248 PHOTOSYNTHESIS 



current of air is made to pass over the leaf at a constant rate, and the 

 amount of carbon dioxide in the air stream after passing over the leaf in 

 the dark is established. This value is higher than that for the amount of 

 carbon dioxide originally in the air current, for through the process 

 of respiration the plant emits carbon dioxide to the air passing over 

 it. The difference between the amount of carbon dioxide originally in 

 the air current and that found after the air has passed over the leaf in 

 the dark gives a value for the rate of respiration. If the leaf is now 

 illuminated, the air current continuing at a constant rate, the carbon 

 dioxide-content of the air stream after passing over the leaf is lower than 

 was the case before illumination. This decrease is due to the photosyn- 

 thetic appropriation of carbon dioxide by the leaf. The value of this 

 decrease in unit time and per unit area of leaf surface is taken as the rate 

 of photosynthesis. 



This method permits the plant to be under very nearly normal condi- 

 tions, it prevents the accumulation of gases which may be injurious to the 

 leaf as well as large decreases in the amount of carbon dioxide in the 

 atmosphere surrounding the leaf and it is relatively easily manipulated. 



The method was first applied by Kreusler *'^ and has been modified and 

 improved by various workers. 



While the method just referred to is very simple in principle there 

 are many details connected with the regulation of the air-stream, the tem- 

 perature control, the illumination and the determination of the quantity 

 of carbon dioxide which require a great deal of attention both in con- 

 struction of the apparatus and in its manipulation if reliable results are 

 to be attained. The construction of the apparatus is, of course, determined 

 by the nature of the investigation and the problem to be solved. For the 

 detailed discussion of matters relating to construction and manipulation 

 the publications of the investigations here cited must be referred to. 



The air-stream has been produced in several ways by different workers. 

 An aspirator of the Mariot bottle type can be used and the rate of flow 

 measured by an accurate gas meter. (It is very important that the gas 

 meters be kept in a constant temperature bath.) The whole air-stream 

 system may also be kept in a chamber of constant temperature. It is often 

 desirable that the air which is used should contain more carbon dioxide 

 than that which is normally present in the atmosphere. For this purpose 

 the air may be drawn from a gasometer to which carbon dioxide has been 

 added or from a gas mixture previously compressed into cylinders. 

 In the latter case the air is allowed to escape from the cylinders and the 

 rate determined by means of a bubble counter and flow meter. 



In much of the work which has been done by mean's of this method 

 leaves which have been detached from the plant have been used. While 



^•Kreusler Landw. Jahrb., 14, 915 (1885); 16, 711 (1887); 17, 161 (1888); 19, 

 649 (1890) Blackman, Phil. Trans. Roy. Soc, B 186, 485 (1895). Brown and 

 Escombe, Proc. Roy. Sac, B 76, 29 (1905). Willstiitter and Stoll, "Untersuchungen 

 Uber die' Assimilation der Kohlensaure," Berlin, 1918, p. 62. 



