58 EXPERIMENTAL PLANT PHYSIOLOGY 



are not green. It can, however, be shown experi- 

 mentally that the starch is not formed originally in the 

 non-green parts, but in the green leaves. 



By cutting off the leaves of growing daffodils, 

 hyacinths, and snowdrops, as soon as the flowering time 

 is over, it can easily be proved experimentally that the 

 store of food in the underground bulb is dependent 

 on the green leaves above : under such conditions no 

 new bulbs for next year will be formed. 



That the starch formed in the leaves does not remain 

 in them has already been proved by removing a plant 

 from sunlight into darkness (Experiments 29 and 30); 

 but, as yet, no explanation has been given of this. 



It seems natural to suppose that the starch is carried 

 away in the cell-sap ; but starch is not soluble in cell- 

 sap. If, therefore, it is to be removed in this way, it 

 must first be converted into some substance that is 

 soluble in the sap of the leaf. 



Now sugar is a substance which has a similar chemical 

 composition to that of starch, the change therefore of 

 starch to sugar would not be a great one ; and sugar is 

 a soluble substance. 



A test for the presence of sugar will now be given so 

 that the possibility of the conversion of starch in the 

 leaf into sugar may be investigated. 



Experiment 37 



Aim. — To test for the presence of sugar using (a) 

 grape-sugar; (b) cane-sugar. 



Method. — Make solutions of the sugars. Add to each 

 a few drops of copper sulphate solution, then excess of 

 strong potash solution, and boil. If no precipitate is 

 produced add a few drops of hydrochloric acid and boil 

 again. 



Observations. — In the case of the grape-sugar solu- 

 tion a red-brown precipitate is obtained after boiling 

 with potash ; the cane-sugar solution has to be boiled 

 with an acid before the precipitate is formed. 



