d with life. The proof of this proposition is simple; and it
shows at the same time, in the simplest way, what a plant does with the
water and carbonic acid it consumes. Namely, 1st, it is only in sunshine
or bright daylight that the green parts of plants give out oxygen
gas,--then they regularly do so; and 2d, the giving out of this oxygen
gas is required to render the chemical composition of water and carbonic
acid the same as that of _cellulose_, that is, of the plant's permanent
fabric. This shows why plants spread out so large a surface of foliage.
Leaves are so many workshops, full of machinery worked by sun-power. The
emission of oxygen gas from any sun-lit foliage is seen by placing some
of this under water, or by using an aquatic plant, by collecting the air
bubbles which rise, and by noting that a taper burns brighter in this
air. Or a leafy plant in a glass globe may be supplied with a certain
small percentage of carbonic acid gas, and after proper exposure to
sunshine, the air on being tested will be found to contain less carbonic
acid and just so much the more oxygen gas.

452. Now if the plant is making cellulose or any equivalent
substance,--that is, is making the very materials of its fabric and
growth, as must generally be the case,--all this oxygen gas given off by
the leaves comes from the decomposition of carbonic acid taken in by the
plant. For cellulose, and also starch, dextrine, sugar, and the like are
composed of carbon along with oxygen and hydrogen in just the
proportions to form water. And the carbonic acid and water taken in,
less the oxygen which the carbon brought with it as carbonic acid, and
which is given off from the foliage in sunshine, just represents the
manufactured article, cellulose.

453. It comes to the same if the first product of assimilation is sugar,
or dextrine which is a sort of soluble starch, or starch itself. And in
the plant all these forms are readily changed into one another. In the
tiny seedling, as fast as this assimilated matter is formed it is used
in growth, that is, in the formation of cell-walls. After a time some or
much of the product may be accumulated in store for future growth, as
in the root of the turnip, or the tuber of the potato, or the seed of
corn or pulse. This store is mainly in the form of starch. When growth
begins anew, this starch is turned into dextrine or into sugar, in
liquid form, and used to nourish and build up the germinating embryo o