718 PART IV. THE PHYSIOLOGY OF PLANTS. 



The performance of this process can be readily demonstrated. 

 If a water-plant (e.g. a leaf of Potamogeton natans, or a portion of the 



shoot of fjlodea canadensis) be 

 placed in water which holds 

 carbon dioxide in solution, and 

 be exposed to sunshine, it will 

 be seen that from the cut sur- 

 face of the leaf or stem bubbles 

 of gas are given off at regular 

 intervals (Fig. 475). These 

 consist of oxygen. 



FIG. 475. Evolution of oxygen from a 



water-plant (Elodea canadensis) : a the cut The relation of light and of chloro- 



stem ; g a weight that keeps the stem in its h n to the evolution of oxygen by a 



Burftce ** ^'^^ **** **** ** "* green plant is strikingly illustrated by 



means of an aerobic Bacterium (Bac- 

 terium Termo), which is highly sensitive to tbe presence of oxygen. If a fila- 

 mentous Alga be placed under a cover-slip on a slide under the microscope, in 

 water containing numerous Bacteria, the Bacteria will be seen to collect along 

 the filament, attracted by the free oxygen which is being evolved. The same 

 preparation will serve to show which are the rays of light most active in the 

 process. If, instead of ordinary white light, a spectrum be reflected by the 

 mirror of the microscope on to the slide bearing the Alga and the Bacteria, the 

 Bacteria will not be distributed uniformly along the filament, as in white light, 

 but will aggregate at certain points (more especially in the red and in the 

 blue), which correspond with the principal absorption-bands of the chlorophyll- 

 spectrum. 



The relation of light and of chlorophyll to the formation of organic sub- 

 stance by a green plant can be demonstrated by the starch-method. For 

 instance, if a leaf of a starch-forming plant, which has been exposed to bright 

 light for some hours, be removed, decolourised by alcohol and tested with iodine, 

 it will assume a dark blue colour, showing an abundant accumulation of 

 starch. If a leaf, still on the plant, be exposed, not to white light, but to a 

 spectrum, the starch will be found to have accumulated in these portions of the 

 leaf upon which have fallen the rays of light which correspond to the principal 

 absorption-bands of the chlorophyll-spectrum. 



The process under consideration is one of fundamental impor- 

 tance. It is the great process in nature by which organic 

 substance is constructed, and in which kinetic energy absorbed 

 from without is converted into the potential energy of chemical 

 combination. For the energy of the rays of light which is used in 

 the construction of the organic substance is not lost, but is simply 

 converted into another form, and it can be recovered by undoing 

 the chemical work which has been performed. When a piece of 

 wood or of coal is burned, the heat and the light which are given 



