brown in others, &c. Every root, according to Grew, after it has arrived
at a certain age, has a double skin, the one coeval with the other
parts, and exists in the seed; a ring is afterwards sent off from the
bark, which forms the second skin; thus in the root of dandelion,
towards the end of May, the original or outer skin appears shrivelled,
and is easily separated from the new one, which is fresher, and adheres
more firmly to the bark. Perennial plants are supplied in this manner
with a new skin every year; the outer one always falls off in the autumn
and winter, and a new one is formed from the bark in the succeeding
spring.
OF THE VESSELS WHICH ARE CONTAINED BETWEEN THE RIND AND THE BARK.
These are called by Du Hamel the cellular coat, enveloppe cellulaire; by
Hill, the exterior vessels, and the vasa propria exteriora.
It has been already observed, that in trees the juice vessels, or vasa
propria, do not form those constituent parts of the wood of which the
timber consists, but that it is from the nature of these recipient
vessels that it derives its virtues, qualities, and specific
properties.[133] A tree may grow, live, and give shade without them; but
on those its peculiar character and decided virtues depend; these are
greatest where the vasa propria are largest or most numerous; and where
we do not find these, we scarce find any thing that will affect the
taste or the smell. There are different ranges of these vessels between
the several parts, each of which has its allotted place, its peculiar
form, its different structure, and its separate use. Many trees have
them in all their parts, others only in some of them, while others do
not exhibit any.
[133] Hill’s Construction of Timber, p. 73.
On taking off the rind, we find a substance of a deep green colour,
succulent and herbaceous, formed of a prodigious number of filaments
interwoven together in various directions; it is more abundant in some
trees than in others, particularly in the elder, and more succulent in
summer than in winter; it is then also less adherent to the rind. Dr.
Hill thinks the best time of separating the rind, in order to view this
part, is in a living branch, at the time of its swelling for the spring,
or for the midsummer shoot, but much easier by the means of
maceration.[134]
[134] Hill’s Construction of Timber, p. 75.
When the rind is perfectly separated, it leaves the vasa propria of this
class behind it; they scarce adhere to the inner bark, and but little to
the rind; they are disposed in packets, and do not run straight down the
branch, but interweaving with each other, form a kind of net. These
packets may be separated easily from the bark; when a thin transverse
section of one of them is examined, it is found to be composed of
twelve or fifteen distinct vessels with hard rinds. Dr. Hill says, that
with a great deal of patience, a vast number of objects, and a good
microscope, we may see by what means these vessels adhere to the bark;
for we shall find upon the sides small oval depressions which fit
thereto, and that are probably a kind of glands, that separate from the
general store of sap, with which the bark is filled, the juices peculiar
to these vessels.
OF THE BARK.
The bark lies next within the rind, and differs but little from it in
construction, though it holds a more important office in the scale of
vegetation, the growth and qualities of the tree being in a great
measure connected with it. It is, therefore, found to differ
considerably in substance, quantity, and quality, in various kinds.
It is originally the outer membrane, covering the lobes of the seed.
Even there, as in the branch of a tree, it appears in the form of a kind
of spunge, or like a crust of bread, composed of flatted bladders.
Its spunge-like nature may be further inferred from the contraction of
its pores when dry, and the ease with which they dilate when in water.
Grew has called it a most curious and exquisitely fine, wrought spunge.
In the course of its growth, the outer ranges of these bladders drying,
it becomes what we call the rind; for the rind was once bark, and has
only suffered a slight change in separating from it.
By the bark the tree is fed with a continual supply of moisture,
protected from external injuries, and defended from the excesses of
heat and cold; for these purposes it is variously disposed in different
trees. In the hardy and slow growing, as the oak and chesnut, it is
thin; in the quick growing, as willow, poplar, and the like, it is
thick. And what is more particularly to be attended to is, that in some
its inner verge is radiated. There are some trees, and a great many
herbaceous plants, in which this part is continued inward, in form of
rays, through the blea into the wood, and seems to form so many green
wedges, that split as it were the substance of both those parts;[135] a
circumstance which accounts for the vegetation of some particular trees,
which are known to live when deprived of the bark; because they have
rays of the same substance within which answer the purpose, and this in
a degree answering to the nature of their life.
[135] Hill’s Construction of Timber, p. 118. Ibid. p. 120,
The bark appears to be formed, first, of longitudinal fibres, which Du
Hamel considers as so many lymphatic vessels; secondly, by a sort of a
filmy cellular tissue, which has been considered as a kind of bladders
by some, or as parenchymatous by others; thirdly, of the vasa propria
interiora, or interior juice vessels.
The longitudinal fibres are disposed in strata, which lie one over the
other. In that stratum which is next the rind, or rather the cellular
coat, we perceive a net of longitudinal fibres, the meshes of which are
large and easily distinguished, particularly when the cellular tissue
that fills up the interstices is removed. To do this, the branches
should be macerated for a considerable time; some require to be kept in
this state for years. It will then be easy to separate first the rind,
then the cellular coating, and afterwards this pulpy matter. It may
sometimes be easily removed after the branches have been boiled.
The most exterior stratum, when examined by the naked eye, seems to be
formed of simple fibres, which graft, solder, or inosculate one with the
other; but when examined by a microscope, each of these fibres will be
found to be a bundle of filaments, which may be easily separated from
each other.
Grew says, that each filament, like the nerves in animals, consists of
twenty or thirty small contiguous tubes, which run uniformly from the
extremity of the root, without sending off any branches, or suffering
any change in their size and shape. Hence the bark may be torn or
divided lengthwise, with greater ease than in an horizontal direction;
when macerated, they are capable of a very great degree of subdivision.
The filaments of a cortical vessel are to be looked on, agreeable to
what we have already observed, as so many little bundles placed near
together, and at first growing parallel to each other; but soon quitting
this direction, the filaments of one fascicle parting from that to which
they originally belonged, and inclining more or less obliquely towards
another, sometimes uniting with it, at others, bending backwards, and
uniting again with that from which it proceeded, or with some one that
it meets with. In this manner new fascicles are often formed, while
other parcels are increased or diminished by the additions of new
filaments; by these means, a kind of irregular net is formed, and the
fibres proceed in a serpentine line from the top to the bottom of the
tree.
The thickness of the bark is entirely formed of strata of these
longitudinal fibres, which lie one over the other; each of these strata
is similar to the exterior one, only the meshes are smaller, and the
fibres finer, in proportion as they are more interior, insomuch that at
last the meshes are almost annihilated, and the fibres seem to lie quite
parallel to each other.
There are some trees, however, where the meshes are not visible, and in
which the fibres lie quite in a straight direction. There are many other
circumstances in which they vary in different trees; in some the meshes
of each stratum correspond with each other, diminishing gradually in
size as they are more interior, and forming as it were so many conical
cells.
We may, I think, conclude from what has been said, that the bark is
composed of several thin membranes, which extend over the whole exterior
surface of the tree. The most exterior membrane is the rind; under this
is what Du Hamel calls the cellular coat; next to this the cortical
stratum or true bark of the tree, which is formed of lymphatic vessels
ranged more or less in a reticular form, and of the vasa propria
interiora. The meshes are so constituted as to form large cavities next
the rind, and small ones near the wood. These cavities are filled with a
parenchymatous substance or the cellular tissue, which being continued
from the wood to the rind, joins and unites the cortical stratum, and
afterwards spreading on the outside thereof, forms what has been termed
the cellular coat.
OF THE CELLULAR TISSUE.
We now proceed to give some account of the substance which fills up the
vacant spaces that are left between the longitudinal fibres. It is
called by Grew the parenchyma or pulp, by Malpighi, the vesicular tissue
or web; both of them consider it as formed of small bladders or
reticles, that are in contact with each other, lying in an horizontal
position, or at right angles to the longitudinal fibres: they do not
suppose them to be all of the same size, or even of the same figure:
Grew compares it to the froth of beer or eggs. The flesh of fruits
consists for the most part of this substance, very much filled with
juice, though with considerable difference in its organization. Be this
as it may, the nature of this substance, its form and structure, are at
present but very little known. It is floccose, and varies in colour in
different species.
OF THE VASA PROPRIA INTERIORA.
Besides the lymphatic vessels and the cellular substance, we find the
juice vessels, or vasa propria, in the bark. In those trees which are
famous for medicinal virtues, they are usually very large; they carry
the milky juices of the sumach, and in them is lodged the finest and
highest-flavoured turpentine in all the kinds of pine. Dr. Hill thinks
that a tree of that genus exhibits them best, and the more, as the
turpentine which fills them may be perfectly dissolved in spirit of
wine. The pinus orientalis is the species in which these vessels are
most distinctly seen.
OF THE BLEA.
This is that part of the tree which is formed into wood, and therefore
lies between it and the bark, and may be separated from them by
maceration.
A longitudinal piece of the blea, when examined by the microscope,
exhibits a number of vessels running parallel to each other, the
interstitial spaces being filled with a floccose, white, formless
substance, of which Dr. Hill suspects even the vessels themselves to be
formed. Innumerable small openings or mouths may be discovered in these
vessels, suited to imbibe the moisture which is so essential to the life
and health of plants. These mouths cannot be well discerned, except
when they are opened by the season of the year, either before the first
leaves of spring, or in the midsummer shooting time; though a small
quantity of moisture will keep them open at that time, yet no quantity
would be sufficient at an improper season.[136]
[136] Hill’s Construction of Timber, p. 47.
The blea is a zone more or less perfect, which lies under the bark, and
covers or surrounds the wood, and is principally distinguished from it
by being less dense. In some species the difference between the blea and
the wood is very remarkable, in others it is less so.
The ancient botanists, struck with the difference they observed between
the wood and the blea, compared this substance to the fat in animals.
Malpighi, Grew, and Du Hamel considered it as the wood not yet arrived
to a state of perfection. It is organized in a manner similar to the
wood, and possessing the same vessels disposed nearly in the same
manner. The juice vessels of this part may be separated from it by
maceration; Dr. Hill says, that in this state they appear perfect
cylinders, with thick white coats, the surface perfectly uniform.
OF THE WOOD.
When the bark and the blea are taken away, we come to the wood, which is
a solid substance, on which the strength of the tree depends, and which
has been considered by naturalists as being to the tree what bones are
to the animal. The wood, in a general view may be considered as formed
of strata, which are inclosed one within the other; these strata consist
of ligneous fibres or lymphatic vessels, the cellular web or tissue,
vasa propria, and what have been called the air vessels. It is more
difficult to investigate the construction of the wood than that of the
other parts, because the texture is in general much harder, and
therefore not so easily separated, requiring very long macerations, and
many subjects, before one may be found fit for examination.
If a transverse section of almost any kind of wood be examined, we shall
perceive these strata very clearly and sensibly distinguished from one
another. It has been generally supposed that each of these is the
product of one year’s growth; though, if we cut the same wood obliquely,
it will be found that each of these strata is compounded of smaller
ones, which are therefore not so easy to discover as the larger. By
macerating rotten pieces of trees, the wood may be divided into an
immense number of leaves or strata, thinner than the finest paper.
If the foregoing strata be examined in their detached state by the
microscope, we shall find them to be composed of longitudinal fibres;
some pieces of rotten wood, after maceration, will divide of themselves
into very fine longitudinal fibres; the existence of these is further
proved by the facility with which wood may be split in the direction of
these fibres. From hence we may collect, that the ligneous strata are
formed of small fibres or vessels, collected together in fascicles, like
the bark: in some trees they are parallel to each other, in others they
are disposed more obliquely, crossing and forming an irregular kind of
network. There is great probability that this reticular disposition
exists in all trees, though it may be difficult to discover it in many
on account of the fineness of the meshes, the hardness of the wood, and
the sameness of colour in the constituent fibres.
We are here only speaking of the lymphatic vessels or ligneous fibres of
the wood, which exist in it as well as in the bark, though in different
states; for the ligneous fibres are always harder and less flexible than
the cortical ones. Malpighi thinks they differ in another particular,
namely, that a juice or fluid issues from the cortical fibres, while
none is found in those of the wood. In this it would appear from the
observations of Du Hamel, that he was mistaken.
A transverse section of wood generally appears formed of a number of
rays proceeding from the corona to the bark, which are intersected at
different distances by concentric circles, interspersed with vessels of
varying magnitude: the variations in this structure afford much pleasure
to the curious observer, and throw considerable light upon the nature
and properties of timber; for it is by means of a variety of strainers
that different juices are prepared from the same mass. Matter,
considered as matter, has no share in the qualities of bodies. It is
from the arrangement of it, or the recipient forms given to it, that we
have so many different substances. According to the modifications that
these receive, we shall find the same light, air, water, and earth,
manifesting themselves in one by a deadly poison, and in another by the
most salubrious food. A lemon ingrafted upon an orange stock, is capable
of changing the sap of the orange into its own nature, by a different
arrangement of the nutritive juices. One mass of earth will give life
and vigour to the bitter aloe, to the sweet cane, the cool house-leek,
and the fiery mustard, the nourishing grain, and the deadly night-shade.
The wood may be considered as composed of two parts, ligneous and
parenchymatous. The former has already been treated of; the latter is
that which is disposed into rays, running as it were between the
ligneous fibres, and interweaving with them; it originates either with
the pith or corona. There is a very great diversity in these radial
insertions; in some trees there are very few, while they abound in
others; in some they are very fine, in others very thick. In texture,
they seem similar to the blebs of the bark, only that here they are so
crowded and stretched out as to appear like parallel threads, somewhat
similar to a net when drawn tight.
OF THE CORONA.
Dr. Hill gives this name to that circle which surrounds the pith, and
separates it from the wood; although in his opinion it differs greatly
from both, and in its composition has no resemblance to either. It is,
according to him, the most important part in the whole vegetable fabric,
by which the propagation and increase of the branches, buds, and shoots,
are carried on.[137]
[137] Hill on the Construction of Timber, p. 55.
It has been usual to suppose the pith of vegetables to be the part in
which these wonderful sources of increase reside, but this is not the
case; and he asserts, that so far from being prior to the other parts,
it is in reality posterior to some of them in the time of its formation.
The corona is not so uniform as the other parts, nor is it constituted
exactly similar in all trees. It is placed between the pith and wood in
all vegetables, forming a ring, whose outline is more or less regulated.
The general circle is cellular, composed of blebs and vessels, like the
bark and the rind, and is perfectly similar to them, only that at
different distances oblong clusters of different vessels are placed
amongst it. These clusters are usually eight or ten in number, and give
origin to the angles of the corona. They are not uniform, or of one kind
of vessels, as in the bark, but each has two distinct sorts, the
exterior one answering to the blea, and the interior, to the wood of
trees; and within each of these are disposed vessels not unlike those in
the blea and wood, though often larger than they are found in those
parts.
Thus each cluster is composed of all the essential parts of the
succeeding branch, and the intermediate parts of the circle are
absolutely bark and rind; they are ready to follow and clothe the
cluster when it goes off in the form of a shoot, because it will then
need their covering and defence, though in its present inclosed state it
does not. It is from this construction, that a tree is ready at all
times and in all parts to shoot out branches, and every branch in the
same manner to send out others; for the whole trunk, and the branch in
all its length, have this course of eight or ten clusters of essential
vessels ready to be protruded out, and the proper and natural
integuments as ready to cover them. In some trees, these parts are more
evident, in others more obscurely arranged. Dr. Hill says, the bocconia,
or parrot-wood of the West-Indies, and the greater celandine, are proper
subjects for opening this great mystery of nature. On the corona and its
clusters depend that property of vegetables, that they can be produced
entire from every piece. These clusters follow the course of the other
portions of the tree; they are therefore everywhere; they are always
capable of growing, and their growth, even in a cutting of the smallest
twig, cannot produce a leaf, or any other part of a vegetable alone, but
must afford the whole; for they are complete bodies, and the whole is
there waiting only for the opportunity of extension, by obtaining
sufficient nourishment. For the knowledge we have of this part we are
altogether indebted to Dr. Hill. It remains for future observers to
confirm, or disprove his observations.
OF THE PITH.
The pith is found in the center of every young shoot of a tree; it is
large in some, less in others, but present in all. It is placed close
within the corona.
It seems to be nothing more than a congeries of the cellular tissue; it
is generally found near the center of the tree, inclosed as it were
within a tube; in general, the cells of the pith are larger than those
of the cellular tissue, with which, according to Du Hamel, it
communicates. For the rays which extend from the pith to the bark are,
in his opinion, produced from it. Thus, though it may differ in name
from the parenchymatous parts of the bark, and the radial insertions in
the wood, yet it is of the same nature and texture, and is continuous
with them; so that, according to this idea, the skin, the parenchyma,
the insertions, and the pith, are all one piece of work, filled up in
divers manners with the vessels.
The bark and the wood grow thicker every year, while the pith, on the
contrary, grows more slender, so that in a branch of one year it is of a
larger size than it is in the same branch when two years old, and so on.
In very young branches, while in an herbaceous state, the pith forms the
greatest part of its substance; but when the fibres are stronger, the
pith becomes less succulent, and surrounded with a tube of wood; when
the branch has arrived to a certain age, it is so compressed as to be
almost annihilated. In examining different branches that proceed from
others in their first state, a small communication between the pith of
the one and the other will be found; but this communication is generally
entirely closed up in the second or third year.[138] The cells of which
the pith is formed are at first entirely one connected body; but as the
plant grows up, it is often so broken and ruptured, as to remain no
longer a continuous substance.
[138] Du Hamel Physique des Arbres, tom. 1, p. 38.
This, as well as many other particulars in the history of the pith,
corroborates the opinion of Dr. Hill,[139] who thinks it is formed for
the purpose of moistening the clusters of the corona, and regulating its
extension; it has been supposed coeval with, or primordial to all the
other parts, but he thinks it is postnate, and comes after them in the
order of time, as well as in its uses; that exhaled air gives origin to
its blebs, and the thickness of the juices cloathing the bubble, gives
it form and substance. The first season is the time of its greatest use,
and it immediately after begins to decay.
[139] Hill’s Construction of Timber, p. 66.
The pith has in general been represented as much more complex than it
really is. It consists of a range of bladders lying one over the other.
The membrane is simple, the outline single; but as it is very difficult
to procure it in this simple state, it is often seen and represented
under a variety of irregular, though pleasing forms, which are
occasioned by the intersections of the outlines of the blebs, as seen
one over another.
A cluster in any part of the corona, protruding itself onward and
outward in the growing season,[140] carries a part of the circle out
with it. The cluster itself is a perfect piece of the wood and blea, and
the bark which follows it out in its progress perfectly clothes it; thus
is the first protrusion of the shoot made, but all this while there is
no pith. The continuation of growth is made by the extension of all the
parts obliquely upwards; in the course of this extension they hollow
themselves into a kind of cylinder, of the form of the future branch,
and by this disposition a small vacancy is made in their center. This
enlarges as they increase, and as it enlarges it becomes filled with the
exudation of those little bladders which remain and constitute the pith,
fed from the inner coat of the pith, which already begins to form itself
into a new corona. Grew seemed to think, that in some instances the pith
was of posterior growth to the other parts, and derived its origin from
the bark; and that the insertions of the bark running in between the
rays of the wood meet in the center, and constitute the pith.
[140] Hill’s Construction of Timber, p. 99.
OF THE SAP VESSELS.