THE FOOD OF THE PLANT 53 



Pick a leaf. Make a careful drawing of it, showing 

 accurately the position of the green and the white 

 portions. Then test for starch in the usual way. 



Observation. — Only that part of the leaf which was 

 originally green is turned blue by the iodine (Fig. 26). 



Inference. — Starch is formed in the green part of 

 the leaf only. 



Experiment 34 



Aim. — To determine whether the absorption of air 

 by a leaf is essential to the formation of starch in it. 



Method. — The leaves of a calceolaria or other small 

 potted plant are depleted of starch in the usual way. 



Three of the leaves are then smeared with vaseline ; 

 one on the upper surface, one on the lower, and the 

 third on both surfaces. 



The plant is put again into the light, and, when a 

 sufficient time has elapsed, the leaves are tested for 

 starch. 



It will be necessary in this case to get rid of the 

 vaseline before applying the iodine test. To this end 

 the leaves may be put into xylol or petrol until the 

 vaseline is dissolved. 



Observations. — Starch has been formed plentifully in 

 the leaf whose upper surface is vaselined. Very little 

 is found in the leaf that had vaseline on the lower 

 surface. In the leaf smeared with vaseline on both 

 surfaces no starch reaction is obtained. 



Inference. — No starch is formed if the absorption of 

 air is prevented. 



Three conclusions have now been drawn relating to 

 the formation of starch in the leaf : 



1. It cannot be formed in the dark. 



2. It cannot be formed in the absence of chlorophyll. 



3. It is formed in green leaves exposed to light under 

 ordinary atmospheric conditions. 



Now starch is made up of the three elements carbon. 



