SENSITIVENESS. 65 
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 may be 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 Arrangemenis,—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 
