TRANSPIRATION 175 



into direct contact with the 'atmosp'lieric gases which pass by 

 diffusion through the stomata.. The mesophyll cells have thin 

 walls, which are not cutinized and which permit the free passage 

 of both liquids and gas^. 



Stomatal Production and Movement. — The number of sto- 

 mata is not constant in plants of the same species but varies with 

 the moisture content of the air and soil, the light, teomperature 

 and other factors. The guard cells undergo certain movements 

 which result in the opening and closing of the opening. These 

 movements of the guard cells are due to turgor and axe not 

 directly related to the moisture condition of the air. They are 

 usually open when the plant is receiving a sufficient amount of 

 moisture and are usually closed when the moisture is insufficient, 

 biit they do not open and close in anticipation of conditions 

 favorable or unfavorable to the plant. They are also influenced 

 to some extent by the intensity of the light. 



Conditions Influencing Transpiration. — Humidity, temper- 

 ature, light, wind velocity and leaf modifications are the most 

 important factors influencing transpiration. 



(a.) Transpiration will tate place unless the humidity of the 

 air is equal to or gTeater than the humidity of the air in the 

 intercellular spaces of the plant. In general, it may be said that 

 the transpiration increases in piroportion to the dryness of the 

 surrounding atmosphere. 



(6) Transpiration usually increases with the rise of 

 the temperature of the air, provided other conditions remain 

 the same. 



(c) Transpiration usually increases vsdth the increase in the 

 intensity of light. This is due to the fact that a part of the 

 energy of sunlightr absorbed by the leaf is transformed to heat. 



