yS General Botany 



justed to dry conditions is by a decrease in the total leaf area. 

 When a plant is brought into the house in autumn, some of its 

 leaves usually fall off. The air inside houses being much drier 

 than the air outside, transpiration is greatly increased. As the 

 water supply remains about the same, the loss of a few leaves 

 restores the water balance of the plant. Some trees, hke the 

 Cottonwood, lose part of their leaves during a summer drought. 

 If a wet period follows, more leaves may be added, and in this 

 way a nearly uniform water balance is maintained. 



(4) Hairs. Hairs are frequently described as structures that 

 greatly reduce transpiration. Some of the very broad scale- 

 hairs may reduce the rate slightly but experiments show that the 

 hairy covering of the common mullein, which is exceedingly 

 dense, has little or no effect on its rate of transpiration either in 

 still air or in wind. 



(5) Resin, wax, and mucilage. Some plants produce resin, 

 wax, or mucilage, which retard transpiration. For example, the 

 horsechestnut has a coating of resin on its buds. The bayberry 

 has wax covering its fruits. The tissues of cactus contain muci- 

 laginous substances that have a great water-holding capacity. 



External factors that modify transpiration. That plants grow- 

 ing under moist conditions have larger leaves and more leaves 

 than the same kinds of plants growing under dry conditions has 

 been noted by every one. Experiments show that their rates of 

 transpiration are far greater than when grown under dry condi- 

 tions. Humidity of the air directly affects transpiration, because 

 when the air is nearly saturated the difference between the 

 humidity inside the leaf and outside is so small that water vapor 

 passes out through the stomata into the air very slowly. 



The amount of available water in the soil and the humidity of 

 the air are important because they determine the amount of water 

 in the cells of the plant. The condition of the mesophyll cells 

 in turn regulates the rate of water loss to the intercellular spaces. 

 A dearth of water in the plant may also prevent the opening of the 



