22 STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



the "Canada Wonder" bean have answered the requirements 

 admirably, and we have been able to grow this material in the green- 

 house during most of the year. Naturally, care was exercised to 

 use only perfect leaves, free from infections and other imperfections. 

 By thus concentrating our efforts we hoped to be able to gain deeper 

 and more detailed information which might serve to elaborate some 

 general principles of plant metabolism. 



For most of the experiments leaves which had been cut from the 

 plant were used. The method of cutting is described later. 



Although experiments with excised leaves apparently represent 

 highly artificial conditions, this method possesses many distinct 

 advantages. An entire plant usually requires so much space that 

 it is exceedingly difficult and cumbersome to construct respiration 

 chambers which will accommodate the plant and at the same time 

 permit accurate control of the temperature. By using excised 

 leaves the space can be very materially reduced and the requirements 

 of temperature control adequately met. A relatively small volume 

 of the respiration chamber has another advantage in that with a given 

 rate of the air-stream it permits the more perfect removal of the 

 gases. By cutting the leaf from the plant the factor of translocation 

 is also eliminated. Thus materials which are formed in the leaf 

 during the course of respiration do not migrate to other parts of the 

 plant, and the only source of supply of food material is that which is 

 stored in the leaf itself. The general effect of these conditions as 

 compared with the attached leaves is, as it were, to shorten the time 

 of certain reactions or accentuate their intensity. By removal from 

 the rest of the plant, the leaf has thus been severed from its base 

 of supplies as well as from the receiver of its surplus products. 



The use of excised leaves also offers the only satisfactory method 

 of feeding to leaves substances the behavior of which it is desired to 

 study. In comparison with animals, physiological work with plants 

 is at a greater disadvantage in this respect. It is not possible to 

 ingest into plants a definite quantity or kind of food material, as 

 can be done with animals. However, it has been established that 

 many organic substances are taken up into the leaves in a short time 

 when the petioles are placed in solutions of these substances. On 

 the other hand, under the conditions of our experiments there was 

 no migration of materials, such as sugars, from the leaf into the 

 nutrient solution. 



The plants used in these investigations were grown in a green- 

 house in a loam soil to which no fertilizer had been added. Care 

 was taken to use leaves of about the same age, size, and development. 

 It was endeavored to study the behavior of mature, well-functioning 

 leaves; so both rapidly growing and old leaves were discarded. 



