6 PHYSIOLOGY OF NUTRITION 



mentally important substance: (i) a comparatively small part of it is surely 

 produced in the body of the plant, by respiration, and (2) an uncertain (perhaps 

 sometimes very significant) part of it is taken into the plant through its root 

 peripheries, along with water from the soil, in which carbon dioxide is often 

 plentiful. Carbon dioxide absorbed from the soil must mainly find its way to 

 the leaves or other green parts, where it should meet the same fate as though it 

 had been produced by respiration or had entered from the air. With high rates 

 of transpiration and water absorption, the amount of carbon dioxide derived 

 from the soil should be considerable in many instances. This question deserves 

 attention. 



Most of the oxygen produced by photosynthesis in ordinary green leaves 

 probably passes directly to the air, but some of it surely moves inward and 

 downward through the plant body, being used in respiration. Some of it may 

 even pass out through the root peripheries into the soil. (In this connection 

 see: Cannon, W. A., Experimental studies on roots. Carnegie Inst. Washington 

 Year Book 24, 289-298. 1925. See especially p. 292-294. Also see: Idem, 

 Physiological features of roots. 168 p. Carnegie Inst. Washington Publ. 368, 

 1925.)— Ed.] 



§3. Chlorophyll. — Since the decomposition of carbon dioxide is effected exclu- 

 sively by the green parts of plants, the properties of the green pigment — called 

 chlorophyll by Pelletier and Caventou 1 — must be studied. Chlorophyll can be 

 extracted from leaves by alcohol, but the solution thus obtained also contains 

 several other pigments, as well as colorless substances, for the removal of which 

 various methods have been devised. 2 The method of Fremy involves the 

 precipitation of the alcoholic extract with barium hydroxide; the green precipi- 

 tate is collected upon a filter and treated with alcohol until the yellow pigments, 

 xanthophyll and carotin, are completely removed. The remaining precipitate 

 is then decomposed by potassium hydroxide, according to the method of 

 Timiriazev. 3 The green solution thus obtained is treated with ether, and dilute 

 acetic acid is gradually added, with shaking, to neutralize the potassium 

 hydroxide. As long as the reaction is alkaline the ether remains without 

 color, but as soon as the hydroxide is neutralized the lower layer becomes yellow, 

 since all the green pigment passes into solution in the ether above. The color 

 of the ether solution is emerald green, more intense than that of the alcoholic 

 solution; it is cherry red, however, in reflected light, while the yellow solution 

 shows no fluorescence. Timiriazev was the first to succeed in separating chlo- 

 rophyll from yellow pigments, out of the alcoholic chlorophyll extract. This 

 chlorophyll is not the normal pigment, however, for it has been changed by the 

 teratment employed in separating it. 



1 [Pelletier], [Joseph], and Caventou, [J. B.], Sur la matiere verte des feuilles. Ann. chim. et phys. II, 

 9: 194-196. 1818.] 



2 Willistatter, Richard, Chlorophyll und seine wichtigsten Abbauprodukte. Abderhalden's Handb. 

 biochem. Arbeitsmeth. 2: 671-716. Berlin, 1910. Willstatter, Richard, and Hug. Ernst, Isollierung des 

 Chlorophylls. Liebig's Ann. Chem. u. Pharm. 380: 177-21 1. 1911. 



3 Timiriazev, K. A., Spectral analysis of chlorophyll. [Russian.] St. Petersburg, 1871.* [Haas and 

 Hill give methods for obtaining chlorophyll, and present a good discussion. See: Haas, Paul, and Hill, 

 T. G., An introduction to the chemistry of plant products. London, 1 921.] 



