THE mo DUCTS OF PHOTOSYNTHESIS 215 



desiccated plants. The same was the case with pentosans ; starch was not 

 determined separately. 



b. Disaccharides and Monosaccharides. The First Sugar Formed. 



Although it has been very easy to demonstrate that starch accumulates 

 in the leaves as a result of photosynthesis, the question as to what is the 

 first carbohydrate formed in the process has been a subject of much dis- 

 pute. As Sachs showed, starch is the "first visible product of photo- 

 synthesis," but such a conclusion naturally is unsatisfactory when it is 

 desired to gain a clearer conception of the kinetics of the chemical reac- 

 tions which evidently start with such simple substances as carbon dioxide 

 and water and end with such a highly complex molecule as starch. In 

 view of the fact that starch yields d-glucose on hydrolysis, the sugges- 

 tion was made long ago that this sugar was the first product. This view 

 gained support from the observations of Bohm and of Meyer that the 

 hexoses, glucose and fructose, constitute the best material for starch 

 formation, when leaves are floated on solutions of various organic sub- 

 stances, and the rate of starch formation in the leaves in the dark is fol- 

 lowed. It should be stated, however, that Brown and Morris ^^ in their 

 experiments upon artificial nutrition of leaves by solutions of carbo- 

 hydrates found that sucrose surpassed all other sugars in the formation of 

 starch.^" The fact that the emission of oxygen commences almost im- 

 mediately on illumination of a green leaf and that the O2 : CO2 ratio is 

 very close to unity, while some time is always required for the appearance 

 of starch, gave further support to the view that glucose is a precursor in 

 the formation of starch. This view was also favored by the experimental 

 evidence which showed that starch was formed only when the concentra- 

 tion of glucose in the cell was above a certain concentration.^^ 



The theory that glucose is the first carbohydrate formed in photosyn- 

 thesis certainly has the advantage of relative simplicity and doubtless the 

 development of this idea was to some extent influenced by the conceptions 

 of carbohydrate chemistry which were being developed at the time. 

 Boussingault ^^ was probably the first to make a definite statement that the 

 first product elaborated from carbon dioxide and water by the leaves is 

 glucose. In 1870 Baeyer had proposed that formaldehyde was the reduc- 

 tion product of carbonic acid, and, based upon the discovery of Butlerow, 

 that formaldehyde is condensed to a hexose by means of alkali, a method 

 of synthesizing hexoses was developed by Fischer and Loew. Whether 

 these chemical reactions can find direct application to the synthesis of 

 carbohydrate in the leaf we shall leave for a later discussion. They have, 

 nevertheless, found wide application among chemists and plant physiolo- 



'' Brown and Morris, Jour. Chcm. Soc, 57, 484 (1890). 



"^Bohm, Bot. Zeitg., 41, 2,2,, 49 (1883). Meyer, ibid., 44, 81, 105. 129, 145 

 (1886) ; 43, 417, 433, 449, 465, 481, 497 (1885). 

 "'Schimper, Bot.'Zeitg., 43, 737 (1885). 

 "* Boussingault, Agronomic, etc., 4, 399 (1868). 



