ASSIMILATION OF CARBON 29 



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 pre- 

 sent its amount decreased under these conditions. The chloroplasts of some 

 plants do not form starch at all, as is the case with leaves of Allium cepa 

 (onion), A. fistulosum, Asphodelus luteus, Orchis militaris, and Lactuca sativa 

 (lettuce), but in all these instances glucose is formed instead of starch. 



According to whether starch ((C6Hio06)„) or glucose (C6H12O6) 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: 



(i) 6 CO2 + 5 H2O = CeHioOs + 6 O2. 



(2) 6 CO2 + 6 H2O = CfiHisOe + 6 O2. 



I 



Timiriazev^ 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 

 decreasing in intensity and ceasing not far beyond the D line. .Thus starch 

 is produced by those wave-lengths of light that cause the decompostion of 

 carbon dioxide, the rays between B and C being most effective in both cases. 



Briosi^ was unable to find starch in the leaves of Musa (banana) and Strelitzia, 

 but found oil instead, and expressed the opinion that the latter was the first 

 product of photosynthesis in these plants. HoUe' and Godlewski* were able 

 to prove, however, that this supposition is untenable. 



Baeyer^ advanced the hypothesis that formaldehyde is really the first prod- 

 uct of photosynthesis, and that carbohydrates arise from this by progressive 

 condensation or polymerization. The formation of f orm^dehyde thus supposed 

 is represented by the equation, CO2 + H2O = CH2O + O2. Baeyer based his 

 supposition upon a discovery by Butlerow^ that qxymethylene (CsHeOa) is con- 



' TimiriazeS, C, Enregistrement photographique de la fonction chlorophyllienne par la platite vivante. 

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



' [Briosi, Giovani, Ueber normale Bildung von Fettartiger Substanz im Chlorophyll. Bot. Zeitg. 31 : 529- 

 S33. 545-550. 1873] 



3 HoUe, H. G., Ueber die Assimilationsthatigkeit von Strelitzia regina. Flora, u. R. 3S: 113-120, 154- 

 160, 161-168, 184-192. 1877. 



* &odlewski, £mil, 1st das Assimilationsprodukt der Musaceen Del oder Starke? Flora, n. R. 35; 215- 

 220. 1877. 



5 Baeyer, Adolf, Ueber die Wasserentziehung und ihre Bedeutung fur das Pflanzenleben und die Gah- 

 rung. Ber. Deutsch. Chem. Ges. 3 : 63-75- 1870. 



' [Butlerow, A., Bildung einiger Zuckerarten durch Synthese. Liebig's Ann. Chem. u. Pharm. 120: 295- 

 298. 1861. Idem, Formation synth^tique d'une substance sucrfe. Compt. rend. Paris 53 ; 145-147. 1861.I 



