278 
PHYSIOLOGY. 
BOOK II. 
till it finally decays. But if it is placed in moist earth above 
the temperature of 32°, and screened from the action of light, 
its integument gradually imbibes moisture and swells; the 
tissue is softened, and acquires the capability of stretching ; 
the water is decomposed, and a part of its oxygen, combining 
with the carbon of the seed, forms carbonic acid, which is 
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 elevating themselves above it, acquiring a green 
colour by the decomposition of the carbonic acid they absorb 
from the earth and atmosphere, and unfolding in the form of 
two opposite roundish leaves. This is the first stage of vege- 
tation : the young plant consists of little more than cellular 
tissue ; only an imperfect developement 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 preparation for the medullary sheath, 
and consists of vertical tubes passing through and separated 
by cellular tissue. 
The young root is now lengthening at its point, and ab- 
sorbing from the earth its nutriment, which passes up to the 
summit of the plant by the cellular substance, and is, in part, 
impelled into the cotyledons, where it is aerated and evapo- 
rated, but chiefly urged upwards against the growing point or 
plumule. 
II. Forced onwards by the current of sap, which is con- 
tinually impelled upwards from the root, the plumule next 
ascends in the form of a little twig, at the same tinie sending 
downwards, in the centre of the radicle, the earliest portion of 
wood that is deposited, and compelling the root to emit little 
ramifications ; and simultaneously the process of lignification 
is going on in all the tissue, by the deposit of a peculiar secre- 
tion in layers within the cells and tubes. 
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 
