128 INTAKE AND DISTRIBUTION OF GASES 



free from carbon dioxide the stomata have still been found to 

 open on exposure to the light. 



We have noted the necessity of stomata for the admission 

 of carbon dioxide for food construction, and the fact that in 

 most cases they are not needed to let in oxgyen for respiration. 

 Are they a necessity in the elimination of water or transpiration ? 

 This process of course hastens the flow of water from roots to 

 leaves, and soil solutes are swept along in this current, and 

 the intake of the solutes is consequently hastened. It seems 

 possible that, in tall plants at least, the passage of solutes from 

 the roots to the crown by diffusion alone would be too slow 

 for the needs of nutrition, and the stomata are doubtless very 

 useful in this. But it may also be true that while the stomata 

 are standing open to let in carbon dioxide during the hours of 

 daylight a great deal more water incidentally passes through 

 them than is needed for the ascent of solutes. Indeed, when 

 the water supply in the soil is running low its transpiration 

 through the stomata becomes a real danger to the plant, and 

 the ability to close the stomata when the guard cells lose their 

 turgidity, as they would under such conditions, becomes a safe- 

 guard against harm and even death from drying up. 



Even in bright sunlight the stomata will close when water 

 evaporates through the guard cells faster than it can be taken 

 in. The sensitiveness of the guard cells to variations in the 

 water supply is shown when a plant with open stomata is taken 

 from the moist air of a greenhouse to the dryer air out of doors; 

 then the stomata close in spite of the fact that the plant may be 

 in bright sunlight. Some plants, such as the willows and alders^ 

 are found to have lost the power of closing the stomata, and 

 the lack of this check on the transpiration stream may be the 

 reason for the restriction of these plants to habitats where the 

 roots find a perennial supply of water ■ along streams and in 

 wet soil. 



The stomata as a rule occur on both sides of leaves, but with 

 greater frequency on the lower side. There are instances, as 

 in the rubber leaf, and some oaks, plums and apples, where the 



