SENSITIVENESS. 05 



paratively rarely that the plant is killed outright, farmers 

 wisely choosing those varieties which experience has 

 shown to be the hardiest. If the cold is sufficient to kill 

 the leaves or any portion of them, the leaves become limp 

 and blackened. The limpness is easily accounted for by 

 the causes we have mentioned,, as well as by the stoppage 

 of supplies of water from the root. The discoloration is 

 the effect of some molecular change in the chlorophyll at 

 present not understood. 



Action of Excessive Heat. Too high a temperature 

 also arrests or perverts all the functions of the leaf. 

 Where transpiration is excessive, and the absorption of 

 fresh supplies not in proportion, the leaves speedily 

 wither, as maybe seen in a field of mangels on a hot day, 

 when the evaporation of watery vapor from the surface 

 is greater than the absorption of moisture by the root. 

 On the other hand, during the night, while the roots are 

 still at work, the transpiring power of the leaf is lessened, 

 and drops of water exude from the leaves. Where the 

 temperature is so high as to kill the plant or leaf out- 

 right, it is the protoplasm which dies ; its constitution 

 and molecular construction become changed, its power 

 of absorbing water destroyed, and thus the turgid condi- 

 tion of the cells is lost. 



Defensive Arrangements. Prejudicial effects, either 

 of a too low or a too high temperature, are moderated 

 by the conformation of the leaf, the thickness of its skin, 

 the arrangement of its tissues, the presence of hairs, and 

 other structural endowments. These circumstances ren- 

 der the selection of the particular variety most suitable 

 for any special locality a matter of the greatest moment. 

 In the case of wheat, for instance, some varieties are 

 much more tender than others. Bearded wheats are as 

 a rule hardier than the beardless ones. A variety known 



