DOMESTIC GREENHOUSES. 
99 
organs by whi.n 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 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 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* 
