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 46 of 313)
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garded as being analogous to the heart and brain of animals, as related by Malpighi ; who did not him-
self adopt it, but believed the pith to be like the cellular tissue, the viscera in which the sap is elaborated
for the nourishment of the plant, and for the protrusion of future buds. Magnol thought that it pro-
duces the flower and fruit, but not the wood. Du Hamel regarded it as being merely an extension of
the pulp or cellular tissue, without being destined to perform any important function in the process of
vegetation. But Linnajus was of opinion that it produces even the wood ; regarding it not only as the
source of vegetable nourishment, but as being also to the vegetable what the brain and spinal marrow
are to animals, the source and seat of life. In these opinions there may be something of truth, but they
have all the common fault of ascribing to the pith either too little or too much. M. Lindsay of Jamaica
suggested a new opinion on the subject, regarding it as being the seat of the irritability of the leaves of
the mimosa, and Sir J. E. Smith says he can see nothing to invalidate the arguments on which this
opinion is founded. Plenck and Knight regard it as destined by nature to be a reservoir of moisture to
supply the leaves when exhausted by excess of perspiration. Hence it appears that the peculiar function
of the pith has not yet been altogether satisfactorily ascertained ; and the difficulty of ascertaining it has
been thought to be increased from the circumstance of its seeming to be only of a temporary use in the
process of vegetation, by its disappearing altogether in the aged trunk. But although it is thus only
temporary as relative to the body of the trunk, yet it is by no means temporary as relative to the process
of vegetation ; the central part of the aged trunk being now no longer in a vegetating state, and the pith
being always present in one shape or other in the annual plant, or in the new additions that are an-
nually made to perennials. The pith then is essential to vegetation in all its stages : and from the
analogy of its structure to that of the pulp or parenchyma which is known to be an organ of elabor-
ation, as in the leaf, the function of the pith is most probably that of giving some peculiar elaboration
to the sap.

767. The generation of the layer of wood in woody plants, or of the parts analogous to wood in the case of
herbaceous plants, has been hitherto but little attended to. If we suppose the rudiments of all the
different parts to exist already in the embryo, then we have only to account for their developement by
means of the intro-susception and assimilation of sap and proper juice ; but if we suppose them to be
generated in the course of vegetation, then the difficulty of the case is augmented : and at the best we
can only state the result of operations that have been so long continued as to present an effect cognizable to
the sense of sight, though the detail of the process is often so very minute as to escape even the nicest
observation. All, then, that can be said on the subject, is merely that the tubes, however formed, do, by
virtue of the agency of the vital principle operating on the proper juice, always make their appearance at
last in a uniform and determinate manner, according to the tribe or species to which the plant belongs,
uniting and coalescing so as to form either a circular layer investing the pith, as in woody plants ; or a
number of divergent layers intersecting the pith, as in some herbaceous plants ; or bundles of longitudinal



174 SCIENCE OF GARDENING. Part II.

and woody fibre interspersed throughout the pith, as in others. In the same manner we may account for
the formation of the layer of bark.

768. Perennials and their annual layers. If a perennial is taken at the end of the
second year and dissected as in the example of the first year, it will be found to have in-
creased in height by the addition of a perpendicular shoot consisting of bark, wood, and
pith, as in the shoot of the former year ; and in diameter by the addition of a new layer
of wood and of bark, generated between the wood and bark of the former year, and cover-
ing the original cone of wood, like the paper that covers a sugar-loaf: this is the fact of
the mode of augmentation about which phytologists have not differed, though they have
differed widely with regard to the origin of the additional layer by which the trunk is in-
creased in diameter. Malpighi was of opinion that the new layer of wood is formed from
the liber of the former year.

769. The new layer of wood Linnaeus considered as formed from the pith, which is absurd, because the
opinion goes to the inversion of the very order in which the layer is formed, the new layer being always
exterior to the old one. But according to the most general opinion, the layer was thought to be formed
from a substance oozing out of the wood or bark first, a limpid fluid, then a viscid pulp, and then a thin
layer attaching itself to the former ; the substance thus exuding from the wood or bark was generally
regarded as being merely an extravasated mucilage, which was somehow or other converted into wood and
bark : but Du Hamel regarded it as being already an organised substance, consisting of both cellular and
tubular tissue, which he designated by the appellation of the cambium, or proper juice.

770. Knight has thrown the highest degree of elucidation on this, one of the most obscure and intricate
processes of the vegetable economy, in having shown that the sap is elaborated, so to render it fit for the
formation of new parts in the leaf only. If a leaf or branch of the vine is grafted even on the fruit-stalk
or tendril, the graft will still succeed ; but if the upper part of a branch is stripped of its leaves the bark
will wither as far as it is stripped ; and if a portion of bark furnished with a leaf is insulated by means of
detaching a ring of bark above and below it, the wood of the insulated portion that is above the leaf is
not augmented : this shows evidently that the leaf gives the elaboration necessary to the formation of new
parts, and that without the agency of the leaf no new part is generated: Such then is the mode of
the augmentation of the plant in the second year of its growth. It extends in width by a new layer
of wood and of bark insinuated between the wood and bark of the former year ; and in height by
the addition of a perpendicular shoot, or of branches, generated as in the shoot of the first year.
But if the plant is taken and dissected at the end of the third year, it will be found to have aug-
mented in the same manner ; and so also at the end of the succeeding year as long as it shall continue
to live ; so that the outermost layer of bark, and innermost layer of wood, must have been originally
tangent in the first year of the plant's growth ; the second layer of bark, and second layer of wood, in the
second year ; and so on in the order of succession till you come to the layer of the present year, which will
in like manner divide into two portions, the outer forming one or more layers of bark, and the inner
forming one or more layers of wood. And hence the origin of the concentric layers of wood and of bark
of the trunk. But how are we to account for the formation of the divergent layers, which Du Hamel
erroneously supposed to proceed from the pith ? The true solution of the difficulty has been furnished by
Knight, who, in tracing the result of the operation of budding, obs?rved that the wood formed under the
bark of the inserted bud unites indeed confusedly with the stock, though still possessing the character and
properties of the wood from which it was taken, and exhibiting divergent layers of new formation which
originate evidently in the bark, and terminate at the line of union between the graft and stock.

771. But how is the formationlof the wood that now occupies the place of the pith to be accounted for >
It appears that the tubes of which the medullary is composed do, in the process of vegetation, deposit a
cambium, which forms an interior layer that is afterwards converted into wood for the purpose of filling
up the medullary canal.

772. Conversion of the alburnum into perfect wood. In consequence of the increase of the trunk by means
of the regular and gradual addition of an annual layer, the layers whether of wood or of bark are ne-
cessarily of different degrees of solidity in proportion to their age ; the inner layer of bark, and the outer
layer of wood, being the softest; and the other layers increasing in their degree of solidity till you reach
the centre on the one hand, and the circumference on the other, where they are respectively the hardest,
forming perfect wood or highly indurated bark, which sloughs or splits into chinks, and falls off in thick
crusts, as in the plane-tree, fir, and birch. What length of time then is requisite to convert the alburnum
into perfect wood, or the liber into indurated bark ; and by what means are they so converted ? There is no
fixed and definite period of time that can be positively assigned as necessary to the complete induration
of the wood or bark, though it seems to require a period of a good many years before any particular
layer is converted from the state of alburnum to that of perfect wood ; and perhaps no layer has received
its final degree of induration till such time as the tree has arrived at its full growth. The induration
of the alburnum, and its consequent durability, are attributed by many to the loss of sap which the
layer sustains after the period of its complete developement ; when the supply from the root diminishes,
and the waste by evaporation or otherwise is still kept up, inducing a contraction or condensation of
its elementary principles that augments the . solhiity of the layer, in the first degree, and begins the
process that future years finish. But Knight believes the induration of the alburnum as distinguishable
in the winter to be owing rather to some substance deposited in it in the course of the preceding summer,
which he regards as being the proper juice in a concrete or inspissated state, but which is carried off again
by the sap as it ascends in the spring.

773. Circulation of vegetable juices. After the discovery of the circulation of the
blood of animals, phytologists, who were fond of tracing analogies between the animal
and vegetable kingdoms, began to think that there perhaps existed in plants also a circu-
lation of fluids. The sap was supposed to be elaborated in the root. The vessels in
which it was propelled to the summit of the plant were denominated arteries ; and the
vessels in which it is again returned to the root were denominated veins. Du Hamel,
while he admits the ascent of the sap, and descent of the proper juice, each in peculiar
and appropriate vessels, does not however admit the doctrine of a circulation ; which
seems, about the middle of the last century, to have fallen into disrepute. For Hales, who
contended for an alternate ascent and descent of fluids in the day and night, and in the
same vessels, or for a sort of vibratory motion as he also describes it, gave no countenance
whatever to the doctrine of a circulation of juices. I3ut the doctrine, as it appears, has
been again revived, and has met with the support of some of the most distinguished of



Book I. PROCESS OF VEGETABLE DEVELOPEMENT. 175

modern phytologists. Hedwig is said to have declared himself to be of opinion, that plants
have a circulation of fluids similar to that of animals. Corti is said to have discovered a
species of circulation in the stem of the chara, but confined, it is believed, within the
limits of the internodia. Willdenow has also introduced the subject, and defended the
doctrine {Principles of Botany, p. 8.5.); but only by saying he believes a circulation to
exist, and that it is impossible for the leafless tree to resist the cold if there be not a cir-
culation of fluids. Knight has given his reasons somewhat in detail ; and though his
doctrine of a circulation should be false, yet the account which he gives of the progress
and agency of the sap and proper juice, short of circulation, may be true. Tlie sum of the
account is as follows : When the seed is deposited in the ground under proper condi-
tions, moisture is absorbed and modified by the cotyledons, and conducted directly to the
radicle, which is by consequence first developed. But the fluid which has been thus con-
ducted to the radicle, mingling no doubt with the fluid which is now also absorbed from
the soil, ascends afterwards to the plumelet through the medium of the tubes of the albur-
num. The plumelet now expands and gives the due preparation to the ascending sap, re-
turning it also in its elaborated state to the tubes of the bark, through which it again
descends to the extremity of the root, forming in its progress new bark and new albur-
num ; but mixing also, as he thinks, with the alburnum of the former year, where such
alburnum exists, and so completing the circulation.

774. Decomposite organs. To the above brief sketch of the agency of the vital prin-
ciple in the generation or growth of the elementary and composite organs, there now re-
mains to be added that of the progress and mode of the growth of the decomposite organs,
or organs immediately constituting the plant, as finishing the process of the vegetable de-
velopement. This will include the phenomena of the ultimate developement of the root,
stem, branch, bud, leaf, flower, and fruit.

775. The root. From the foregoing observations and experiments, it appears that the roots of plants,
or at least of woody plants, are augmented in their width by the addition of an annual layer, and in their
length by the addition of an annual shoot, bursting from the terminating fibre. But how is the develope-
ment of the shoot effected ? Is it by the intro-susception of additional particles throughout the whole of
its extent ; or only by additions deposited at the extremity ? In order to ascertain the fact, with regard
to the elongation of the root, Du Hamel instituted the following experiment : Having passed several
threads of silver transversely through the root of a plant, and noted the distances, he then immersed the
root in water. The upper threads retained always their relative and original situation, and the lowest
thread which was placed within a few lines of the end was the only one that was carried down. Hence
he concluded that the root is elongated merely by the extremity. Knight, who from a similar experiment
obtained the same result, deduced from it also the same conclusion. We may regard it then as certain,
that the mode of the elongation of the root is such as is here represented, though in the progress of its
developement, it may affect a variety of directions. The original direction of the root is generally perpen-
dicular, in which it descends to a considerable depth if not interrupted by some obstacle. In taking up
some young oak-trees that had been planted in a poor soil, Du Hamel found that the root had descended
almost four feet, while the height of the trunk was not more than six inches. If the root meets with an
obstacle it then takes a horizontal direction, not by the bending of the original shoot, but by the sending
out of lateral shoots. The same effect also follows if the extremity of the root is cut off', but not always
so, for it is a common thing in nursery-gardens, to cut off the tap-roots of drills of seedling oaks without
removing them, by a sharp spade, and these generally push out new tap-roots, though not so strong as the
former. When a root ceases of its own accord to elongate, it sends out also lateral fibres which become
branches, and are always the more vigorous the nearer they are to the trunk, but the lateral branches of
horizontal roots are the less vigorous the nearer they are to the end next the trunk. In the former case,
the increased luxuriance is perhaps owing to the easy access of oxygen in the upper divisions ; but in the
latter case, the increased luxuriance of the more distant divisions is not so easily accounted for, if it is not
to be attributed to the more ample supply of nutriment which the fibres meet with as they recede from
the trunk, particularly if you suppose a number of them lying horizontally and diverging like the radii of
a circle. But the direction of roots is so liable to be affected by accidental causes, that there is often but
little uniformity even in roots of the same species. If plants were to be sown in a soil of the same density
throughout, perhaps there might be at least as much uniformity in the figure and direction of their roots,
as of their branches ; but this will seldom happen. For if the root is injured by the attacks of insects, or
interrupted by stones, or earth of too dense a quality, it then sends out lateral branches, as in the above
cases ; sometimes extending also in length by following the direction of the obstacle, and sometimes ceas-
ing to elongate, and forming a knot at the extremity. But where the soil has been loosened by digging or
otherwise, the root generally extends itself to an unusual length, and where it is both loosened and en-
riched, it divides into a multiplicity of fibres. This is also the case with the roots of plants vegetating in
pots, near a river, but especially in water. Where roots have some considerable obstacle to overcome they
will often acquire a strength proportioned to the difficulty : sometimes they will penetrate through the
hardest soil to get at a soil more nutritive, and sometimes they will insinuate their fibres into the crevices
even of walls and rocks which they will burst or overturn. This of course requires much time, and does
much injury to the plant. Roots consequently thrive best in a soil that is neither too loose nor too
dense ; but as the nourishment which the root absorbs is chiefly taken up by the extremity, so the soil is
often more exhausted at some distance from the trunk than immediately around it Du Hamel regards
the small fibres of the root which absorb the moisture of the soil as being analogous to the lacteals of the
animal system, which absorb the food digested by the stomach. But the root is rather to be regarded as
the mouth of the plant, selecting what is useful to nourishment and rejecting what is yet in a crude and
indigestible state ; the larger portions of it serving also to fix the plant in the soil and to convey to the
trunk the nourishment absorbed by the smaller fibres, which ascending by the tubes of the alburnum, is
thus conveyed to the leaves, the digestive organs of plants. Du Hamel thinks that the roots of plants are
furnished with pre-organised germs by which they are enabled to send out lateral branches when cut,
though the existence of such germs is not proved ; and affirms that the extremities of the fibres of the
root die annually like the leaves of the trunk and branches, and are again annually renewed ; which last
peculiarity Professor Willdenow affirms also to be the fact, but without adducing any evidence by which
it appears* to be satisfactorily substantiated. On the contrary, Knight, who has also made some observ-
ations on this subject, says, it does not appear that the terminating fibres of the roots of woody plants die
annually, though those of bulbous roots are found to do so. But the fibres of creeping plants, as the com-
mon crowfoot and strawberry, certainly die annually, as do those of the vine.



17G SCIENCE OF GARDENING. Part II.

776. "The stem. The stem, like the root, or at least the stem of woody plants, is also augmented in
width by the addition of an annual layer, and in length by the addition of an annual shoot bursting from
the terminating bud. Is the developement of the shoot issuing from the stem effected in the same man-
ner also ? The developement of the shoot from the stem is not effected in the same manner a6 that of the
root by additions to the extremity only, but by the intro-susception of additional particles throughout
its whole extent, at least in its soft and succulent state : the longitudinal extension diminishing in pro-
portion as the shoot acquires solidity, and ceasing entirely when the wood is perfectly formed ; though
often continuing at the summit after it has ceased at the base. The extension of the shoot is inversely as
its induration, rapid while it remains herbaceous, but slow in proportion as it is converted into wood.
Hence moisture and shade are the most favorable to its elongation, because they prevent or retard its in.
duration; and hence the small cone of wood which is formed during the first year of the plant's growth
increases no more after the approach of winter, neither in height nor thickness. Such is the mode of the
growth and developement of the trunk of perennial and woody plants, to which tiere exists a striking
exception in the growth of the trunk of palms. Their internal structure has been already taken notice of
as presenting no concentric or divergent layers, and no medullary canal, but merely an assemblage of large
and woody fibres, interspersed without order in a pulp or parenchyma, softer at the centre and gradually
becoming harder as it approaches the circumference. When the seed of the palm-tree germinates, it pro-
trudes a circular row of leaves, or of fronds, which crowns the radicle, and is succeeded in the following
year by a similar row issuing from the centre or bosom of the former leaves, which ultimately die down
to the base. This process is continued for four or five /ears successively without exhibiting as yet any
appearance of a stem, the remaining bases of the 'eaves oi frond forming by their union merely a sort of
knob or bulb. At last, however, they constitute oy their nion an incipient stem, as thick the first year
as it ever is after ; which in the following year is augmented in, height as before, and so on in succession
as long as the plant lives, the leaves always issuing from the summit and crowning the stem, which is a
regular column, but decaying at the end of the year, and leaving circular marks at the points of insertion,
which furrow the surface of the plant, and indicate the years of its growth.

777. The branches, in their mode of growth and developement, exhibit nearly the same appearances as
the trunk from which they issue. They originate in a bud, and form also a cone that consists of pith,
wood, and bark ; or rather they form a double cone. For the insertion of the branch into the trunk
resembles also a cone whose base is at the circumference, and whose apex is at the centre, at least if it is
formed in the first year of the plant's growth, or on the shoot of the present year; but falling short of the
centre in proportion to the lateness of its formation, and number of intervening layers. Branches in their
developement assume almost all varieties of position from the reflected to the horizontal and upright ; but
the lower branches of trees are said to be generally parallel to the surface of the soil on which they grow,
even though that surface should be the sloping side of a hill owing, as it has been thought, to the evo-
lution of a greater number of buds on the side that forms the obtuse angle with the soil, in consequence
of its being exposed to the action of a greater mass of air.

778. The bud, which in the beginning of spring is so very conspicuous on the trees of this country as to
be obvious to the most careless observer, is by no means common to all plants, nor to plants of all climates ;
shrubs in general, and annuals universally, are destitute of buds as well as all plants whatever growing
within the tropics, the leaf being in them immediately protruded from the bark. It is only in the woody
plants of cold climates, therefore, that we are to look for buds ; and in them no new part is added, whether
proper to the leaf or flower, without the intervention of a bud. For when the young shoot is produced, it
is at the same time furnished with new buds, which are again extended into new shoots in the following
spring ; and thus the bud is to be regarded as forming, not only the cradle but also the winter quarters of
the shoot, tor which its coat of tiled and glutinous scales seems admirably well adapted. It is found chiefly
in the extremity, or on the surface of the young shoot or branch, and but rarely on the stem, except it be
at the collar where it produces suckers. It is also generated for the most part in the axil of the leaves,
as may be seen by inspecting the annual shoot of almost any tree at random, though not universally so
tor to this rule there exists a curious and singular exception in the bud of the platanus, which is gene-



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 46 of 313)