II.] STARCH FORMATION BY THE LEAF 29 



convenient for the purpose. Late at night, when the 

 removal of starch has been in progress for some time, 

 about three hundred leaves with their stalks attached 

 should be gathered and brought indoors. From the 

 heap of leaves pairs of approximately equal size are 

 picked out, and as each pair is made up one member of 

 it is placed in one heap which we will call A, the second 

 on another heap, B. When a hundred pairs have thus 

 been divided, the two heaps are weighed and will be 

 found approximately equal, because the errors of match- 

 ing each pair of leaves will neutralise one another with 

 so large a number as a hundred pairs. However, there 

 will probably be some small difference in weight which 

 must be noted. The next step is to put one heap (A) of 

 one hundred leaves into the oven to dry, the other 

 hundred are set with their stalks in bottles of water in a 

 position which will expose them to full daylight on the 

 following morning. At the close of that day they also 

 are put in the oven to dry. On comparing these sets of 

 dry weights (making any necessary allowance for initial 

 differences of green weight) the leaves that have been 

 exposed to the air for a whole day will be found to have 

 gained in weight, because of the starch they have formed 

 under the action of the light 



We have already described one experiment to show 

 that the leaf of a plant is also carrying on a second 

 process, one of respiration or breathing, which in its 

 result is exactly opposite to assimilation, in that it burns 

 up some of its carbonaceous matter, combining it with 

 the oxygen of the air by which it is surrounded to form 

 carbon dioxide and water. It is possible to show that 

 each part ^of the plant is producing carbon dioxide, 

 merely by gathering such parts in quantity, shutting 

 them up in a jar for an hour or so, and then testing the 

 gas within the jar with lime water for carbon dioxide ; 



