236 
PHYSIOLOGY. 
BOOK II. 
the temperature of 32°, and screened from the action of hght, 
its integument gradually imbibes moisture and swells, the 
tissue is softened, and acquires the capability of stretching, 
oxygen is absorbed, carbonic acid expelled, nutritious food for 
the young parts is prepared by the conversion of starch into 
sugar, and the vital action of the embryo commences. It 
lengthens downwards by the radicle, and upwards by the 
cotyledons ; the former penetrating the soil, the latter elevat- 
ing themselves above it, acquiring a green colour by the 
deposition of the carbon they absorb from the atmosphere, and 
unfolding in the form of two opposite roundish leaves. This 
is the first stage of vegetation : the young plant consists of 
little more than cellular tissue ; only an imperfect develope- 
ment of vascular and fibrous tissue being discoverable, in the 
form of a sort of cylinder, lying just in the centre. The 
part within the cylinder, at its upper end, is now the pith, 
without it the bark ; while the cylinder itself is the prepara- 
tion for the medullary sheath, and consists of vertical fibres 
passing through and separated by cellular tissue. 
The young root is now lengthening at its point, and 
absorbing from the earth its nutriment, which passes up to the 
summit of the plant by the cellular substance of the pith, and 
is thence impelled into the cotyledons, where it is aerated 
and evaporated, and urged upwards against the growing 
point or plumule : such of it as is not fixed in the cotyledons 
passes down through the bark into the root. 
Forced onwards by the current of sap, which is continually 
impelled upwards from the root, the plumule next ascends in 
the form of a little twig, at the same time sending roots 
downwards in the centre of the radicle, in the form of fibres, 
which become the earliest portion of wood that is deposited : 
these fibres, by their action, now compel the root to emit 
little ramifications. Previously to the elongation of the 
plumule its point has acquired the rudimentary state of a leaf : 
this latter continues to develope as the plumule elongates, until, 
when the first internode of the latter ceases to lengthen, the 
leaf has actually arrived at its complete formation. When 
fully grown it repeats in a much more perfect manner the 
functions previously performed by the cotyledons : it aerates 
