RESPIRATION AND OXYGEN. 69 



dioxid in the case of Prunus, and Warburg but little in Nerium at 

 5 to 25 per cent, while Schiitzenberger and Quinquand found 5 to 10 

 per cent the optimum for Elodea, and Warburg observed that photo- 

 synthesis was markedly reduced in Hoplophytum at 10 to 15 per 

 cent, and was but slight in Bryophyllum at 12 per cent. Although 

 Brown and Escombe showed that photosynthetic activity was 

 increased with 2 to 4 times the normal amount of C0 2 in the air, the 

 gain in dry weight was less, the stems and leaves were reduced, and 

 the flowers were almost completely suppressed. 



The significance of the injurious effect of carbon dioxid on photo- 

 synthesis upon the question of the greater amount of this gas in the 

 atmosphere during earlier geological periods was first pointed out by 

 Bohm, who regarded it as probable that the composition of the 

 atmosphere has remained the same since the beginning of plant life 

 on the land. This view receives further support from the conclusion 

 of Brown and Escombe that a sudden increase in the carbon dioxid 

 of the atmosphere to 2 to 4 times the present amount would destroy 

 most flowering plants. 



TRANSPIRATION. 



Saussure (1804) was the first to observe that pea plants wilted in 

 an atmosphere of carbon dioxid, as well as in mixtures containing 

 three-fourths and two-thirds of it when this gas was led into the 

 nutrient solution in which they were growing, and Wolff (1870 : 134) 

 confirmed this in the case of barley and beans. Barthelemy (1873) 

 also found that transpiration was reduced by the action of dry 

 carbon dioxid. In a study of the effect of very dilute acids upon 

 transpiration, Burgerstein (1876 : 202) determined the influence of 

 carbon dioxid. In three preliminary experiments with corn seed- 

 lings in which the amount of C0 2 was not known, the transpiration 

 generally was much greater for the plants in distilled water. In the 

 series proper, the solutions contained respectively 0.08 and 0.04 per 

 cent of carbon dioxid. The plants in the stronger solution trans- 

 pired slightly more than those in distilled water, and those in the 

 weaker slightly less in the case of corn. All the other plants studied, 

 peas, beans, pumpkin, broad beans, Celtis, Fagus, Tilia, Cratcegus, 

 and Salisburia, regularly showed considerably greater water-loss in 

 the C0 2 solution. 



In a study of the absorption of free nitrogen by legumes, Kosso- 

 witsch (1892 : 702) found that an atmosphere of 80 per cent C0 2 

 and 20 per cent oxygen worked injury to the root-system. When 

 the roots of peas had been in such an atmosphere only 2 days, the 

 plants began to wither and they grew no further. As soon as the 

 CO 2 was removed from the inclosed soil and the latter aerated, the 

 peas regained their normal turgescence. He also found that when 

 the soil was penetrated by a stream of mixed oxygen and hydrogen, 

 the plants were not injured. 



