354 



EXPEMIUESTS WITH PLANTS 



plants with a mulch, or, in the case of trees and 

 shrubs, by bending them down (the roots must be cut 

 in such a way as to permit this) on the approach of 

 winter and covering them with straw or brush i. 



Spraying or sprinkling with water at nightfall is a 

 very effective means of protection ; this depends on 

 the fact that moisture in the air prevents the radiation 

 of heat and consequent cooling of the trees and soil ; 

 it acts, so to speak, as a trap for the heat. Filling the 



air with dense smoke 

 from bonfires of tar, resin 

 or similar substances 

 answers the same pur- 

 pose. 



The pale appearance 

 of young leaves during 

 cold weather, which is 

 very noticeable in Winter 

 Wheat, etc. (see Fig. 204 

 of the Ivy Geranium) , is 

 due to the fact that for 

 the formation of chloro- 

 phyll a higher tempera- 

 ture is required than for 

 growth. The leaves con- 

 sequently develop but cannot turn green. 



It is noticeable that some kinds of plants stand far 



1 See an article by Galloway in the Year Book of the U. S. Department of 

 Agriculture for 1895, 



204. Branch of Ivy Geranium after a few 

 days of cold weather ; the young leaves 

 have developed without being able to 

 turn green. 



