11 



concert. Mr. Darwin inclines to the first of these explanations. 

 I do not propose to discuss theories, but call your attention to 

 tliese and similar facts; and would suggest to our younger mem- 

 bers that they should read Mr. Darwin's book on " The Power of 

 movement in Plants," as giving them much information presented 

 in an agreeable form. 



M. A. Pauchon has conducted a prolonged series of experi- 

 ments to ascertain the influence which light has on the ger- 

 mination of seeds. Failing to obtain satisfactory results by 

 the direct method, he compared the respiratory activity of seeds 

 under different conditions of light and obscimty, measuring this 

 activity by their absorption of oxygen. Taking identical parcels 

 of seeds of equal number and equal weight, he arrived at the 

 following conclusions : — 



1. Light constantly accelerates the absorption of oxygen by 

 gcrmiuating seeds. The advantage in favour of light varies from 

 a quarter to one-third of the amount of oxygen absorbed by the 

 seeds kept in the dark. 



2. There is a relation between the degree of illumination and 

 the amount of oxygen absorbed. The influence of light is very 

 manifest under a clear sky and bright sun ; with a cloudy sky it 

 gradually decreases. 



3. The acceleration produced by exposure to light continues 

 for some hours after the seeds are placed in the dark. 



4. The difference in quantities of oxygen absorbed in light 

 and darkness are greater in winter than in summer. Hence it 

 would appear that the influence of light upon this respiration is 

 more intense at low temperatures. 



Professor Pringsheim, by concentrating solar light on vege- 

 table tissue under a microscope, finds that the absorption of oxygen 

 increases with the intensity of light, and especially vnth the 

 intensity of the chemical rays. But the increasing intensity of 

 respiration involves danger to the vegetable tissue ; and the light 

 which is necessary for accumulating carbon becomes hurtful as soon 

 as oxidization exceeds power of assimilation. Chlorophyll, by its 

 luminous absorption, helps to balance these two functions ; by 

 its preference for chemical rays, it diminishes the respiratory 

 effort, and thus acts as a protecting screen or regulator of respi- 

 ratory action. 



