THE LEAF 57 
content of the soil. This condition is very evident in many crops 
during long dry seasons. Delicate plants, such as many ferns, 
cannot grow in dry places, because they lose more water by 
transpiration than they can absorb from the soil. The bareness 
of desert regions is due to excessive evaporation and the scarcity 
of water in the soil. If it were not for the high rate of evapora- 
tion, however, there would be enough water in the soil in many 
desert regions to support a fair growth of vegetation at all times. 
Excessive loss of water is one of the greatest dangers to which 
many plants are exposed, and is one of the most frequent causes 
of the death of plants. The harmful effects of excessive tran- 
spiration account for the fact that the leaf is protected by a heavily 
cutinized epidermis and by stomata which close as the result of - 
the loss of water. 
Transpiration and temperature. The evaporation of water has 
the effect of cooling the object from which it evaporates, and so 
the transpiration of water from plants tends to reduce their tem- 
perature. This fact explains why living leaves remain relatively 
cool when exposed to intense sunlight, while dry objects, such 
as paper, become hot. If it were not for this cooling effect of 
evaporation, leaves would be overheated on hot, sunny days. 
Animals, like plants, may be benefited by the cooling effect of 
evaporation. The reason why we feel cool when in front of an 
electric fan is that the wind increases the rate of evaporation 
of water from our bodies. 
One danger of high leaf temperature is illustrated by the 
injury known as scalding. On hot days this injury sometimes 
results from the heating effect of intense sunlight following a 
shower which has left a high percentage of moisture in the air. 
The humidity lowers the rate of transpiration, with the result 
that the leaf becomes overheated. The injurious effects of high 
temperatures are, however, not confined to such conspicuous 
injuries as the one just mentioned. Many physiological processes, 
including growth, are affected by excessive heat. 
Transpiration and mineral matter. When the chlorenchyma 
cells of the leaf lose water through transpiration, the osmotic 
