OAK. 185 



these are performing healthy functions and are in 

 full vigour the action of sunlight causes them to 

 part with their oxygen. But as they approach the 

 season for their fall the active functions of assimi- 

 lation and exhalation become retarded. The oxygen 

 absorbed at night is not freely given off during 

 the daytime and its retention in the cellular tissue 

 causes, under the sun rays, the exquisite tinting 

 of Autumn. How much these striking effects of 

 colour may be partly dependent upon chemical 

 substances, other than oxygen, absorbed into the 

 tissues of plants from their roots towards the 

 approach of the season for the fall of the leaf, and 

 how much upon the action of light upon all these 

 substances, science has not yet been able to accu- 

 rately determine. It has been discovered that 

 there are a number of distinct pigments or colour- 

 ing matters of the nature of chlorophyll in the 

 tissues of plants. The presence of chlorophyll in 

 the superficial cells of leaves causes them, under 

 the action of light, to assume their green hue ; 

 and similarly the presence in varying proportions 

 of the other pigments, to which Mr. Alfred Russell 

 Wallace gives the collective name of chromophyll, 



