PLANTS. 
tain? also longitudinal vessels, and is visibly 
porous in some plants, and particularly the 
cane. 
2. On removing the cuticle, tire true bar.-: 
appears, and may be considered as a conge* 
ri.es of pulp or cellular substance, in which 
are placed a number of vessels, as well as 
longitudinal fibres. Tire vessels of the bark 
are differently situated, and destined for 
various uses, in different plants. In the bark 
of the pine, for instance, the inmost are 
lymph-ducts, exceedingly minute; those 
nearest the surface are gum or resiniferous 
vessels, for the secretion of the turpentine, 
and these are so large as to be visible to the 
naked eve. 
3d. The wood lies between the bark and 
the pith. Its substance is denser than that 
of the bark, and its structure more difficult 
to be understood. It is however generally 
supposed to consist ot two substances, the 
parenchymatous or cellular, and the ligneous. 
The ligneous parts are no more than a con- 
geries of old dried lymph-ducts. Between 
the bark and the wood a new ring of these 
ducts is formed every year, which gradually 
loses its softness as the cold season ap- 
proaches, and towards the middle of winter 
is condensed into a solid ring of wood. These 
annual rings, which are visible in most trees 
when cut transversely, serve as marks to de- 
termine their age. They seem to decrease 
in breadth, as the tree advances in age ; and 
as they are found to be very unequal in size 
throughout, their breadth probably varies 
according as the season is favourable or other- 
wise. 
Dr. Darwin distinguishes the wood into 
two parts, the sap-wood or alburnum, and 
the heart. The former is much less durable, 
and is most abundant in thriving trees. In an 
oak-tree the division between these parts is 
very distinguishable. The alburnum is gra- 
dually converted into heart; but we do not 
recollect to have met with any observations 
which determine the number ol years in 
which this conversion takes place. 
Dr. Darwin attributes to the sap-wood the 
office of nourishing the embryon buds. 
“ We may conclude,” says this author, 
“ that the umbilical vessels of the new bud 
are formed along with a reservoir of nutri- 
tious aliment about midsummer in the bark, 
which constitutes the long caudex of the 
parent bud, in the same manner as a reservoir 
of nutritious matter is formed in the root or 
broad caudex of the turnip or onion, for the 
, nourishment of the rising stem ; and that 
these umbilical vessels of the embryon bud, 
and the reservoir of nutriment laid up for it, 
which is secreted by the glands of the pa- 
rent bud, and now intermixed with the pre- 
sent bark of the tree, become gradually 
changed into alburnum, or sap-wood, as the 
season advances, in part even before , the end 
of the summer, and entirely during the win- 
ter months. 
“ That the alburnum of trees, which exists 
beneath the bark both of the trunk and roots 
of them, contains the nutritious matter de- 
posited by the mature leaves, or parent buds, 
for the use of the embryon buds, appears not 
only from the saccharine liquor which oozes 
from the wounds made in the vernal months 
through the bark into the alburnum of 'the 
birch and maple, betula et acer; but also 
from the following experiment, which was 
conducted in the winter, before the vernal 
sap-1 nice rises. 
“ Fart of a branch of an oak tree in Ja- 
nuary was cut off, and divided carefully into 
three parts; the bark, the alburnum, and the 
heart. These were shaved or rasped, and 
separately boiled for a time in water, and 
then .set in a warm room to ferment; and it 
was seen that the decoction of the alburnum 
or sap-wood passed into rapid fermentation, 
and became at length acetous, but not either 
of the other, which evinces the existence 
both of sugar and mucilage in the alburnum 
during the winter months; since a modern 
French chemist has shewn by experiments, 
that sugar alone will not pass into the vinous 
fermentation, but that a mixture of mucilage 
is also required ; and from this experiment 
it may be concluded, that in years ot scarcity 
the sap-wood of those trees which are not 
acrid to the taste, might afford nutriment by 
the preparation of being rasped to powder, 
and made into bread by a mixture ot flour, 
or by extracting their sugar and mucilage 
by boiling water. These observations have 
been since confirmed by the very accurate 
experiments of Mr. Knight, who has shewn 
that all the saccharine matter of fruit trees is 
elaborated in the leaves of the preceding 
year, and deposited in the alburnum, whence 
it is drawn in the following spring for the 
perfecting of the flower and the fruit. An 
essential caution (by the way) to unskilful 
pruners (such as the bulk ot common gar- 
deners are), who in cutting oil the new wood, 
or alburnum, in the spring, just cut oft so 
much of the fruit (see Pruning); and when 
gardeners (falsely so called) pull off the 
leaves of vines, and other fruit-trees, they 
destroy the crop of the. succeeding year. 
One striking difference between the wood 
aud the bark is,- that the former is possessed 
of spiral vessels which run from one end of 
the tree to the other. From the great re- 
semblance of these vessels to the air-vessels 
of insects, they are supposed to be subservi- 
ent to the same function. The stem ot some 
plants is entirely hollow ; partly, it is suppos- 
ed, from these plants, which are generally 
of a quick growth, requiring a more than oi- 
dinary supply of air. 
Dr. Darwin considers the spiral vessels 
above alluded to as lymphatics. lie admits 
that air is observed to issue both from green 
and dry wood cut transversely, which is dis- 
tinctly seen by plunging the wood in water, 
and removing the pressure ot the atmosphere 
by the air-ppmp. This circumstance, how- 
ever, he attributes to the rigidity of the fibres 
of wood, which, when divided, sutler the sap 
to escape, when, as the vessels cannot col- 
lapse, the air consequently enters in its place, 
lie illustrates and confirms his opinion with 
his accustomed ingenuity, and among other 
observations relates the following experi- 
ment; “I placed, in the summer of 1731, 
some twigs of a fig-tree with leaves on them, 
about an inch deep in a decoction of madder, 
and others in a decoction of log-wood, along 
with some sprigs cut off from a plant ot picris. 
"These plants were chosen because their 
blood is white. After some hours, on the 
next day, on taking out either of these, and 
cutting off from its bottom about an eighth 
of an inch ot the stalk, an internal circle ot 
red points appeared, which 1 believed to be 
the ends of absorbent vessels coloured red 
1 3 K 2 
44S 
with the decoction, and which probably exist- 
ed in the newly-formed alburnum, or sap- 
wood ; while an external ring of arteries was 
seen to bleed out hastily a milky juice, and at 
once evinced both the absorbent and arterial 
system. • . 
" Dr. Darwin admits the existence of air- 
vessels which pass through the bark to the 
sap-wood; but these run transversely, and 
not in the direction of the trunk or arms, 
j Du Hamel likewise observed large vessels, 
I some round and some oval, which in the 
birch-tree stand prominent, and pierce the 
outer bark. 
4th. The pith is situated in the centre ot 
the stem, and in young plants it is very abun- 
dant. It is said by some authors to consist 
of exactly the same substance as the paren- 
chyma or cellular substance of the bark ; and 
to be composed of small cells or bladders, 
generally of a circular figure, though in some 
plants, as the borage and thistle, they are 
angular. In most plants the pith gradually 
dies away as they approach to maturity ; and 
in old trees it is almost entirely obliterated. 
The pith appears to be essential to the lile 
of the other parts in young shoots. In those 
plants which have hollow stems, this central 
cavity, though not filled with the pith or 
medulla, appears to be lined with it. 
! Such are the solid parts of plants ; but to 
| render their organization more clearly under- 
j stood, in Plate Miscel. fig. 186, is the section 
ofa branch of ash cut transversely, asit appears 
to the eye. Fig.' 187, is the same section mag- 
nified : A A the bark ; BB an arched ring of 
'■ sap-vessels next the cuticle; CCM the cel- 
lular substance of the bark, with another 
arched row of sap-vessels ; DD a circular 
line of lymph-ducts next the wood ^ EE the 
wood ; F the first year’s growth ; G the se- 
cond ; FI the third; III the true wood ; KK 
the great air-vessels ; LL the lesser air-ves- 
sels ; N the pith. 
There is reason to believe that the proper 
entrance of the air to plants, is through the 
cuticle; which is proved to be a vascular 
’substance, since, when under an exhausted 
receiver, it issues directly through the cuticle. 
That the air is necessary to the sustenance 
of plants, appears from the experiments of 
Dr. Bell. In the winter season he covered 
several young trees with varnish, leaving the 
tops of the branches only exposed to the air. 
They remained in this situation during the 
following summer, when some of them lived, 
though in a languid state; but those from 
which the air had been more accurately ex- 
cluded, died without a single exception. To 
this proof the same author adds, that trees 
overgrown with moss have few leaves, weak 
shoots, and scarcely any fruit; and that it 
is the common practice of ail judicious gar- 
deners to strip the moss from the bark of 
aged trees, which by admitting the air gene- 
rally restores them to vigour and f: uitfulness. 
II. The root, which fixes the plant to the 
earth, and is the chief source of its nourish- 
ment, differs much in different spec es of 
vegetables. All roots agree in being fibrous 
at their extremities, and it is by their fibres 
chiefly that they are iitted to draw nourish- 
ment from the earth. The internal struc- 
ture of the root, or rather of its fibres, differs 
not very materially in general from that of 
the stem. It consists of a cuticle, bark, 
wood, and commonly ol a small portion of 
