28 PHYSIOLOGY OP NUTRITION 



and the higher the temperature, the smaller is the amount of chlorophyU formed 



by the plant. 



§6. Products of Photosynthesis."— The simplest equation that may repre- 

 sent the exchange of gases in photosynthesis is CO2 = C + O2. The carbon is 

 retained by the plant, combined with other elements in the form of organic sub- 

 stances. The question now arises as to what are to be considered as the first 

 products of photosynthesis, and the investigations of Sachs^ showed that the 

 first visible product is starch. If leaves are kept for several days in darkness the 

 starch completely disappears from the chlorophyll bodies, and if the leaves are 

 then returned to Hght starch soon appears again. Small traces of starch may 

 be recognized by the method of Bohm, whereby leaves are 

 first decolorized by alcohol and then treated with caustic 

 potash and iodine solution; the starch grains, greatly swollen 

 by potassium hydroxide, are stained by iodine and thus be- 

 come visible. If a part of the leaf is covered with tinfoil be- 

 fore it is exposed to light, and if, after the exposure, the leaf 

 is decolorized with alcohol and then treated with iodine, the 

 portion that was shaded becomes yellowish brown, while the 

 rest of the leaf is blue or black, according to the amount of 

 starch present (Fig. 17). The experiment becomes particu- 

 larly striking if the whole leaf is covered with a piece of 

 Fig. 17.— Ac- tinfoil, or cardboard, from which the letters of the word starchy 

 cumulation of starch etc., have been cut out as in a stencil; after the treatment 

 t"orti!m 'li'^tillt described above, the letters stand out blue against a brown 



The light-colored background.'' 



brwSorindiht According to Famintsyn,' algee may be very satisfactorily 

 starch has been employed in this connection ; the presence of starch may be 

 stained by iodine. ^^^^^ ^^^^^ ^^j^ ^^-^^ ^^ ^^^,^ illumination from a bright 



lamp. Accordijig to Kraus,* algse may form starch in sunlight within a period 

 of five minutes. As Godlewski^ has shown, starch can be formed in light only 



1 Brown, H. T., and Morris, G. H., A contribution to the chemistry and physiology o£ foliage leaves. 

 Jour. Chem. Soc. London 63 : 604-677. 1893. 



' Sachs, J., Ueber den Einfluss des Lichtes auf die Bildung des Amylums in den Chlorophyllk6rnem. 

 Bot. Zeitg. 20: 365-373. 1863. Idem, Ueber die Auflosung und Wiederbildung des Amylums in den 

 Chlorophyllkornern bei wechselnder Beleuchtung. Ibid. 221 289-294. 1864. 



' [Famintzin, A., Die Wirkung des Lichtes auf Algen und einige andere ihnen nahe verwandte Organismen. 

 Jahrb. wiss. Bot. 6: 1-44. 1867. See P. 34.] 



« [Kraus, Gregor, Einige Beobachtungen uber den Einfluss des Lichts und der Warme auf die Starkeerzeug- 

 ung im Chlorophyll. Jahrb. wiss. Bot. 7: 511-531. 1868. 1 



' Godlewski, Emil, Abhangigkeit der Starkebildung in den ChlorophyUk6mem von dem Kohlensauregehalt 

 der Luft. Flora, n. R. 31 : 378-383. 1873. 



•■ The experiment should be performed in such manner that access of the carbon dioxide of 

 the air to the stomata is clearly not hindered; otherwise the conclusion given is not logically 

 substantiated. (See Ganong, W. F., A laboratory course in plant physiology. 2 ed., New- 

 York, 1908. P. 86-88, 89-90.) It is usually best to transfer the decolorized leaves from 

 alcohol to water, then to an aqueous solution of potassium hydroxide, after which an aqueous 

 solution of potassium iodide and iodine is added to bring out the color reaction. The iodine 

 solution may be prepared by dissolving 5 g. of the iodide in water, then dissolving i g. of iodine 

 in this, and diluting the resulting double solution to a volume of 1000 cc. or less. — Ed. 



