4 PHYSIOLOGY OF NUTRITION 



CO 



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



U2 



and Mangin 1 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. 1), 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 not 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. 3 



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, 191 7. (This is reprinted from a series of articles having same 

 title, in New phytol. 14-16. 1915-17.) Stiles seems subsequently to have adopted the word 

 photosynthesis to indicate the formation of carbohydrate by green plants, from carbon dioxide 

 and water. Every student of plant physiology ought to be familiar with the following: 

 Stiles, Walter, Photosynthesis; the assimilation of carbon by green plants. 268 p. Longmans, 

 Green and Co., London and New York, 1925. — Ed. 



1 Bonnier, Gaston, and Mangin, Louis, L'action chlorophyllienne separee 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 of the hydrogen and of some of the oxygen being water, taken up from 

 the soil. 



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

 Braunschweig, 1877. Winkler, C. A., Lehrbuch der technischen Gasanalyse. 1885. [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. from 3d German ed. London, 1902.I For physiological experiments, see especially: Doyere, M. L., 

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

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

 acid exchanges of plants. Phil, trans. Roy. Soc. London 6186:485-503. 1896. [Idem, same title. II. 

 On the paths of gas-eous exchange between aerial leaves and the atmosphere. Ibid. 8186:503-562. 1896.] 

 Palladin, W., and Kostytschew, S., in Abderhalden's Handbuch der biochemischen Arbeitsmethoden 3 :479. 

 Berlin, 1910. 



