AGRICULTURAI, BOTANY, CHEMISTRY AND PHYSIOLOGY OF PLANTS IO99 



leaves of the plant were covered with strips of black paper. On the follow- 

 ing day the starch reaction was tested on some of the leaves, and it was found 

 that the leaves of the plants cultivated in the sugar solution only showed 

 a starch reaction clearly on their parts not covered with black paper, while 

 on those of the control plants the reaction was negative. 



In order to ascertain whether starch was formed at the expense of the 

 glucose by direct synthesis, or indirectly by a photosynthetic process at 

 the expense of the carbonic acid as a result of its complete oxidation, it was 

 desired to ascertain which part of the solar spectrum might possess most 

 influence on the phenomenon. For this purpose, the pre\'ious experiment 

 was repeated, but the colourless glass of the vessel was replaced by blue 

 glass. For control of this experiment, a plant cultivated in a solution 

 free from glucose was placed in a glass vessel of a similar colour to the preced- 

 ing one, but without caustic potash, and carbonic acid introduced. In 

 this plant, as in that of the previous experiment, the starch reaction was ne- 

 gative on some leaves; on others slight traces of colouring were shown. 



This suggests that for the formation of starch in plants growing in 

 a sugar solution, the same light rays are essential which are responsible 

 for the chlorophyll function. The reaction obtained with the blue glass 

 was much below that obtained with white light, and was equal to the reac- 

 tion shown by the control plants kept in the coloured vessel in the presence 

 of carbonic acid. 



It was then thought that if, with a plant growing in a sugar solution, 

 the formation of starch takes place in consequence of the complete oxida- 

 tion of glucose, b}- removing from the atmosphere not only the carbonic 

 acid but the oxygen as well, the starch would be prevented from forming 

 as the glucose could no longer oxidise. A plant grown in a sugar solu- 

 tion was therefore placed in a hydraulically closed bomb containing a 

 tube full of stick caustic potash and filled with pure nitrogen. The 

 starch reaction was negative. The experiment repeated several times 

 yielded the same result, although the plants still retained their \'itality. 



In order to check the preceding experiment, the plant was kept in the 

 bomb without the tube of caustic potash, and in an atmosphere deprived 

 of oxygen, containing in addition to the nitrogen little carbonic acid ; 

 the starch reaction was always positive. 



The results of these experiments therefore show that : 



i) Plants cultivated in glucose solution show the presence of 

 starch in their leaves, even in the absence of carbonic acid, but onh^ 

 when in the light ; 



2) The region of the solar spectrum which is most effective in the 

 formation of starch, under the above conditions, is the same one which is 

 most active in the chlorophyllian function ; 



3) Plants grown in a sugar solution in an atmosphere free of 

 carbonic acid and oxygen, do not form starch in their leaves even 

 when exposed to light. 



The logical conclusion is that the sugar absorbed by the roots is oxidised 

 in the interior of the plant itself by the atmospheiic oxygen until carbonic 



