SENSITIVENESS. C5 



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 sup- 

 plies not in proportion, the leaves speedily wither, as may 

 be seen in a field of mangels on a hot day, when the 

 evaporation of watery vapour 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 tempera- 

 ture is so high as to kill the plant or leaf outright, it is the 

 protoplasm which dies ; its constitution and molecular con- 

 struction become changed, its power of absorbing water 

 destroyed, and thus the turgid condition 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 render 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 



F 



