ASSIMILATION OF CARBON 2Q 



becomes particularly striking if the whole leaf is covered with a piece of tin- 

 foil, 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, 1 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, 2 alga? may form starch 

 in sunlight within a period of five minutes. As Godlewski 3 has shown, starch 

 can be formed in light only in the presence of carbon dioxide. In a closed 

 chamber, illuminated but free from this gas, no starch was formed; indeed, 

 if starch had been originally present its amount decreased under these con- 

 ditions. The chloroplasts of some plants do not form starch at all, as is the 

 case with leaves of Allium cepa (onion), A.fistulosum, Asphodelusl uteus, Orchis 

 militaris, and Lactuca saliva (lettuce), but in all these instances glucose is 

 formed instead of starch. 



According to whether starch ((C 6 Hi O 5 )„) or glucose (C 2 Hi 2 6 ) is con- 

 sidered as the first product of photosynthesis, the chemical equation represent- 

 ing the process may take one or the other of the two forms given below: 



(1) 6 C0 2 + 5 H 2 = C 6 H 10 O 5 + 60 2 . 



(2) 6 C0 2 + 6 H 2 = C 6 H 12 6 + 6 2 . 



Timiriazev 4 showed by direct experiment that the formation of starch in light 

 is brought about by the same rays of the spectrum as are effective in the decom- 

 position of carbon dioxide. By means of a heliostat, a spectrum was thrown 

 upon a leaf of a plant that had been previously exposed to darkness so as to 

 free the leaves of starch; two strips of paper were fastened across the leaf with 

 the spectrum falling between them, and upon these strips were recorded the 

 positions of the Fraunhofer lines in the spectrum. At the end of the experi- 

 ment, after the leaf had been decolorized by alcohol and stained with iodine, it 

 became evident that starch formation had occurred exactly in the regions cor- 

 responding to the absorption bands of chlorophyll. In such an experiment the 

 band between lines B and C is especially pronounced, and a fainter iodine- 

 starch color is noticeable in the orange-yellow region, this coloration gradually 



1 [Famintzin, A., Die Wirkung des Lichtes auf Algen und einige andere ihnen nahe verwandte Organ- 

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



" [Kraus, Gregor, Einige Beobachtungen liber den Einfluss des Lichts und der Warme auf die Starkeer- 

 zeugung im Chlorophyll. Jahrb. wiss. Bot. 7: 51 1-53 1. 1868.] 



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

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



4 Timiriazeff, C, Enregistrement photographique la de fonction chlorophyllienne par la plante vivante. 

 Compt. rend. Paris no: 1346-1347. 1890. 



r 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. 



