SECT. II 



PHYSIOLOGY 



215 



gaseous exchange between a plant and the atmosphere constitutes the 

 chief source of its nourishment ; and it required the courage of a firm 

 conviction to derive the thousands of pounds of carbon accumuhited 

 in the trees of a forest from the small proportion (0'03 per cent) of 

 carbon dioxide contained in the atmosphere. 



By means of the gas-bubble method it is easy to bring proof of 

 the statement made above that 

 only the green j^arts of plants, and 

 these only in light, are able to 

 assimilate CO^. Thus, the stream 

 of bubbles from an Elodea which 

 goes on briskly at a brightly lit 

 window becomes slower as the 

 plant is brought into the middle 

 of the room, and ultimately ceases 

 when the intensity of the light is 

 still such as to allow our eyes to 

 read. Within certain limits as- 

 similation increases in proportion 

 to the intensity of the light. 

 Similar experiments may be carried 

 out using artificial sources of light. 

 They show that all the methods of 

 illumination in common use may 

 be effective in the assimilation 

 of CO.,. When, however, mono- 

 chromatic light, from whatever 

 source, is employed it is found that 

 rays of difterent wave-length are 

 by no means of equal use in assimi- 

 lation. In spite of some opinion 

 to the contrary it may be stated 

 with certainty that red light (wave- pj,, 194 

 length A=680/xju.) has the maxi- 

 mum effect. Rays of short wave- 

 length are less effective, but recent 

 work appears to show that the blue 

 rays are more active in assimilation 

 than are the yellow or green rays. 



Since sunlight is in nature the indispensable factor in CO^-assimila- 

 tion it becomes at once clear why certain organs of the plant, the 

 foliage-leaves, have a flat expanded shape. Their large surface fits 

 them to absorb the light. If their function of CO.^-assimilation is 

 to be well performed the foliage leaves must not only have a large 

 surface but also be thin. Practically it appears that light which has 

 passed through one or two foliage leaves is unable to exert any further 



Evolution of oxygen from assimilating 

 plants. In the glass cylinder C', filled witli 

 water, arc placed shoots of Elodea canadensis ; 

 the freshly cut ends of the shoots are intro- 

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

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

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

 port the test-tube. 



