THE LADIES* MAGAZINE OF GARDENING. 
335 
in the experiments of De Saussure with common air the production and 
decomposition of carbonic acid by plants in sunshine must have been 
continually going on, yet, in all the analyses which he made, the air was 
found unchanged, either in purity or in volume; in other words, the 
processes of formation and decomposition of this acid gas exactly counter¬ 
balanced each other. 
Of the two processes which have been now described, each may be 
considered as in its nature and purpose quite distinct from the other; 
hence, their effects may be readily distinguished; neither do they neces¬ 
sarily 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 
temperature in which to display itself; and when that temperature falls 
below a certain point, which is very variable in regard to different 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 something more, for if light be excluded, vegetation proceeds 
imperfectly, 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 affected 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 produces in the air, this process closely resembles the function of 
respiration in animals, and may thus with propriety be deemed a physio¬ 
logical 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 support 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 cliromule, 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 
