GROWTH AND MOVEMENT 483 



Unwise pruning <>f 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 bitched t<> the trees, frequently open the 



way for infection by some deadly parasite. Ice storms, bail, wind-, and 

 lightning all contribute to serious mechanical injuries at times, whose 

 direct effects are less to be feared than the indiret t. 



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, espei ially 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, t ven 

 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 (— 25o°(\),' 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 an' 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 of the absolute zero, — 273 ) if enough is ever obtained for such a test. 



