54 MISC. PUBLICATION 257, U. S. DEPT. OF AGRICULTURE 



The more C0 2 is present, the more water molecules are combined to 

 form foods, and the less is the amount of water available for transpi- 

 ration. At the same time the osmotic pressure of the cell sap is 

 increasing. 



It has been noted by KosarofT (ISO) that C0 2 also depresses tran- 

 spiration when present in large quantities around the roots. He grew 

 plants in flasks with enough water present for growth and conducted 

 C0 2 through the flasks (containing the roots), measuring the transpi- 

 ration by weighing. When pure C0 2 was led through the flasks, tran- 

 spiration was lowered about half. When a gas mixture of 80 to 90 

 percent of C0 2 and 20 to 10 percent of oxygen was put into the flasks, 

 the decrease in transpiration was not so pronounced, leading Kosaroff 

 to the conclusion that C0 2 had both a direct and indirect effect on 

 the water relations, the former being due to the specific effect of C0 2 

 and the latter to the deprivation of oxygen. The depressing action 

 of C0 2 was shown not only in decreased absorption but also in de- 

 creased transpiration. The dissolved C0 2 depressed not only the 

 activity of the root cells but, as it penetrated into the plant, affected 

 also the stomata. As has been stated above, however, since the 

 amount of C0 2 present either in the soil or in the air does not change 

 very much in nature, the effect of C0 2 is probably not of great im- 

 portance from the point of view of silviculture. 



SULPHUR DIOXIDE 



Sulphur dioxide is important in connection with silviculture chiefly 

 through the addition of this gas to the air in the vicinity of smelters, 

 certain types of factories, etc. The question of smelter-smoke damage 

 is one appearing frequently in the literature, and there is now little 

 doubt of the harmfulness of this compound on vegetation. Schroder 

 (183) studied especially the effect of S0 2 on the water relations of' 

 plants, which he found lost the ability to transpire normally when in 

 an atmosphere of 0.02- to 0.10-volume percent S0 2 . In some cases 

 cut twigs and in others rooted plants of Acer platanoides , Castanea 

 vesca, Fagus sylvatica, Quercus, and Abies pectinata were studied. 

 As a result of the decreased transpiration, less water is absorbed and 

 the general nutrition of the plant is thus thrown out of balance. 

 He found also that the effect of S0 2 on transpiration varied fairly 

 directly with the amount of S0 2 present. In the presence of light, 

 high air temperature, and low relative humidity more S0 2 was taken 

 in by the plants with more serious after effects than in the dark, at 

 low temperatures, and in humid air. He also noticed that the tran- 

 spiration of conifers is not so visibly depressed as is that of broad- 

 leaved trees in air containing equal amounts of S0 2 , which he thought 

 entered the leaf not only through the stomata but also through the 

 cuticle. 



In other experiments, Schroder and Reuss (184) studied the effect 

 of relatively high concentrations of S0 2 . Three similar twigs of Acer 

 platanoides, A, B, and C, were treated in the following fashion: A was 

 put into an atmosphere containing no S0 2 ; B in an atmosphere con- 

 taining 1 part S0 2 in 8,300 of air; and C in an atmosphere containing 1 

 part S0 2 in 1,666 of air. During the following 46 hours A transpired 

 25.19 g per 1,000 cm 2 ; B, 18.62 g; and C, 13.85 g. 



