84 PHOTOSYNTHESIS 



toxic to phanerogams.^'' Bottomley and Jackson,^*' however, report that 

 this gas can to some extent replace carbon monoxide in photosynthesis. 

 Boussingault also tried hydrocarbons without success. 



c. The Evolution of Oxygen. 



As has already been pointed out, one of the first facts which was 

 observed in connection with the phenomenon of photosynthesis was that 

 in the light carbon dioxide was taken up by the plant and oxygen was 

 emitted. Thus the emission of oxygen as well as the absorption of car- 

 bon dioxide have been utilized as a means of studying the phenomenon. 

 While a variety of qualitative methods, based upon the evolution of 

 oxygen have been devised, relatively little work has been done on the 

 laws governing this phase of the phenomenon. The methods which have 

 been used in studying the emission of oxygen are described in Chapter 4. 

 That gas is emitted in the process of photosynthesis can be observed 

 most easily in submerged aquatic plants. These absorb the dissolved car- 

 bon dioxide and emit the oxygen in form of minute bubbles. Leaves 

 of land plants are not always suited to this demonstration, because the 

 gas is not allowed to escape from the minute stomata on account of the 

 capillary surface formed by the water. Leaves covered with a waxy sur- 

 face are therefore more suitable. The demonstration of the evolution 

 of oxygen in a striking manner is made possible by the use of the leuco 

 compound of methylene blue or indigo carmine. An aquatic plant is 

 allowed to remain over night in water the surface of which is covered 

 with a layer of paraffin oil. When the oxygen has been removed from 

 the water by the respiratory activity of the plant, a small quantity of 

 the leuco dye is added to the water and the plant is illuminated. The 

 evolution of oxygen from the illuminated plant is made evident by the 

 formation of the bright colored dye close to the plant. 



It should be stated at once that the gas which is thus emitted is 

 not pure oxygen, but contains from 25-85 per cent of this gas with ad- 

 mixtures of nitrogen. Since it is impossible to obtain an exact analysis 

 of the gas as it is emitted from the leaves of land plants, the results 

 obtained with aquatic plants give more reliable data on this point. In 

 land plants the path of oxygen escape is through the stomata, while in 

 aquatic plants under normal conditions the oxygen escapes from the 

 whole leaf surface by diffusion into the surrounding medium. The 

 diffusion coefficient of oxygen is slightly higher than that of carbon 

 dioxide. According to Carlson ^^ kco„ =1-378 and ko„ =1.607 (per 

 sq. cm. per 24 hrs. at 16°) giving a ratio between the coefficients of oxygen 

 and carbon dioxide of 1.166. 



"^ Richards and MacDougal, Bull. Torrey. Bot. Club, 31, 57 (1904). 

 '"Bottomley and Jackson, Proc. Rov. See, 72, 130 (1903). Krascheninnikoff, 

 Rev. Gen. Bot., 21, 177 (1909). 



"Carlson, /. Am. Chem. Soc, 33, 1027 (1911). 



