45] CHAP. II. PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 195 



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

 shoot of Elodca canadensis) be placed in water which holds carbon 

 dioxide in solution, and be exposed to sunshine, it will be seen that 

 from the cut surface of the leaf or stem bubbles" of gas are given off 

 at regular intervals (Fig. 128). These consist of oxygen. 



The relation of light and of chlorophyll to the formation of 

 organic substance 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 those 

 portions of the leaf upon which have fallen the rays of light which 

 correspond to the principal absorption-bands of the chlorophyll- 

 spectrum. 



It is, generally speaking, only 

 plants possessing chlorophyll 

 which can create organic sub- 

 stance. Inasmuch, therefore, as 

 organisms, whether plants or 

 animals, which do not possess 

 chlorophyll require for their nu- 

 trition more or less complex 



. , Fie. 128. Evolution of oxygen from a 



organic substances, they are water . plant (Elodea canadensis) : a the cut 



entirely dependent for their food stem ; g a weight that keeps the stem in its 



upon organisms which do pos- **"** 

 sess chlorophyll. 



This process is also of great importance in another direction. 

 All living organisms, speaking generally, absorb free oxygen and 

 evolve carbon dioxide in respiration. Those organisms which 

 possess chlorophyll prevent the excessive accumulation of carbon 

 dioxide in the atmosphere, and keep up the supply of free oxygen, 

 in that, under the influence of light, they absorb the former gas 

 from the air, and replace it by an equal volume of the latter. 



The characteristic difference between the anabolic capacity of 

 plants which do and of those which do not possess chlorophyll is 

 then this, that the former can produce, from carbon dioxide and 

 water, organic substances containing the elements C, H, and 0, 

 whereas the latter cannot produce these, but must be supplied 

 with them as food. From this point onwards the anabolic 



