354 



EXPERIMENTS 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 1 . 



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. 





