XI. THE PLANT 231 



What has been said about absorbed carbon dioxide is equally true 

 of the evolved oxygen in assimilation or of carbon dioxide in respira- 

 tion. 



Diffusion is also quite capable of accounting for the transpiration 

 of water through the stomata, and the outward movement of water 

 or oxygen would thus not interfere with the inward passage of the 

 carbon dioxide. 



With reference to the chemical reactions which attend the assimi- 

 lation of carbon dioxide by plants much work has been done. The 

 chlorophyll granules frequently enclose starch granules, and for a 

 long time it was thought that starch was the first body formed in the 

 assimilative act, and that sugar, also detected in leaves, was formed 

 entirely from starch by hydrolysis. 



It was shown in 1886 by Meyer l that leaves floated on solutions 

 of sugar were able to produce starch. From levulose (10 per cent 

 solution) almost all the leaves tried produced starch, a smaller pro- 

 portion were able to utilise dextrose, while still fewer could form 

 starch from galactose. Baeyer, in 1870, suggested that the formation 

 of carbohydrates by leaves was probably effected by the formation of 

 formaldehyde, 0=CH 2 , and its subsequent polymerisation. Very 

 little evidence in favour of this view was forthcoming, however ; 

 indeed Bokorny in 1888 2 showed that formaldehyde itself would not 

 act as a source of starch owing to its poisonous action. In 1891 3 he 

 succeeded in using as a nutrient a dilute solution of sodium hydroxy- 

 methylsulphonate, CH 2 (OH).S0 3 Na, which readily splits up into for- 

 maldehyde and sodium hydrogen sulphite 



CH,(OH).S0 3 Na = OCH 2 + NaHSO 3 . 



By the addition of sodium or potassium phosphate the injurious ef- 

 fect of the acid sulphite upon the plant could be prevented, and he 

 then found that the leaves of Spirogyra majuscula were able to form 

 starch from a very dilute solution of this formaldehyde derivative. 

 Baeyer's hypothesis was thus confirmed. 



According to Brown and Morris 4 cane sugar is probably the first 

 sugar formed in the process of assimilation. This view is supported 

 by the recent work of Parkin on the assimilation of the leaves of 

 the snowdrop. 5 Its formation proceeds until the cell-sap attains a 

 certain concentration, varying in different plants, when starch granules 

 begin to form from it. These are intended as reserve materials and 

 become attacked by the diastase, present in all leaves, as soon as the 

 sugar solution, by diffusion to other portions of the plant, sinks below a 

 certain concentration. It is thought that inversion of the cane sugar 

 into dextrose and levulose precedes its translocation from cell to cell 

 and that maltose is the chief product of the diastatic action on starch ; 

 that, of the invert sugar formed in the plant, the dextrose is first used 



Zeit., 1886, Nos. 5 and 6 ; Jour. Chem. Soc., 1886, Abstracts, 902. 

 a Landwirt. Versuchs-Stat., 1889. 



3 Ber. deut. hot. Gesell., 1891, 103; Jour. Chem. Soc., 1891, Abstracts, 1539. 

 4 Jour. Chem. Soc., 1893, Trans., 604. 

 5 Biochem. Jour., 1911, 6, 1 ; Jour. Chem. Soc., 1911, Abstracts, ii. 1127. 



