THE INTAKE AND UTILIZATION OF WATER 



25 



but that when the stomata are closed transpiration practically ceases except for . • 

 cuticular water loss. ^ , 



Variabilitii in Number of St07nata in Accordance with External Condi- . 

 iions. — The number of stomata on the leaf surface is not determined .*" 

 in the unopened bud, but varies with the conditions under which the 

 leaf develops. 



Table 14 presents data showing the reduction in the number of stomata -" 

 per square millimeter when the available water supply in the soil is reduced. 

 Though the reduction in the number of stomata per unit of leaf surface 

 is not exactly proportional to the reduction in soil moisture, there is a very 

 material decrease, indicating a marked ability on the part of the plant to adjust 

 itself to its water supply. This flexibility is perhaps one of the reasons why 

 many plants are able to thrive under wide ranges of soil moisture and atmospheric 

 humidity. 



Table 14. — The Influence of Soil Moisture Upon Number of Stomata 



{After Duggar*^) 



However, the number of stomata cannot be modified after the leaf 

 once attains full size. This means that its ability to adapt itself 

 in this way to extremes of soil or atmospheric moisture must be exercised 

 early in the season. Foilage developing in the spring on a plant in a 

 moisture laden soil will probably transpire somewhat more per unit of 

 area later in the season, than it would had it developed under drier 

 conditions, though the increase will probably not be proportional to 

 the increase in number of stomata. However, the extra amount of water 

 required by such plants during the summer is due mainly to the in- 

 creased leaf area. If trees are so handled early in the season as to develop 

 large water requirements the cultivator should recognize the fact that 

 this demand will be more or less continuous through the summer and 

 shape his cultural practices accordingly. 



Factors Influencing Rate of Transpiration. — The rate of transpiration 

 is influenced by a number of factors, such as the character of the 

 cuticle, the age of the leaf, defoliation, wind velocity, light and tempera- 

 ture. 



