DOMESTIC GREENHOUSES. 



99 



organs by which the consumption of oxygen gas is effected are 

 the leaves ; and its purpose, in great part at least, seems to be 

 that of producing some necessary change in the sap during its 

 transmission through those organs, on its way from the vessels of 

 the wood to those of the inner bark, whereby it may be rendered 

 fit for the purposes of nutrition and growth. In its nature and 

 object, therefore, as well as in the specific change which it pro- 

 duces in the air, this process closely resembles the function of 

 respiration in animals, and may thus with propriety be deemed a 

 physiological process. The second, or purifying process, in which 

 oxygen gas is evolved, differs in all respects from that which has 

 just been described. It is in a great measure independent of 

 temperature ; at least it proceeds in temperatures too low to sup- 

 port vegetation, provided light be present — an agent not required 

 for germination, nor essential to vegetable development. The 

 organs by which this process acts on the air are, as before, the 

 leaves ; not, however, by changing the qualities of the sap in the 

 vessels of those organs, but by producing changes in the chromule, 

 or colorable matter, in their cells, to which it imparts color and 

 other active properties. In doing this, it does not convert the 

 oxygen gas of the air into carbonic acid, but, by decomposing that 

 acid gas, restores to the air the identical portion of oxygen of 

 which the former process had deprived it. The former process, 

 carried on by the agency of the oxygen gas of the air, was essen- 

 tial to living action, and affected the w T ell-being of the whole 

 plant ; that exercised by the agency of light is not necessary to 

 life, is local, not general in its operation, and is capable of pro- 

 ceeding in circumstances and under conditions incompatible with 

 living action. By withdrawing the air altogether, or depriving it 

 of oxygen gas, vegetation soon ceases through the whole plant ; 

 but the exclusion of light from any part of the plant affects that 

 part only ; and even the total exclusion of that agent only de- 



