26 FUNDAMENTALS OF FRUIT PRODUCTION 



Character of Cuticle. — The cuticular water loss of plants is affected materially 

 by the thickness and character of the cuticle. The presence of a waxy coating 

 diminishes the amount of water loss. The effectiveness of such a coating may be 

 seen from data given by Boussingault^® which show that an apple loses 0.05 

 grams of water per cubic centimeter per hour through its cuticle, but that when 

 the cuticle is removed it loses 0.277 grams or 55 times as much from the same 

 surface. Cork is also an effective protection against water loss. A peeled 

 potato loses water 64 times as rapidly as an unpeeled potato. ^"^ 



Age of Leaf. — The amount of transpiration from a leaf hkewise varies with 

 age. The youngest leaves transpire most for the cuticle is thin and permeable; 

 as the leaf grows older the cuticle thickens and permeabiUty decreases, and with 

 it the rate of transpiration. Later this rate rises to a second maximum, lower 

 than the first, as the result of the development and functioning of the stomata. 

 Then as the leaf ages further there is another decline in the rate of transpiration 

 due possibly to changes in the properties of the epidermis. 



Defoliation. Summer Pru7iing. — Data presented in Table 15 show 

 that the rate of transpiration is also affected by defoliation. This indi- 

 cates that, though midsummer or late summer pruning to protect a 

 plant and its fruit from drought injury will reduce its water requirements 

 somewhat, the reduction will not be directly proportional to the percent- 

 age of the foliage that is removed. 



Tabt-e 15. — The Influence of Defoliation Upon Rate of Transpiration in a 



5-YEAR Old Fir Tree 



(After Hartig ^^) 



Percentage Evaporation per Square Meter of 



OF Foliage Surface (Grams) 



100 270 



60 272 



30 460 



10 607 



Wind Velocity/. Windbreaks. — The agencies thus far mentioned as 

 affecting rate of transpiration have been internal to the plant. Various 

 external factors also have their effects. The most important of these 

 external factors are atmospheric humidity, wind, light and temperature, 

 which together determine the evaporating power of the air. It has been 

 found that the rate of water loss is a simple linear function of the evapo- 

 rating power of the air and that the leaf gives off water as if the water 

 were at a temperature about 1°C. higher than the surrounding air.^" 



Water loss in wind is greater than in still air. This is brought out 

 by the data presented in Table 16. Attention has been called already to 

 the fact that one of the functions of the windbreak is to reduce the water 

 requirement of the plants in its shelter. Probably the decrease in the 

 amount of water required for a given unit of dry matter is not directly 

 proportional to the lessened rate of transpiration consequent upon de- 



