4 PHYSIOLOGY OF NUTRITION 



found out how the ratio -7^-^ varies, independently of respiration. Bonnier 



and Mangini investigated this and found the value of the ratio to be really 

 somewhat less than unity. So the plant gives off not only the equivalent of all 

 the oxygen originally contained in the absorbed carbon dioxide, but also a 

 smaller portion of oxygen arising from the water that is decomposed in 

 photosynthesis. 2 



As to methods of investigation, the decomposition of carbon dioxide can be 

 detected in the following manner. A cut leaf is placed in a calibrated glass 

 tube (Fig. i), the upper end closed and the lower, open end dipping into mercury. 

 Then a part of the air is removed by a rubber tube and the level of the mercury 

 rises. The volume of the remaining air is read, after which some carbon dioxide 

 is admitted from a gasometer and the gas volume is again determined.- The 

 apparatus is now placed in light and after some time the gas volume is once 

 more recorded.. The remaining carbon dioxide is removed by injecting some 

 concentrated potassium hydroxide solution, and the diminished gas volume is 

 again read; pyrogallol is next introduced, and a final reading, after the removal 

 of oxygen by the pyrogallol, gives the amount of nitrogen that remains. The 

 numbers obtained permit the determination of the amounts of carbon dioxide 

 absorbed and of oxygen liberated. ^ 



A less exact method consists in counting the number of gas bubbles 



1 Bonnier, Gaston, aild Mangin, Louis, L'action chlorophyllienne s^par6e de la respiration. Ann. 

 sci. nat. Bot. VII, 3 : S-44. 1886. 



2 It will be seen later that hydrogen and oxygen are actually assimilated, as well as carbon, in the 

 photosynthetic process, the source oi the hydrogen and of some of the oxygen being water, taken up from 

 the soil. 



"For precise methods of gas analysis see: Bunsen, Robert W. E., Gasometrische Methoden. 2te 

 Aufl. Braunschweig, 1877. Winkler, C. A., Lehrbuch der teohnischen Gasanalyse. 188s. [idem, 

 Handbook of technical gas-analysis containing concise instructions for carrying out gas-analytical methods 

 of proved utility. Translated with a few additions by George Lunge. London, 1885. Idem, same title,' 

 2d English ed. rfom 3d German ed.' London, 1902.] For physiological experiments, see especially: DoySre, 

 M. L., Etudes sur la respiration. Ann. chim. et phys. /J/, 28: 5-50. 1850. Blackman, F. Frost, Experi- 

 mental researches on vegetable assimilation and respiration. I, On a. new method for investigating the 

 carbonic acid exchanges of plants. Phil, trans. Roy Soc. London B186: 485-502. 1896. [idem, same title. 

 II. On the paths of gaseous exchange between aerial leaves and the atmosphere. Ibid.BiB6: 503-562. 

 1896.] Palladin, W., and Kostytschew, S., in Abderhalden's Handbuch der biochemischen Arbeitsmethoden 

 3: 479. Berlin, 1910. 



translation of PfefEer's Plant Physiology (i : 302. Oxford, 1900), but his objections do not 

 appear valid as against the use of photosynthesis. Of course, this should include all possible 

 forms of chemical synthesis brought about through the action of light, but the formation of 

 carbohydrate out of carbon dioxide and water is by far the most important form of photosyn- 

 thesis, and the term may readily be qualified whenever need arises. Thus, we may distinguish 

 chlorophyll photosynthesis of carbohydrate from other photosyntheses. The word assimilation 

 has been employed in so many different senses that to attempt its use as a precise term in this 

 connection here seems unadvisable. — Jorgensen and Stiles prefer, however, to employ the 

 "rather non-committal expression," carbon assimilation, and they do so in their recent 

 very excellent monograph on this subject, which should be referred to in connection with 

 this entire chapter, and which should become familiar to every student of plant physiology. 

 See: Jorgensen, Ingvar, and Stiles, Walter, Carbon assimilation, a review of recent work 

 on the pigments of the green leaf and the processes connected with them. New phytolo- 

 gist reprint No. 10. London, 1917. (This is reprinted from a series of articles having 

 same title, in New phytol. 14-16. 1915-17. — Ed. 



