28 



PHYSIOLOGY OF NUTRITION 



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

 by the plant. 



§6. Products of Photosynthesis. 1 — The simplest equation that may repre- 

 sent the exchange of gases in photosynthesis is CO" = C + Oo. 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. The 

 investigations of Sachs 2 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 light starch 

 soon appears again. Small traces of starch may be recog- 

 nized 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 

 become visible. If a part of the leaf is covered with tinfoil 

 before 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, accord- 

 ing to the amount of starch present (Fig. 17). The 

 experiment becomes particularly striking if the whole leaf 

 is covered with a piece of tinfoil, or cardboard, from which 

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

 the treatment described above, the letters stand out blue against a brown 

 background/ 



According to Famintsyn, 3 algae may be very satisfactorily employed in this 

 connection; the presence of starch may be shown after only half an hour's 

 illumination from a bright lamp. According to Kraus, 4 algae may form starch 

 in sunlight within a period of five minutes. As Godlewski 5 has shown, starch 



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

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



2 Sachs, J., Ueber den Einfluss des Lichtes auf die Bildung des Amylums in den Chlorophyllkornern. 

 Bot. Zeitg. 20: 365—373. 1862. Idem, Ueber die Auflosung und Wiederbildung des Amylums in den 

 Chlorophyllkornern bei wechselnder Beleuchtung. Ibid. 22: 280-294. 1864. 



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

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



4 [Kraus, Gregor, Einige Beobachtungen uber den Einfluss des Lichts und der Warme aud die Starkeer- 

 zeugung im Chlorophyll. Jahrb. wiss Bot. 7: 511-531. 1868.] 



6 Godlewski, Emil, Abhangigkeit der Starkebildung in den Chlorophyllkornern von dem Kohlensaurege- 

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



T 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. 86-90.) 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 1 g. of 

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



Fig. 17. — Accu- 

 mulation of starch 

 in the illuminated 

 portion of a leaf. 

 The light-colored 

 portion was shaded 

 by tinfoil and the 

 starch has been 

 stained by iodine. 



