226 PHOTOSYNTHESIS 



undertaken without the slightest scrutiny of the special work it is sup- 

 posed to perform. 



Furthermore, it is not infrequent that the biologist "strains out gnats 

 and swallows camels" in that the most refined methods of physical meas- 

 urement are used while some other manipulation of equal importance does 

 not approach the former in accuracy. 



The methods which have been used to detect and measure the photosyn- 

 thetic activity of illuminated chlorophyllous leaves and other parts of plants 

 are based upon three different principles. These are : 



1. The liberation of oxygen. 



2. The absorption of carbon dioxide. 



3. The formation of organic substance — the gain in dry weight. 



It is essential to bear in mind that these three methods do not always 

 yield the same results ; that the three methods are based upon different 

 chemical reactions, each of which represents a step or a series of steps 

 in a long chain of reactions which are, of course, more or less dependent 

 upon each other, but nevertheless fall in the class of step-reactions. We 

 are evidently not dealing with a single reaction, but with a long series of 

 reactions involving the manufacture of highly complex organic substances 

 from the most stable carbon compound. We are able to observe the raw 

 materials, carbon dioxide and water, which enter the factory. Also we 

 note the end products, oxygen and carbohydrates, and have some idea of 

 the power or energy necessary to run the factory. So far all attempts 

 to establish the intermediate steps and thus determine the course of the 

 various reactions have failed. At present we are then confined to deter- 

 mining the nature and course of the process by inference from careful 

 quantitative investigation of the relation of the factors : intake of raw 

 material, carbon dioxide and water, formation of synthesized product, 

 carbohydrate, "waste product," oxygen, and energy consumed. ^ 



Furthermore, as has been repeatedly emphasized, respiration always 

 accompanies photosynthesis. At best we can measure only the difference 

 between two reactions which are in general running in opposite directions. 

 As yet it has not been possible to separate clearly the two processes. The 

 intensity or rate of the respiratory activity can, of course, be established 

 by parallel experiments, or still better with the same plant during periods 

 of darkness. Here the assumption must be made that the rate of respira- 

 tion does not change in the light. In such experiments of alternate periods 

 of light and darkness it is also essential that these periods be not too 

 long. The question of the separation of the two processes of respira- 

 tion and photosynthesis has been already dealt with. 



A very larii^e number of qualitative methods have been devised to 

 detect photosynthetic activitv ; most of these are based upon tests for 

 oxygen. However, as the development of the subject .is dependent upon 

 a more intense and exact studv, the emphasis is here laid upon those 

 methods which are applicable to quantitative investigation. 



