ADJUSTMENT TO WATER 63 



in the vial. Remove the leaves from the vials and premit them to 

 dry out, noting the time required in each case. 



80. The influence of physical factors upon transpiration. 



Three factors, humidity, water content, and Ught, affect trans- 

 piration directly. Their influence is seen not only in their ability 

 to cause the stomata to open and close, but also in determming 

 the rapidity of transpiration when the stomata are open, and, 

 indeed, though in much smaller measure, when the latter are 

 closed. We have already seen that light causes stomata to open 

 in the morning and close at night, thus resulting regularly in less 

 transpiration at night than during the day. In addition the 

 greater part of the light energy absorbed by the chloroplast, 

 usually more than 95%, is converted into heat, and produces 

 water loss or, as it is sometimes called, chlorovaporization. A 

 marked decrease in humidity or in water content, as well as the 

 two acting together, ordinarily causes the stomata to close, 

 while subsequent increase tends to open them. It is usually im- 

 possible to distinguish clearly between these two factors hi con- 

 nection with water loss, since they practically always act together. 

 For example, the stomata of water plants remain open even when 

 the humidity falls to a minimum, and the}' likewise sta}^ open in a 

 saturated atmosphere, even though the water content be low. 

 W^ith reference to the amount or rate of transpiration, however, 

 each factor has its own effect. Increased humidity checks water 

 loss; decreased humidity promotes it. A reduction in water 

 content decreases transpiration, and an increase tends to augment 

 it. Transpiration is at a maximum when the water content is 

 high or excessive and the humidity very low. 



Temperature pressure, altitude, and wind affect transpiration 

 only in so far as they change the humidity or, through the latter, 

 the water content. These relations have already been discussed, 

 and it is only necessary to state again the facts. High air tem- 

 peratures increase water loss; low temperatures decrease it. Soil 

 temperatures below freezing decrease transpiration by rendering 

 water content less available. High altitudes, i.e., low pressures, 

 promote transpiration; low altitudes, i.e., high pressures, reduce 

 it. Dry winds increase water loss; moist winds decrease it. 

 Apart from their humidity, winds increase transpiration l^y re- 

 moving the increasingly humid air above the leaf and by aiding 

 in, the movement of the air in the air-spaces and through the stomal a. 



