MANUAL FOR SUGAR GROWERS. 13 



and conveyed down the leaf -stalk into the stem, to 

 be used as food in the development of new tissues ; 

 or stored up as a reserve food-supply in the form of 

 sugar (in some plants as starch) to support growth 

 when the daily supply of carbon is insufficient to 

 meet some rapid development. An experiment 

 shows the formation of starch in the leaf, the in- 

 fluence of light on its production, and the fact of its 

 removal. The leaf of a plant — such, for instance, as 

 a rose — is placed in spirit for a short time until all 

 the green colouring matter is removed and the leaf 

 left white ; the leaf is now washed with water and 

 finally placed in water containing iodine in solution ; 

 the effect of the iodine on the starch is to tui'n it 

 blue, so that the leaf becomes of a blue-black colour 

 from the presence of starch. A small plant is taken 

 and excluded from sunlight for from twelve to 

 twenty-four hours, by being covered with a box, 

 or by other suitable means : if the leaves of this 

 plant be treated in the manner described above, 

 they will not be coloured blue by iodine, showing 

 the absence of starch, — the starch formed dui^ing the 

 period of exposure to sunlight being removed from 

 the leaf by vital processes, and used as a food- 

 material in the ordinary course of nutrition and 

 growth. 



Another important function of the leaf is transpi- 

 ration, or the evaporation of the superfluous water of 

 the crude sap in the form of vapour. The quantity of 

 water given off from the leaves of various crops has 

 been made the subject of experiment ; the amount 

 transpired is found to be very great, — as much as two 



