THE FOOD OF PLANTS. <J.~> 



require twelve hours, others twenty-four or 

 even more) they will be found to show no blue 

 colour indicating that all the starch they con- 

 tained has been used up during the time they 

 have been in the dark. 



4. Using the plant which we now know 

 to have no starch in its leaves we can 

 prove that starch is only formed in the parrs 

 actually exposed to the light. Take some tin- 

 foil or lead-paper (such as is used for ta 

 packages), 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. 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 sunlight. If this is done 

 in the morning the plant may be tested in the 

 afternoon, and it should then be found on boil- 

 ing the leaf and putting it in iodine solution, 

 that we obtain a pattern, in blue, on the leaf 

 exactly similar to the portion exposed. 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. 



