Temperature and Light Intensity 
IN RELATION TO TRANSPIRATION. Excessive light intensity 
raises the temperature of the plants thus increasing the loss of 
water from the leaves. If the rate of water loss is faster than the 
rate of water intake through the roots, the plant will wilt. We see 
here the danger of drying plants between waterings especially 
under conditions of high light intensity. 
When the sun isshining on the leaf, the leaf temperature is raised 
above that of the surrounding air and plants lose water to the 
atmosphere even when it is saturated with moisture (100% relative 
humidity). This is a proven and accepted fact. Obviously, the 
best way to cut the excessive loss of water from the plantis to 
coolthe leaves and the house so that there would not be much 
difference between their temperatures. One way of doing thisis 
to increase humidity by spraying water over the plants, the walks 
and benches. It has been found, however, that increasing the 
humidity in this manner cools the house for less than 30 minutes; 
The effect is so temporary since the water on the leaf surfaces 
soon evaporates and the atmosphere itself gets dry again, 
especially when there is good air circulation. Furthermore, ‘so 
long as the leaf receives light, its temperature will always be 
higher than that of the surrounding air and hence, the plant will 
continue to lose water regardless of humidity. 
Shading prevents the sun's rays from striking the leaf andcools 
the house, reducing the difference between the leaf and the house 
temperatures throughout the day. Shading, therefore, is amore 
effective way of reducing water loss than increasing. humidity. 
Spraying water is not only ineffective but a good way of spreading 
diseases. High humidity and high temperature together createa 
condition that is ideal for the growth of many destructive fungi 
and bacteria. 
IN RELATION TO GROWTH. Within certain limits and providing 
other factors as water, light, etc., are sufficient, a “Pisemm 
temperature usually increases food production, respiration and 
rate of growth. 
The energy necessary for ‘growth is given off in respiration, a 
process which uses the food manufactured by the plant. If little 
food is available, it is used up in a short time and growth ceases. 
This situation is aggravated by low light and high temperature. 
A plant may not grow at 40° F. because of the very low rate of 
