MEASURING PHOTOSYNTHETIC ACTIVITY 249 



experiments with excised leaves apparently represent highly artificial 

 conditions, the method possesses many distinct advantages. An entire plant 

 requires so much space that it is exceedingly difficult to construct apparatus 

 which permits adequate control of temperature during illumination and 

 the change of position of the leaves relative to the direction of the light 

 and various other conditions are impossible to control. By using excised 

 leaves or at least single leaves the amount of light which is absorbed by 

 the leaf as well as the gas exchange can be more accurately determined. 



The fact must not be overlooked, however, that when excised leaves 

 are used rate of photosynthesis is quite different from that obtaining in at- 

 tached leaves. In excised leaves the factor of translocation of materials 

 formed in the leaf to other parts of the plant has been eliminated. It 

 would be expected that with continued photosynthetic activity and conse- 

 quent accumulation of starch and sugars there would occur a diminution 

 in the rate of photosynthesis. As a matter of fact Brown and Escombe *^ 

 found that contrary to expectations the leaf attached to the plant shows a 

 lower, rate than the excised leaf. The latter is in the mean about 44 

 per cent higher than the former. Brown and Escombe ascribe this differ- 

 ence in the behavior of the two sets of leaves to difference in the stomatal 

 openings in the two cases. The petiole of the excised leaf, in these experi- 

 ments, was in water, while in the attached leaf the water was supplied to 

 the leaf through the roots. It is conceivable that in the former case condi- 

 tions were more favorable to a wide opening of the stomata and consequent 

 freer access of carbon dioxide. Whatever the explanation of the differ- 

 ence in behavior of the two sets of leaves may be, it is essential to bear 

 these facts in mind before drawing too wide conclusions from observations 

 made with excised leaves. 



Various types of leaf containers have been devised which permit the 

 leaves to be held at right angles to the source of illumination, to be sub- 

 merged in a constant temperature bath and through which the air-stream 

 may be passed. Willstiitter and Stoll have described such a chamber to 

 contain a number of leaves and in Figure 16 is shown a leaf frame de- 

 signed by Spoehr for a single leaf. 



In this method the estimation of the rate of photosynthesis depends 

 upon the determination of small dift'erences in the amount of carbon 

 dioxide. In order that the carbon dioxide in the air-stream be not too 

 greatly reduced, and the rate of photosynthesis consequently also affected, 

 it is usually advisable to enrich the air-stream with carbon dioxide. Under 

 certain experimental conditions, to make sure that the carbon dioxide 

 is not the limiting factor, it is necessary to add considerable quantities of 

 carbon dioxide. In this way relatively large quantities of carbon dioxide 

 are contained in the air-stream whether the leaves are illuminated or not. 

 As a consequence the accuracy of this method depends upon deterniining 

 small differences between two large values in the amount of carbon dioxide 

 in the air-stream. 



"Brown and Escombe, /. c, p. 51. 



