CAEBONIC ACID BY LIVING PLANTS. 
625 
of differences differs from the wth term in the “ Air ” column by a mean of T49 and 1-89 
respectively. 
It must therefore be concluded that part of the carbonic acid exhaled from vegetable 
tissues arises from a source independent of oxidation. It must therefore be due to a 
separation of the gas from one or several of the proximate constituents of their tissues 
in consequence of the chemical changes therein taking place. 
This is a view that is by no means novel. Rochleder* and others have already 
advanced it as an hypothesis. Indeed the former appears inclined to attribute the evo- 
lution of carbonic acid to this cause, even in some cases where it is ordinarily regarded 
as a direct product of the so-called plant respiration. Moreover there are several facts 
in the known chemistry of plant-growth which harmonize with this view. It is easily 
seen that the subtraction of the elements of carbonic acid from any vegetable principle 
will have the result of leaving a residue richer in both carbon and hydrogen, and hence 
of higher force-value. 
There are grounds for believing that woody tissue is derived from the starch secreted 
under the influence of sunlight in the leaves of the plant. But as these tissues contain 
a larger percentage of carbon, and have a higher force-value than starch, this change 
would be effected by the separation of a substance having a less force-value, as is the 
case with carbonic acid, in which the energy of combination is nearly exhausted. (A 
decomposition of this nature occurs in the slow changes by which wood is converted into 
lignite and coal.) The conversion of starch into fats in plants is a change which 
there are experimental grounds for believing to be one of constant occurrence. It has 
been observed that during the ripening of oily seeds the starch disappears as the oil is 
formed. This change can scarcely be explained without assuming the subtraction of 
carbonic acid as in the decomposition by which alcohol, glycerine, and fatty acids are 
derived from sugar in vinous fermentation. The direct conversion of sugar into wax, 
with the evolution of carbonic acid, is known to occur by the agency of bees. 
Sachs has shown that some seeds which contain no tannin develope that substance 
during the first growth after germination, with a simultaneous disappearance of starch. 
The period at which it occurs would preclude the direct formation of tannin from 
external sources. The tannins contain a less percentage of oxygen than the class of 
carbohydrates ; and their derivation from the latter, attended by the separation of car- 
bonic acid, is an hypothesis supported by the facts of the case. Without assuming 
unusual or improbable decompositions, it is scarcely possible to explain such conversions, 
unless this separation be assumed. 
The state in which the oxygen absorbed by the plant remains, until it is finally excreted 
in combination with carbon, is a point of great interest. In order to ascertain whether 
it might not possibly be occluded by the tissues, an experiment was performed. A con- 
siderable quantity of willow bark was placed in a flask connected with a Sprengel pump, 
and so arranged that it could be exhausted while heated in an oil-bath. At temperatures 
* Chem. u. Phys. d. Pflanzen, 1858, p. 113 und 151. 
