charring are borne by dry seeds, while a few min- 
utes' exposure at 70 C. will kill soaked ones. Similarly, plants of firm 
texture and little sap withstand unfavorable temperatures better than 
watery ones. 
1 Doubtless they will endure the temperature of liquid helium (probably within five 
or six degrees cf the absolute zero, 273) if enough is ever obtained for such a test. 
