THE LEAF 95 



ages), cut out a cross or other pattern, and wrap the foil 

 round a leaf (attached to the plant) so that the only 

 portion of the leaf which can be seen is that which shows 

 through the cut-out pattern (see Fig. 14). Press the tin- 

 foil tightly down on the leaf, to prevent light getting under 

 the edge of the cut portion, and expose the plant to sun- 

 light. If this is done in the morning the plant may be 

 tested in the afternoon, and it should then be found on 



FIG. 14. Experiment to show that starch is only formed 

 in the portions of leaves exposed to sunlight. The 

 left-hand figure shows the leaf wrapped in tinfoil, 

 from which a cross has been cut. The right-hand 

 figure shows the leaf after exposure to sunlight and 

 tested for starch. 



boiling the leaf and decolorising it in spirit, and putting 

 it in iodine solution, that we obtain a pattern, in blue, on 

 the leaf exactly similar to the portion exposed to the 

 light. That is to say, the part of the leaf exposed to 

 the light, and that part only, has been able to form 

 starch, or, in other words, assimilation only goes on in 

 the light. 



The coloration due to the iodine soon fades away, 

 but the leaves may be preserved for any length of time 



