PHYSIOLOGY 



219 



to the starch abundantly present in the cells. In Fig. 199 the result 

 of the iodine reaction is shown on a leaf, part of which had been 

 covered with a strip of dark paper or tinfoil. The cells darkened by 

 the overlying paper or foil formed no starch, while those exposed to 

 the light are shown by the iodine reaction to be full of it. A green 

 leaf kept in air devoid of carbonic 

 acid, although fully exposed to the 

 light, will similarly form no starch. 

 The bye-product arising from 

 the assimilatory process is PURE 

 OXYGEN. The volume of oxygen 

 thus set free is nearly equal to (in 

 most cases it somewhat exceeds) 

 the volume of carbonic acid 

 taken in. If plants assimilate in 

 a known quantity of air containing 

 carbonic acid gas, its volume will 

 therefore remain .nearly the same. 

 The chemical process of assimila- 

 tion resulting in the decomposition 

 of the carbonic acid may be thus 

 expressed : 



6C0. 2 + 6H.,0 = C 6 H ]2 B + 60., 

 (Glucose). 



From this chemical equation( 30 ) 

 it is evident that the ELEMENTS OF 



WATER ARE REQUISITE FOR THE 

 PROCESS OF ASSIMILATION (p. 189). 



In the transformation of the mono- 

 saccharides into poly-saccharides 

 (starch, cellulose) which afterwards , 



Fio. 'JltO. Evolution ot oxygen from assimilating 

 plants. In the glass cylinder C, filled with 

 water, are placed shoots of Elodea canadensi? ; 

 the freshly cut ends of the shoots are intro- 



takes place, small amounts of 

 water are again liberated. 



duced into the test-tube B, which is also full 

 of water. The gas-bubbles E, rising from the 

 cut surfaces, collect at S. H, stand to sup- 

 port the test-tube. 



The oxygen given off by green 

 plants, although not perceptible 

 when they are growing in the open 

 air, becomes apparent in the case 

 of water plants. It was indeed through the evolution of bubbles of 

 oxygen from water plants that INGENHOUSS first had his attention 

 called to the assimilatory activity of leaves. To see this process, it 

 is only necessary to place a cut stem of a water plant in a vessel of 

 water exposed to the sunshine, when a continuous series of small 

 bubbles of gas will at once be seen to escape from the intercellular 

 passages intersected by the cut. The gas thus evolved may be 

 collected with little trouble (Fig. 200), and will be found to be chiefly 



