THE MEASURE]VIENT OF PHOTOSYNTHESIS 325 



respiration. Since in rapidly photosynthesizing tissues the rate of photosyn- 

 thesis is often ten to twenty times as great as the rate of the respiration, the 

 apparent photosynthetic rate is often not greatly less than the true rate. In 

 some experiments upon photosynthetic rates, the values obtained are corrected 

 by adding to them values supposed to represent the quantity of carbohydrates 

 consumed in respiration during the period of the determination. The values 

 used for such corrections are obtained by measuring the respiration rates of 

 the same plant or organ w^hen so treated that photosynthesis cannot occur, 

 as for example after transference to a dark room. 



Quantitative measurements of photosynthesis in terrestrial plants, based 

 on determinations of the rate of oxygen evolution or the rate of carbon dioxide 

 consumption are made with the plants or excised leaves enclosed in glass 

 chambers. In most of the more recent applications of this method outside 

 air is passed through the chambers at a relatively rapid rate. The air is 

 collected upon emergence from the chamber and analyzed for oxygen, or 

 carbon dioxide, or both. Another procedure is to analyze small samples of 

 the air for these gases at frequent intervals. By a comparison of the results 

 of these analyses with similar analyses of the outside atmosphere the rate of 

 carbon dioxide consumption or oxygen evolution can be computed, either of 

 which will serve as a measure of the rate of photosynthesis. Investigations 

 in which photosynthesis was measured by this method are described in the 

 next chapter. 



A commonly used simple method of measuring the rate of photosynthesis 

 in water plants by the rate of evolution of oxygen is the so-called "bubble- 

 counting" method. When cut shoots of water plants such as the waterweed 

 (Elodea canadensis) are illuminated, bubbles may be observed to rise in more 

 or less rapid succession from the cut ends of the stems. Bubbles released 

 from Elodea stems usually contain from 30 to 60 per cent oxygen. The 

 greater the rate of photosynthesis, the more rapid the rate of bubble emission. 

 If suitable precautions are taken to minimize sources of error the number of 

 bubbles appearing in a unit time may be taken as a roughly quantitative 

 measure of the rate of photosynthesis. The method will not yield exact 

 results but is satisfactory for demonstrating the general effect of certain ex- 

 ternal factors upon photosynthesis. The principal sources of error in this 

 method are: (i) the bubbles may vary considerably in size, (2) the oxygen 

 content of the bubbles is not constant but increases as the rate of emission, 

 (3) a part of the oxygen liberated diffuses directly into the surrounding 

 water and thus escapes detection, and (4) movement of the plant or agita- 

 tion of the circumambient liquid may have a marked effect upon the rate of 

 bubble emission (Wilmott, 1921). 



