60 AIR AND LIGHT. 



The conversion of sap into different kinds 

 of secretion is effected by the combined action 

 of Air, Light, and Temperature. 



Mr. Knight is of opinion, founded like all his 

 opinions, on well conducted experiments, that 

 the motion given to plants by the wind, enables 

 their fluids to circulate more freely and is thus 

 beneficial. 



Air and Light. 



When an embryo plant is formed within its 

 integuments, it is usually colorless, or nearly 

 so ; but, as soon as it begins to grow, that part 

 which approaches the light (the stem) becomes 

 colored, while the opposite extremity (the root) 

 remains colorless. 



The parts exposed to the air absorb oxygen 

 at night, absorb carbonic acid and part with 

 oxygen again in daylight ; and thus in the day- 

 time purify the air, and render it fit for the re- 

 spiration of man. 



The intensity of this latter . operation is in 

 proportion to the intensity of solar light to 

 which leaves are directly exposed. 



Its cause is the decomposition of carbonic 

 acid, the extrication of oxygen, and the acquisi- 



