Chapter 4 

 The Methods of Measuring Photosynthetic Activity 



1. General Considerations 



"When you can measure what you are speaking about and express it 

 in numbers, you know something about it, and when you cannot measure 

 it, when you cannot express it in numbers, your knowledge is of a meagre 

 and unsatisfactory kind. It may be the beginning of knowledge, but you 

 have scarcely in your thought advanced to the stage of a science." ^ 



As was pointed out in Chapter 2, our knowledge of the phenomenon 

 of photosynthesis is most intimately associated with the development 

 of information concerning the gases of the atmosphere. Practical ex- 

 perimental work as well as the broader conceptions of the process were 

 entirely dependent upon an understanding of the chemical composition 

 of the air. With the discovery of oxygen and its relation to oxidative 

 processes the cosmical function of green plants was quickly elaborated 

 by the genius of de Saussure. Furthermore, improvements in the tech- 

 nique of constructing and manipulating apparatus for the analysis of gases 

 had direct application to the problem of photosynthesis and were reflected 

 in more precise knowledge. 



The modern biologist has a doubly difficult task. Not only must he 

 be familiar with physical and chemical methods and manipulation, but 

 he must also be able to apply these methods to his particular problems 

 and be sufficiently conversant with the principles and theory of such 

 physical and chemical methods to be able to apply them to his demands. 

 Just as the practical application of some chemical discovery often requires 

 far more labor and ingenuity than was entailed in the purely scientific 

 work, so the application of physical and chemical methods to specialized 

 biological investigation demands particular scrutiny and cross-checking. 

 It is easy enough to set up a piece of physical apparatus and get num- 

 berless readings and numerical results. Whether these mean anything 

 in the biological process under investigation is another question. There 

 is probably no branch of science, with the possible exception of medicine, 

 which is so susceptible to fads in methods and concepts as biology. This 

 is undoubtedlv due to the fact that there is still an enormous amount 

 of groping in the dark : the method of trial and error is still the prevalent 

 one. The introduction of a physical or chemical method is all too often 



^From Lord Kelvin, quoted by :\Iillekan, R. A., "The Electron," University of 

 Chicago Press, p. 4. 



225 



