STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 21 



METHODS AND APPARATUS. 



1. The Experimeiital Material. 



For the study of respiration and photosynthesis the leaves of all 

 plants are not equally well suited. That is, in some plants, due to 

 structural peculiarities, these processes exhibit complications and 

 complexities which make the interpretation of observational data 

 extremely difficult. The fundamental feature of both processes 

 is that they depend upon the ingress and egress of gases. The most 

 satisfactory methods of measuring the rate of respiration and photo- 

 synthesis are based upon the gaseous exchange, and any factor 

 which at all influences the easy diffusion of these gases interferes 

 with the accuracy of the determinations. For this reason, thin 

 leaves are more easily worked with than those which are succulent 

 or possess protective devices against the loss of water. 



Moreover, in a study of respiration, analytical data are of great 

 importance and the leaf material should not present undue diffi- 

 culties for the chemical determination of the materials intimately 

 associated with the respiratory activity. The plant material should 

 also be capable of being easily grown in quantity and should not be 

 susceptible to infection or disease. Further insight into the mechan- 

 ism of the processes of respiration and photosynthesis can be gained 

 only by means of experimentation, that is, by subjecting the plant 

 to a variety of external conditions and at will altering some of the 

 internal constituents. For such a procedure the plant must not be 

 too sensitive nor easily affected by shght variations of external 

 conditions. In view of the fact that these studies were directed 

 primarily toward an attack on the problem of photosynthesis, it 

 was desirable to work with leaves in which the chlorophyll apparatus 

 was well developed, and that these leaves should be autotrophic in 

 the sense that, aside from mineral nutrients, they should be entirely 

 capable of producing their own food material. 



It would, of course, be desirable to gain information relative to 

 the respiratory and photosynthetic processes of a large variety of 

 species. We are fully aware that different species exhibit great 

 variation in their behavior. Similarly, a knowledge of the behavior 

 of these plants in the field, under natural conditions, would probably 

 be of greater significance than the information gained under such 

 highly artificial conditions. However, these ideals are not amenable 

 to the experimental method and can be attempted only at too great 

 a sacrifice of precision and intensity of investigation. 



We were therefore constrained to confine our studies to a very 

 limited number of plants which fulfilled the requirements mentioned. 

 The wild sunflower of southern Arizona, Helianthus annuus, and 



