184 PHOTOSYNTHESIS 



starch is a product of photosynthesis and is formed in the chloroplasts. 

 These organs have the power of forming starch in the Hght and again 

 dissolving it in the dark. Sachs developed the macrochemical method 

 of determining the amount of starch in a leaf on the basis of the colora- 

 tion produced with iodine. By means of this method he also demonstrated 

 the necessity of light, chlorophyll and carbon dioxide for starch forma- 

 tion. He showed that starch represents a reserve food material and his 

 investigations indicate that starch formation is the result of an accumula- 

 tion of the products of photosynthesis. 



That starch is deposited in the chloroplasts, even in the dark, when 

 leaves are floated on solutions of soluble carbohydrates was demonstrated 

 by Bohm.* This was an exceedingly important observation. Already at 

 that time, as still, there existed an active controversy regarding the first 

 product of photosynthesis. Sachs took the stand that starch was the 

 first visible product. This method of producing starch in plants kept in 

 the dark, from soluble carbohydrates has been extensively investigated.^ 

 Starch is formed under these circumstances in the plastids, whether they 

 contain chlorophyll or not. Starch formation in photosynthesis is there- 

 fore probably a secondary reaction and cannot be regarded as a direct 

 product of photosynthesis, but is formed when the concentration of the 

 simple, soluble carbohydrates attains a sufficiently high concentration. 



Plants form starch in the dark, not only from the carbohydrates, 

 glucose, fructose, galactose, mannose, sucrose and maltose, but also from 

 other organic compounds. Thus alcohols such as mannitol, dulcitol, 

 erythritol and glycerine also are capable of being used for starch forma- 

 tion in the plant. The pentose, arabinose, is also capable of conversion 

 into starch according to reports of Polonovski '^ and Morillez. The fact 

 that such a variety of substances can be utilized by the plant for starch 

 formation would indicate that it is fallacious to draw conclusions re- 

 garding the first product of photosynthesis from the observation that 

 any particular substance produces starch. Nor can it be concluded that 

 all substances which form starch are normally the products of photo- 

 synthesis. 



The independence of starch formation and photosynthesis is further 

 illustrated by the fact that some plants, although they carry on active photo- 

 synthesis, never produce starch. Bohm showed this to be the case and 

 Meyer made a classification of the plants which form starch and those 

 which do not. The plants which form no starch contain disaccharides 

 and monosaccharides. There are also some plants which store their 

 carbohydrate in the form of inulin instead of starch. Bohm also showed 

 that some plants in which normally no starch is found, e.g. Galanthus, 



*Bohni, Bot. Zcitq., 41, iZ, 49 (1883). 



"Meyer, Bot. Zeitg., 44, 81, 105. 129, 145 (1886). Laurent, Bull. Sac. bot. 

 Belgium, 26, 243 (1887). Klebs, Untcrs. bot. Inst. Tiibmgen., 2, 489 (1888). 

 Bokorny, Biol. Centrabl., 17. 1 (1897). Saposchnikoff, Bcr. bot. Gcs., 7, 259 (1889). 

 Winkler, Jahrb. zviss. Bot., 32, 525 (1898). 



•Polonovski and Morillez, C. A., 19, 2064. 



