“Se 
; The Life of the Plant. ee 
solved in the spring at the commencement of the period of 
vegetation, and thea employed in the production of new 
organs, especially the leaves. 
The decomposition of the carbon dioxide contained in the atmosphere 
is the main source of the carbon required for building up the tissues of 
plants. Only those cells which contain chlorophyll, and then only under 
the influence of sunlight, have the power of decomposing the carbon 
dioxide which they take up, and of producing organic compounds out 
of its elements and those of water, with elimination of an equal volume 
of oxygen, or in other words of assimilating. It is highly probable that 
in this process the carbon dioxide loses only one half of its oxygen, the 
other half of the oxygen which is evolved resulting from the decompo- 
sition of water. It is only living chlorophyll that has the power of de- 
composing the carbon dioxide ; and in general both sides of the leaf take 
part in this process, though with different energy. Light and heat are 
absolutely necessary for it ; a small deficiency of heat may be compen- 
sated by more intense light, but there are limits which cannot be passed 
(see Fig. 373, p. 213). Plants which do not contain chlorophyll have no 
power of decomposing carbon dioxide, and hence of assimilating; they 
must therefore obtain their nutriment from substances already assimi- 
lated; and hence grow either on or in animals or other plants as parasites, 
and obtain their nutriment from them, or live [as. saprophytes] on 
organic substances actually undergoing decomposition, and absorb their 
organic constituents before they are completely decomposed. 
The mode of formation of albumznozds is still unknown ; it has not 
even been accurately ascertained at what spot they are produced. The 
most probable solution is that they are formed at those spots where | 
calcium oxalate is stored up, because this salt is formed by the decom- 
position of calcium sulphate, the sulphur of which passes over into 
the albuminoids. (See p. 168). 
Neither is it yet known how cellulose is formed, although access of 
atmospheric oxygen appears essential to its production. It is tolerably 
certain that starch, sugar, inuline, and oil are the materials out of 
which the protoplasm constructs the cell-wall ; at least substances of this 
description which are stored up in the cells always disappear when an 
energetic formation of cellulose is going on. 
Starch is invariably formed originally within the grains of chlorophyll. 
Light is necessary for this ; but the very small intensity of light which 
" suffices for the formation of chlorophyll is not sufficient for that of starch 
or of the drops of oil which sometimes replace it. In direct sunshine 
starch is produced in the green parts of plants almost at the very | 
