48 EXPERIMENTAL PLANT PHYSIOLOGY 



Inference. — Starch is present in a green leaf that 

 has been picked in the afternoon of a sunny day. 



Now it will be remembered that the aim stated on 

 page 46 was to investigate the conditions under which 

 starch can be formed in the plant, and to ascertain, if 

 possible, the part taken by the air in its formation. 



Experiment 29 



Aim. — To find out if light and darkness influence 

 the formation of starch in the leaf. 



Method. — Over both sides of a leaf of a grow- 

 ing plant pin carefully pieces of silver paper. The 

 paper should be turned over at the edges and fast- 

 ened so that the pins 

 do not injure the leaf. 

 Cover another leaf 

 with two pieces of 

 paper, each of which 

 has a hole cut in the 

 centre. The two holes 

 must be alike in size 

 and shape, and the 

 papers must be placed 

 over the leaf-surfaces 

 so that the holes ac- 

 curately coincide. 

 In this way one leaf is completely in the dark ; the 

 other is darkened except for the central portion. The 

 leaves should be left for two days, or longer if the 

 weather is not sunny ; then they should be picked in 

 the afternoon and tested for starch. 



Observations. — No starch is present in the leaf that 

 was wholly covered. 



Starch is present in the uncovered part of the second 

 leaf (Fig. 22). 



Inference. — Starch is formed only under the influ- 

 ence of light ; it cannot he formed in darkness. 



