RESPIRATION AND OXYGEN. 67 



PHOTOSYNTHESIS. 



The first studies of the effect of pure carbon dioxid upon photo- 

 synthesis were made by Grischow (1819), who found that this proc- 

 ess was greatly reduced but not wholly prevented in the sunlight 

 and with the gas at the ordinary air-pressure. Boussingault 

 (1868 : 269) confirmed Grischow's results, and concluded that the 

 reduction was due to the excessive partial pressure of the carbon 

 dioxid, rather than to the absence of oxygen, as the latter was con- 

 stantly produced in the sunlight. When the carbon dioxid was 

 mixed with other gases, or its pressure reduced, its depressing effect 

 was less marked. He also found (1865 : 605) that oleander leaves 

 lost the ability to free oxygen from a mixture of CO 2 and ordinary 

 air after 48 hours' retention in carbon dioxid, nitrogen, or marsh- 

 gas in the dark at a temperature of 22° to 23° C. In the case of a 

 single leaf, which had been kept for 48 hours in hydrogen in the dark, 

 2.6 c. c. of carbon dioxid were broken down after 5 hours of insola- 

 tion. In the case of many plants, while photosynthesis was slight 

 in pure carbon dioxid, it was marked in mixtures containing 30 and 

 even 40 per cent. 



Pfeffer (1871) observed that both 8 and 16 per cent of carbon 

 dioxid in the air were without noticeable effect upon the evolution 

 of oxygen from the leaves of Prunus laurocerasus. 



Bohm (1873 : 230) found that leaves of laurel plants, after re- 

 maining several hours in an atmosphere without oxygen, perished 

 or completely lost the capacity to break down carbon dioxid. How- 

 ever, they continued to live, obtaining the necessary energy for their 

 functions through intramolecular respiration. He also studied the 

 influence of CO 2 upon the greening and growth of seedlings, and found 

 that this gas exerted a strikingly injurious effect. In a mixture of 

 2 per cent its unfavorable effect upon chlorophyll formation was 

 noticeable. In air whose oxygen-content corresponded to that of 

 ordinary air, but contained 50 per cent of CO2, not only did no growth 

 occur, but the plants themselves died after a short time. In view 

 of the fact that plants visibly sickened in an atmosphere which con- 

 tained only a few per cent of CO2, it was concluded that the present 

 plant world would perish, at least in part, in an atmosphere not much 

 richer in CO 2 than the existing one. From this it results either that 

 the compostion of the atmosphere on earth has been the same from 

 the beginning, which seems a necessary consequence of its being 

 limitless, or, what was regarded as less probable, that there were 

 plants in earlier periods which could endure a greater amount of 

 CO2 without damage. 



Godlewski (1873 : 243) endeavored to determine the optimum 

 amount of CO 2 for photosynthesis, and found it to lie between 5 and 

 10 per cent for Typha, Glyceria and Nerium, while greater amounts 



