without fresh Supplies of Water and Air. 501 



may be considered as in its nature and purpose quite distinct 

 from the other ; hence, their effects may be readily distinguished ; 

 neither do they necessarily interfere, when actually working 

 together. The first, or deteriorating, process, in which oxygen 

 gas is consumed, goes on at all times and in all circumstances, 

 when vegetation is active. It requires always a suitable temper- 

 ature in which to display itself; and when that temperature falls 

 below a certain point, which is very variable in regard to dif- 

 ferent plants, the process is more or less completely suspended, 

 again to be renewed when the temperature shall again return. 

 This conversion of oxygen into carbonic acid is as necessary to 

 the evolution of the seed as to the growth of the plant, and is all 

 that is required for germination ; but the plant requires some- 

 thing more, for, if light be excluded, vegetation proceeds imper- 

 fectly, and the plant does not then acquire its proper colour, and 

 other active properties which it ought to have. The chief 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 trans- 

 mission 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 temper- 

 ature ; at least, it proceeds in temperatures too low to support 

 vegetation, provided light be present, an agent not required 

 for germination, nor essential to vegetable developement. 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 chro- 

 mule, or colourable matter, in their cells, to which it imparts 

 colour 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 por- 

 tion 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 essential to living action, and affected the well- 

 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 proceeding in circumstances and under con- 

 ditions incompatible with living action. By withdrawing the air 

 altogether, or depriving it of oxygen gas, vegetation soon ceases 



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