PLANTS. 
7 
wood and pith ; but this is not the case with suc- 
culent vegetables, such as carrots, in which the 
bark forms almost one half of the semidiameter of 
the root, and in dandelion it is nearly twice as thick 
as the woody part. 
The bark is composed of two substances; the pulp, 
which is the principal part, and a few woody fibres. 
The pulpy part is full of pores, or vessels, which 
are not pervious so as to communicate with each 
other; but consist of distinct little cells or bladders, 
scarcely visible without the assistance of a micro- 
scope. In all roots these cells are constantly filled 
with a thin watery liquor. In many roots, as the 
horse-radish, asparagus, potatoe, &c, the pulp is of 
an uniform structure. But in others it is more di- 
versified, and puts on the shape of rays, running 
from the centre towards the circumference of the 
bark. These rays generally stand at an equal di- 
stance from each other, in the same plant ; but the 
distance varies greatly in different plants. A num- 
ber of ligneous vessels are dispersed through the 
substance of the root, which are tubular, and serve 
for the conveyance of the sap to nourish the trunk 
and branches. These vessels are disposed in a lon- 
gitudinal direction, and ascend the whole length of 
the plant. Instead of running in a direct line, as we 
should naturally expect to find them ; they incline 
at small distances, towards each other, so as to form 
packets of fibres, connected together by the pulpy 
substance of the root. Thus the vessels pass in an 
oblique direction from one range to another, so as 
