60 SOLAK LIGHT. 



tiality of digestion in the animal organism consists in the conversion 

 of food into a homogeneous fluid, a function which plants do not 

 exercise. But digestion also means the conversion of raw materials 

 into substances capable of being assimilated or rejected ; and in that 

 sense the word is here employed. 



These functions are performed by means of the vital forces 

 of vegetation, which we cannot estimate or comprehend, 

 assisted by the influence of an external agent, the nature of 

 whose action may be understood from its effects. That agent 

 is solar light. 



It is the property of solar light, when striMng upon the leaf 

 of a plant, either directly or indirectly to cause : 1. A de- 

 composition of carbonic acid ; 2. An extrication of oxygen ; 

 and, 3. Insensible perspiration. By their vital forces plants 

 appear to decompose water, independently of the action of 

 light. 



Carbonic acid is originally introduced into the interior of a 

 plant, either dissolved in the water it imbibes by its roots, or by 

 attraction from the atmosphere, or by the combination of oxygen 

 resulting from the decomposition of water or from other sources, 

 with the carbon in its interior. "When a leaf is exposed to the 

 direct influence of the sun, it gives off oxygen, by decomposing 

 the carbonic acid ; whereupon the carbon remains behind in 

 the interior of the leaf in a solid state. In the total absence 

 of solar light, there is little or no extrication of gaseous matter, 

 and what little is given off will be found to be carbonic acid, 

 which plants exhale at all times in small quantities ; oxygen 

 however, which was before expelled, is inhaled. Hence plants 

 decompose carbonic acid during the day, and acquire it again 

 during the night, and, during the healthy state of a plant, the 

 decomposition by day, and recovery by night, of this gaseous 

 matter, is perpetually going on. The quantity of carbonic acid 

 decomposed is in proportion to the intensity of the light which 

 strikes a leaf, the smallest amount being in shady places ; and 

 the healthiness of a plant is, caieris paribus, in proportion to 

 the quantity of carbonic acid decomposed ; therefore, the 

 healthiness of a plant should be in proportion to the quantity 

 of light it receives by day. 



