WATER UTILIZATION BY TREES 53 



regions contain this same type of plant and, furthermore, states that 

 the oil, to which is due the aromatic odors of these plants, is com- 

 monly given off more in damp and foggy weather than in dry weather. 

 It is his opinion that these substances are a protection against animals 

 rather than against drying out. 



It is the opinion of the writer, however, that ecologists very com- 

 monly make mistakes when they try to explain all similar phenomena 

 in the same way. It is quite possible that desert plants are aided in 

 their struggle to retain water by the same substances that keep plants 

 in moist conditions from being eaten by animals. If the gas pressure 

 and the insulating effect of the air surrounding the leaf are increased, 

 transpiration will be diminished; but whether this is an "adaptation" 

 is quite a different matter. 



Griffon (73) calls attention to the fact that, contrary to popular 

 opinion, eucalyptus transpires less than other common trees of France, 

 such as poplar, willow, linden, and walnut. Its use in plantings to 

 dry up marshes and swampy regions is effective not because of its 

 high transpiration rate but because it produces much foliage quickly. 



CARBON DIOXIDE 



Of the other various components of the atmosphere which influence 

 transpiration only two are of sufficient importance to warrant inclu- 

 sion in this review: carbon dioxide and sulphur dioxide. The propor- 

 tion of C0 2 in the atmosphere is very small (only 3 parts in 10,000), 

 but at the same time it is very constant; changes in this percentage 

 occur rather rarely and under exceptional conditions. Soil bacteria 

 sometimes increase noticeably the percentage of C0 2 in the soil air 

 spaces and also in the soil water. Likewise, vegetation on a forest 

 floor, as well as the forest vegetation itself, may locally increase the 

 percentage of C0 2 in the atmosphere of forests at night when photo- 

 synthesis is no longer taking place. These increases, however, are 

 seldom large and are consequently probably unimportant as far as 

 transpiration is concerned, since they occur at a time when transpir- 

 ation is already decidedly checked. 



In the small amount of experimental work that has been done on 

 this subject wuth woody plants, it has been noted that transpiration 

 in air free of C0 2 is always greater than that in air containing C0 2 , 

 and when the C0 2 content is relatively high transpiration is decidedly 

 depressed. The common explanation for this is that the stomata tend 

 to close in an atmosphere containing excessive amounts of C0 2 . This 

 is true, but it is also well known that the main function of stomata 

 is to permit C0 2 to enter the leaf so that it may be used in photo- 

 synthesis. If C0 2 caused the stomata to close, the leaf would not 

 function. The truth of the situation seems to be that as the C0 2 is 

 manufactured into food in the guard cells of the leaf their turgor 

 increases and the stomatal openings enlarge, permitting more C0 2 to 

 enter, especially in light. It is also true, however, that in pure C0 2 

 or in air extremely rich in this compound the stomata close, so that 

 the decrease in transpiration in the presence of excessive amounts of 

 CO 2 may be explained by the smaller stomatal openings. And still 

 another reason for the decrease in transpiration in atmospheres high in 

 CO 2 , in the opinion of the writer, may be sought in the fact that 

 the C0 2 is combined with the water in the cells to form organic foods . 



