J. C. (John Claudius) Loudon.

An encyclopaedia of gardening; comprising the theory and practice of horticulture, floriculture, arboriculture, and landscape-gardening, including all the latest improvements; a general history of gardening in all countries; and a statistical view of its present stat, with suggestions for its future online

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Online LibraryJ. C. (John Claudius) LoudonAn encyclopaedia of gardening; comprising the theory and practice of horticulture, floriculture, arboriculture, and landscape-gardening, including all the latest improvements; a general history of gardening in all countries; and a statistical view of its present stat, with suggestions for its future → online text (page 37 of 313)
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composed of a very fine epidermis enclosing a soft and juicy pulp. The scales of the strobile are composed
of a tough and leathery epidermis, enclosing a spongy but often highly indurated pulp interspersed with
longitudinal fibres that pervade also the axis.

610. The flower-stalk, or peduncle supporting the flower, which is a prolongation of the stem or branch, or
rather a partial stem attached to it, if carefully dissected with the assistance of a good glass, will be found
to consist of the following several parts: 1st, An epidermis, or external envelope ; 2dly, A parenchyma,
or soft and pulpy mass ; odly, Bundles of longitudinal threads or fibres, originating in the stem or branch,
and passing throughout the whole extent of the parenchyma. The several organs of the flower are merely
prolongations of the component parts of the flower-stalk, though each organ does not always contain the
whole of such component parts, or at least not under the same modifications. The epidermis, however,
and parenchyma are common to them all ; but the longitudinal threads or fibres are seldom if ever to be
found except in the calvx or corolla.

611. The leafstalk, or petiole supporting the leaf, which is a prolongation of the branch or stem, or rather
a partial stem attached to it, exhibits upon dissection the same sort of structure as the peduncle, namely, an
epidermis, a pulp or parenchyma, and bundles of longitudinal threads or fibres.

612. Gems. There exist among the different tribes of vegetables four distinct species of gems, two peculiar
to perfect plants, the bud and bulb, and two peculiar to imperfect plants, the propago and gongylus ; the
latter being denominated simple gems, because furnished with a single envelope only ; and the former being
denominated compound gems, because furnished with more than a single envelope.



144



SCIENCE OF GARDENING.



Part II.



Buds are composed externally of a number of spoon-shaped
scales overlapping one another, and converging towards a point
in the apex, and often cemented together by means of a gluti-
nous or mucilaginous substance exuding from their surface. If
these scales are stripped oli' and dissected under the microscope,
they will be found to consist, like the leaves or divisions of the
calyx, of an epidermis enclosing a pulp interspersed with a net-
work of fibres, but unaccompanied with longitudinal threads.
If the scales of a leaf-bud are taken and stripped otf, and the



remaining part carefully opened up, it will be found to <

of the rudiments of a young branch terminated by a bunch of

incipient leaves imbedded in a white and cottony down, being

minute but complete in all their parts and proportions, and

folded or rolled up in the bud in a peculiar and determinate

manner.



Bulbs, which are either radical or caulinary, exhibit in their
external structure, or in a part of their internal structure tliat
is easily detected, several distinct varieties, some being solid,
some coated, and some scaly ; but all protruding in the process
of vegetation the stem, leaf, and llower, peculiar to their
species.

The propago, which is a simple gem, peculiar to some genera
of imperfect plants, and exemplified by Gsertner in the lichens,
consists of a small and pulpy mass forming a granule of no regu-
lar shape, sometimes naked, and sometimes covered with an
envelope, which is a fine epidermis.

The gimgyhu, which is also a simple pern peculiar to some
genera of imperfect plants, and exemplified by Gsertner in the
fuci, consists of a slightly indurated pulp moulded into a small
and globular granule of a firm and solid contexture, and invested
with an epidermis.





613. The term caudex, in its present application, is to be understood as including the whole mass or body-
both of the trunk and root, as distinct from the temporary parts of the plant, or parts already investi-
gated ; and as comprehending both the caudex ascendens, and caudex descendens of Linnajus, or the
trunk and its divisions, with the root and its divisions. In opening up and dissecting the caudex, whether
ascending or descending, the dissector will soon discover that its internal structure, like its external aspect
or habit, is materially different in different tribes of plants.

6" 14. The first general mode of the internal structure of the caudex is that
in which an epidermis encloses merely a homogeneous mass of pulp or
slender tibre,which forms the principal body of the caudex, and becomes some-
what indurated with age, though not woody, without discovering any further
variety of component parts. This, Mirbel observes, is the simplest mode of
internal structure existing among vegetables ; it is exemplified in the lower
orders of frondose and imperfect plants, particularly the alga? and fungi.

615. The second general mode of internal structure of the caudex is that in
which an epidermis encloses two or more substances, or assemblages of
substances, totally heterogeneous in their character. A very common va-
riety of this mode is that in which an epidermis or bark encloses a soft and
pulpy mass, interspersed with a number of longitudinal nerves cr fibres, or
bundles of fibres, extending from the base to the apex, and disposed in a
peculiarity of manner characteristic of a tribe or genus. This mode pre-
vails chiefly in herbaceous and annual or biennial plants, {fig. 48.) The
pulp being solid, as in apsidium filix-mas, and tubu/ar, as in the garden
parsnep or common hemlock. A second variety of this mode is that in which
a strong and often thick bark encloses a circular layer of longitudinal fibres,
or several such circular and concentric layers, interwoven with thin transverse and
divergent layers of pulp, so as to form a firm and compact cylinder, in the centre of
which is lodged a pulp or pith. This mode is best exemplified in trees and shrubs
{fig- 49.), though it is also applicable to many plants whose texture is chiefly cr
almost wholly herbaceous, forming as it were the connecting link between such
plantfl as are purely herbaceous on the one hand, and such as are purely woody on
the other. In the latter case the wood is perfect ; in the former case it is imperfect.
The wood being imperfect in the root of the beet, the common bramble, and burdock j
and perfect in the oak or alder.

616. The appendages of the plant, whether conservative or reproductive, exhibit
nothing in their internal structure that is at all essentially different from that of the
organs that have been already described.

Sect. II. Composite Organs.

617. From the preceding analysis, it appears the decomposite organs are reducible to
one or other of the several following substances, namely, epidermis, pulp, pith, cortical
layers, ligtieous layers, and vegetable fibre. These now remain to be further analysed, under
the title of composite organs, as being still compound, with a view to reach the ultimate
and elementary organs of the vegetable subject.

618. Structure of the vegetable epidermis. The epidermis of the vegetable, which, from its resemblance
to that of the animal, has been designated by the same name, is the external envelope or integument of
the plant, extending over the whole surface, and covering the root, stem, branches, leaves, flower, and
fruit, with their appendages ; the summit of the pistil only excepted. But although it is extended over
the whole surface of the plant, it is not of equal consistence throughout. In the root and trunk it is a
tough and leathery membrane, or it is a crust of considerable thickness, forming a notable portion of the
bark, and assuming some peculiar shade of color ; while in the leaves, flowers, and tender shoots, it is a
fine, colorless, and transparent film, when detached ; and when adherent, it is always tinged with some
peculiar shade, which it borrows from the parts immediately beneath it. Du Hamel, Saussure, Hedwig,
Comparetti, Bauer, and others, have examined the epidermis, and, according to their descriptions, it is
represented as consisting of at least two if not more layers, which in the stem of many plants, are very
easily distinguished, particularly in that of the paper-birch, the bark of which may, perhaps, be regarded
as a succession of individual cuticles.

619. The pulp is a soft and juicy substance, constituting the principal mass of succulent plants, and a notable
proportion of many parts even of woody plants. It constitutes the principal mass of many of the fungi and
fuci, and of herbaceous plants in general. Of those phytologists who have described the pulp, Mirbel is con-
sidered the most accurate. He compares it to clusters of small and hexagonal cells or bladders, con-
taining for the most part a colored juice, and formed apparently of the foldings and doublings of a f ne
and delicate membrane, in which no traces of organisation are to be distinguished. In the trunk of what
are called dicotyledonous plants, he regards the pulp, or cellular tissue, as consisting of two distinct
portions, which he designates by the respective appellations of-the herbaceous tissue, and the parenchyma.
The former is the exterior portion of the cellular tissue, of which the cells always contain a resinous and
colored juice, that communicates its peculiar tinge to the epidermis. The latter is the interior portion of the
tissue, composed also of cells, but differing from those of the herbaceous tissue in containing only a
watery juice without color, because it has not been exposed to the action of the light, though in the calyx
and fruit this watery juice is said to be also often colored. But in the description of the vegetable pulp,
the only distinction necessary to be made is that by which it is divided into two parts, namely, an
apparatus of hexagonal cells or vesicles, and a contained juice, whether colorless or colored, the union of
which substances forms a true pulp.



Book I.



INTERNAL STRUCTURE OF PLANTS.



145




620. The pith, as has been already shown, is a soft and spongy, but often succulent
substance, occupying the centre of the root, stem, and branches, and extending in
the direction of their longitudinal axis, in which it is enclosed as in a tube. The
structure of the pith is precisely similar to that of the pulp, being composed of an
assemblage of hexagonal cells containing a watery and colorless juice, or of cellular
tissue and a parenchyma.

GEL The cortical 'layers, or interior and concentric layers, constituting the mass
of the bark, are situated immediately under the cellular integument, where such
integument exists, and where not, immediately under the epidermis ; or they are
themselves external. They are distinguishable chiefly in the bark of woody plants,
but particularly in that of the lime-tree. They are composed of two elementary
parts bundles of longitudinal fibres constituting a network {fig. 30.), and a mass
of pulp more or less indurated, filling up the meshes. The innermost of the
layers is denominated the liber, and was used by the ancients to write on
before the invention of paper. It is the finest and most delicate of them all,
and often most beautifully reticulated {fig. 51. a), and varied by bundles of
longitudinal fibre (6). But the liber of daphne lagetto is remarkable
beyond that of all other plants for the beauty and delicacy of its network,
which is not inferior to that of the finest lace, and at the same time so
very soft and flexible that in countries of which the tree is a native the
lace of the liber is often made to supply the place of a neckcloth. If the
cortical layers are injured or destroyed by accident, the part destroved is
again regenerated, and the wound healed up without a scar. But if the
wound penetrates beyond the liber, the part destroyed is no longer rege-
nerated. Or if a tree is bent so as to break part of the cortical fibres, and
then propped up in its former position, the fractured fibres will again unite.
Or if a portion of the stem is entirely decorticated and covered with a piece
of bark, even from another tree, the two different barks will unite. Hence
the practicability of ascertaining how far the liber extends. And hence also the
origin of grafting, which is always effected by a union oT the liber of the
graft and stock.

622. The ligneous layers, or layers constituting the wood, occupy the
intermediate portion of the stem between the bark and pith ; and are
distinguishable into two different sorts concentric layers and divergent
layers, {fig. 50.)

62i The concentric layers, which constitute by far the greater part of the mass of the wood, are suffi-
ciently conspicuous for the purpose of exemplification on the surface of a horizontal section of most trunks
or branches, as on that of the oak and elm. But though they are generally described as being con-
centric, they are not always strictly so. For they are often found to extend more on the one side of
the axis of the stem or branch, than on the other. Some authors say the excess is on the north side, but
others say it is on the south side. The former account for it by telling us it is because the north side is
sheltered from the sun ; and the latter by telling us it is because the south side is sheltered from the cold ;
and thus from the operation of contrary causes alleging the same effect, which has been also thought to
be sufficiently striking and uniform to serve as a sort of compass, by which the bewildered traveller
might safely steer his course, even in the recesses of the most extensive forest But Du Hamel
has exposed the futility of this notion, by showing that the excess is sometimes on the one side of the
axis, and sometimes on the other, according to the accidental situation of the great roots and branches ;
a thick root or branch producing a proportionably thick layer of wood on the side of the stem from which
it issues. The layers are indeed sometimes more in number on the one side than on the other, as well
as thicker. But this is the exception, and not the rule. They are thickest, however, on the side on
which they are fewest, though not of the same thickness throughout Du Hamel, after counting twenty
layers on the one side of the transverse section of the trunk of an oak, found only fourteen on the other.
But the fourteen exceeded the twenty in thickness by one fourth part But the layers thus discoverable
on the horizontal section of the trunk are not all of an equal consistency throughout, there being an
evident diminution in their degree of solidity from the centre, where they are hardest, to the circum-
ference, where they are softest. The outermost layer, which is the softest of all, is denominated the
alburnum, perhaps from its being of a brighter white than any of the other layers, either of wood or bark;
from which character, as well as from its softer texture, it is also easily distinguished, though in the
case of some plants, as in that of the poplar and lime-tree, this peculiarity of character is not very ap-
parent From the peculiarity of external character, however, which it possesses in general, it was at one
time thought to be a substance essentially different from that of the layers which it invests. The ancients,
whose phytological opinions were often very whimsical, supposed it to be something analogous to the fat
of animals, and intended perhaps to serve as a sort of nutriment to the plant in winter. But it is now
known to be merely wood in a less condensed state, being yet lighter and softer than the interior layers,
but acquiring strength and solidity with age. It does not, however, acquire its utmost degree of solidity
till after a number of years, as is plain from the regular gradation observable in the solidity of the different
layers. But if a tree is barked a year before it is cut down, then the alburnum is converted into wood
in the course of that year.

624. The divergent layers which intersect the concentric layers in a transverse direction, constitute also a
considerable proportion of the wood, as may be seen in a horizontal section of the fir or birch, or of
almost any woody plant, on the surface of which they present an appearance like that of the radii of a
circle,

625. The structure of the concentric layers will be found to consist of several smaller and component
layers, which are themselves composed of layers smaller still, till at last they are incapable of farther
division. The concentric layers are composed of longitudinal fibres, generally forming a network ; and
the divergent layers, of parallel threads or fibres of cellular tissue, extending in a transverse direction,
and filling up the interstices of the network ; the two sets of fibres being interwoven and interlaced
together, so as to form a firm and compact body in the matured layers ; and thus corresponding exactly
to the description given of them by Grew and Malpighi, in which the longitudinal fibres are compared to
the warp, and the transverse fibres to the woof of a web.

626. The structure of the stem in plants that are purely herbaceous, and in the herbaceous parts of
woody plants, is distinguished by a humber of notable and often insulated fibres passing longitudinally
throughout its whole extent, as* in the stipe of aspidium filix-mas, or leaf-stalk of the alder. These
fibres, when viewed superficially, appear to be merely individuals, but when inspected minutely, and
under the microscope, they prove to be groups or bundles of fibres smaller and minuter still, firmly
cemented together, and forming in the aggregate a strong and elastic thread ; but capable of being spht
into a number of component fibres, till at last you can divide them no longer. If the fibres of the bark
are separated by the destruction of a part, the part is again regenerated, and the fibres are again united,
without leaving behind them any traces of a wound But if the fibres of the wood are separated by the
destruction of a part, the part is never regenerated, and the fibres are never united.



146



SCIENCE OF GARDENING.



Part II.



Sect. III. Elementary or Vascular Organs.
627. From the previous analysis of the composite organs it appears they are all ulti-
mately reducible to fibres, cellular tissue with or without parenchyma, and reticulated mem-
brane, which we must consequently regard as being, under one modification or other, the
ultimate and elementary organs of which the whole mass of the plant is composed. If it
is asked of what the elementary organs are themselves composed, the reply is, they are
composed, as appears from the same analysis, of a fine, colorless, and transparent mem-
brane, in which the eye, aided by the assistance even of the best glasses, can discover no
traces whatever of organisation ; which membrane we must also regard as constituting
the ultimate and fundamental fabric of the elementary organs themselves, and by conse-
quence of the whole of the vegetable body. It has been asked by some phytologists
whether or not plants are furnished with vessels analogous to the blood-vessels of the
animal system. But if it is admitted that plants contain fluids in motion, which cannot
possibly be denied, it will follow, as an unavoidable consequence, that they are furnished
with vessels conducting or containing such fluids. If the stem of a plant of marigold is
divided by means of a transverse section, the divided extremities of the longitudinal fibres,
arranged in a circular row immediately within the bark, will be distinctly perceived, and
their tubular structure demonstrated by means of the orifices which they present, particu-
larly when the stem has begun to wither. The same sort of structure may be observed
in the stem of cucurbitaceous plants also, particularly in that of the gourd, in which there
are besides discoverable several sets of longitudinal tubes situated near the centre, and
of considerable diameter. Regarding it, therefore, as certain that plants are furnished with
longitudinal tubes, as well as with cells or utricles for the purpose of conveying or contain-
ing their alimentary juices, we proceed to the specific illustration of both, together with
their peculiarities and appendages.

628. The utricles are the fine and membranous vessels constituting the cellular tissue of the pith and pulp
already described, whether of the plant, flower, or fruit. Individually they resemble oblong bladders in-
flated in the middle, as in the case of some plants ; or circular or hexagonal cells, as in the case of
others. Collectively they have been compared to an assemblage of threads of contiguous bladders or
vesicles, or to the bubbles that are found on the surface of liquor in a state of fermentation.

629. The tubes are the vessels formed by the cavities of the longitudinal fibres, whether as occurring in the
stem of herbaceous plants, or in the foot-stalk of the leaf and flower, or in the composition of the cortical
and ligneous layers, or by longitudinal openings pervading the pulp itself, as in the case of the vine. They
have generally been characterised under the denominations of proper vessels, lymphatics, and trachea?.
But as this is rather a premature reference to their different uses, which is besides not altogether correct^
we shall adopt, with a little alteration, the denominations introduced by Mirbel, as arising from their
form or structure. The first and primary division founded upon this principle is that by which they are
distributed into large tubes and small tubes.

630. The large tubes are tubes distinguishable by the superior width of the diameter which they present on
the horizontal section of the several parts of the plant.



Simple tubes {.fig-. 52.) are the largest of all the large
tubes, and are formed of a thin and entire membrane,
without any perceptible disruption of continuity, and
are found chiefly in the bark, though not confined to
it, as they are to be met with also in the alburnum
and matured wood, as well as in the fibres of herb-
aceous plants.

Porous tubes resemble the simple tubes in then-
general aspect ; but differ from them in being pierced
with small holes or pores, which are often distributed
in regular and parallel rows. They are found in
most abundance in woody plants, and particularly in
wood that is firm and compact, like that of the oak ;
but they do not, like the simple tubes, seem destined
to contain any oily or resinous juice.

Spiral tubes are fine, transparent, and thread-




like substances, occasionally interspersed with the
other tubes of the plant, Out distinguished from them
by being twisted from right to left, or from left to
right, in the form of a corkscrew. They occur in
most abundance in herbaceous plants, particularly
in aquatics. * r *

False spiral tubes are tubes apparently spiral on a
slight inspection, but which, upon minute examine
ation, are found to derive their appearance merely
irom their being cut transversely bv parallel fissures.

Mured tubes are tubes combining in one individual
twoor moreof the foregoing varieties. Mirbel exem-
plifies them in the case of the butomus umbellatus,
m which the porous tubes, spiral tubes, and false
spiral tubes, are often to be met with united in one.



631. The small tubes are tubes composed of a succession of elongated cells united
like those of the cellular tissue. Individually they may be compared to the stem of the
grasses, which is formed of several internodia, separated by transverse diaphragms and
collectively to a united assemblage of parallel and collateral reeds.

632. Pores are small and minute openings of various shapes and dimensions, that seem to be destined to the
absorption, transmission, or exhalation of fluids. They are distinguishable into the following two sorts :
perceptible pores and imperceptible pores. The perceptible pores are either external or internal and are
the apertures described by Hedwig as discoverable in the network constituting the epiderm" . The im
perceptible pores are pores that are not distinguishable by the eye, unless assisted with the best elas4
but they are known to exist by the evidence of experiment, and have lately been ably delineated and de'
scribed by A. T. Thomson, in his Lectures on Botany. (Vol. i. p 609 ) X aeimeaiea and de -

Jw, fK*' acc r ^ in S t0 Mirbel, are empty, but often regular and symmetrical spaces formed in the in-
Tn th.f C ffi h l meanS ^ a par l ial disru P tion of th e membrane constituting the tub or utricles
SrtM^Vofth^ol&K 60118 P K nt , S ' h .f, gaps are ften inter Pted by transverse diaphragms formed of a
portion of the cellular tissue which still remains entire, as may be seen in the transparent structure of tht

*2tf2$ gfigff** ^ TVanSV6rSe ^ are S3id t0 be ol^WeX^thlS?!^



Online LibraryJ. C. (John Claudius) LoudonAn encyclopaedia of gardening; comprising the theory and practice of horticulture, floriculture, arboriculture, and landscape-gardening, including all the latest improvements; a general history of gardening in all countries; and a statistical view of its present stat, with suggestions for its future → online text (page 37 of 313)