This section is from "The Horticulturist, And Journal Of Rural Art And Rural Taste", by P. Barry, A. J. Downing, J. Jay Smith, Peter B. Mead, F. W. Woodward, Henry T. Williams. Also available from Amazon: Horticulturist and Journal of Rural Art and Rural Taste.
Ik explaining the history of a plant, it is necessary to begin with the most important parts. The root is that organ which, in contradistinction to the stem, seeks to exclude itself from the light and air, and descend into the medium of the earth. Stems and roots, though sufficiently distinct in most plants, are in others often mistaken for one another; but the latter may be readily distinguished from the former by having no buds, and one regular ramification. The principle office of the root seems to be, to supply a copious quantity of nutriment, or sap, for the maintenance of the plant, which it does by "En-dosmosis." This term may be explained by the following example. When two liquids of unequal density are separated by a permeable membrane, the lighter liquid, or the weaker solution, will flow into the denser and stronger with a force proportioned to the difference in density - but at the same time a small proportion of the denser liquid will flow into the weaker, which is called "Exosrmosis." By the examination of any embryo, of the exogenous structure, we may gain a good idea of the peculiarities of the root.
The radicle, or pre-existing axis grows in such a manner as to elongate throughout its whole extent, showing that it is not root itself, but merely the first joint of the stein, which thrusts itself downwards into the soil, while it raises two cotyledons, which supply the place of leaves until the caulineones appear. Contemporaneous with this elongation of the radicle, a new and different growth takes place at the lower extremity, in a downward direction, which forms the root. The root, then, is a new formation of cells from the root end of the radicle; it commences by a number of very lax, tender cellular tissue, resting upon a blunt cone of woodv matter, composed nected with the alburnum. The accumulation of cells is not upon its sides, but at the extremity under the thin epidermis and the superficial cells. The division of the cells from this point proceeds from below onwards, those which lie behind quickly extending to their full size, and remain in that state, while those which approach the apex multiply by continual divisions. In this way the root keeps on growing, and may be compared to an icicle, which lengthens from the point only; the only real difference being that the icicle elongates by continual accretions from the outside, while the growth of the root is from the inside.
As this growth of the root is made from the under side of the extremity alone, it follows that that part, is always clothed with a vitally active tissue. "The new cells,* however, do not occupy the extremity alone, as is commonly but incorrectly stated; this is capped, as it were, by an obtusely conical mass of older cells, consisting of the superficial tissue of the end of the radicle, pushed forward by the cell multiplication that commences behind it, as already mentioned.
"As the orignal cells of this apex wear away or perish, they are replaced by a layer beneath, and so the advancing point of the root, consisting, as inspection plainly shows, of older and denser tissue than that behind it, (the point of every branch of the root is capped in this way,) it follows that the so called spongioles, or spongelets, have no existence. Not only are there no such organs as are commonly spoken of, but absorption does not evidently take place to any considerable extent, through the older tissue of the point itself." Roots absorb nourishment by endosmoais throughout the whole of the newly formed tissue, and especially through the hair-like prolongations, commonly called the fibrils; these capillary tubes are of great tenuity, and have extremely delicate wails, and perform a more important part in absorption than is generally supposed. They perish soon as the growing season is ended, or when the roots become old and hardened - "at the same time the external layer of cells that bears them, at first indistinguishable from the parenchyma beneath, except perhaps in the size of the cells - hardens and thickens into a kind of epidermis, or firmer skin, so as to arrest or greatly restrain the imbibition.
This epidermis of the root consists of less compressed cells than in other parts exposed to the light, and is distributed to stomates, or breathing fibres." The growth of the root keeps pace with the 8tern, as the latter shoots up and becomes clothed with branches and leaves, from which water is exhaled during healthy vegetation; the former grows onward, still renewing the tender hygrometrical tissue, through which the absorption required to restore that which is lost by elaboration, or consumed by growth, is principally effected; hence the danger of removing trees during the summer season, or when the roots are in rapid action. The growth of the branches and roots being simultaneous, while new branches and leaves are developing - the roots are extending at a corresponding rate, and greatly increasing the absorbing points, they cannot now be removed with safety to the tree, and at the very time when their aid is most required. But when the growth of the season is over, the leaves grow languid, and the rootlets also cease to grow, as the tissue of their extremities not being renewed, gradually becomes hardened, and loses its absorbing powers. This marks the season for transplanting, (namely,) before the growth of the season has commenced, or in the fall, after it is made.
This elongation of the roots by their growing points alone, is admirably adapted to the situation in which they are placed, growing, as they do, in such an unequal medium as the soil. If the roots increased in the same manner as stems, they would be forced wherever the elongating force was insufficient to overcome the obstacle, or wherever this force was most powerful, and they would be thrown into all kinds of contorted shapes, very ill adapted to perform the services for which they are required. But, increasing as they do, by their points only, they insinuate themselves into the crevices of rocks, or yielding parts of the soil, and afterwards, by their expansion in diameter, enlarge the cavity, or when arrested by any obstacle, their advancing points follow its surface until it reaches a softer medium. In this manner they rapidly extend from place to place, as. fast as the nourishment in their immediate vicinity is consumed. Thus roots extend in whatever direction the soil proves moat favorable to their growth, without supposing any instinct or pre-science on the part of the vegetable, as we have before stated. "The advancing extremity of the root consists of parenchyma alone, but bundles of vessels and woody tissue appear in the forming root soon after their appearance in the primordial stem above; these form a central woody or fibrous portion, which continues to descend as the growing apex advances, sometimes, although not usually, enclosing a distinct pith, as the wood of the stem does." We have taken the root as an epitome of the whole plant, for in its whole development it produces no other parts, nothing but naked branches emanating from one particular part of the root, but indiscriminately over the whole of the superficial surface, all tending to increase the amount of absorbing surface.
In reply to the statement that roots produce no other organs, there is this abnormal exception, namely, that of producing buds, which spring up into branches, and are clothed with leases. Although the roots are not naturally provided with, buds, yet, under certain circumstances, they will produce them; that for instance when a poplar or apple tree, gorged with sap, is cut down, the root will; send np innumerable branches. The roots of the osage Grange habitually give rise to such stems, hence the utility of it in planting hedges. Some plants present a still more striking phenomena, such as the Bryophyllum, which has been known, to produce buds on the margins of its leaves; all such buds are said to be adventitious.
"The root has been illustrated from the highest class of phenogamous plants, in which the original root, or downward prolongation of the axis continues to grow, at least for a considerable time, and becomes a tap-root, or main trunk, from which branches of a> larger and smaller size emanate. Often, however, this main root nearly perishes, or ceases, to grow, and the branches take its place. In some plants of the highest class, (in the gourd family for example,) and in nearly the whole great classes to which the grasses, and lilies,, and palms belong, there is no one main trunk or primary root, from which the rest proceed; but several Foots spring forth simultaneously from the radicle in germination, and form a cluster of fibres of nearly equal sise. Such plants scarcely exhibit the distinct opposition of growth in the first instance, already mentioned as one characteristic of phenogamous. vegetation. Most phenogamous plants likewise send forth secendary roots from the stem itself, the only kind produced by eryptogamous plants. Roots vary much as regards their duration, and have been divided into three grand classes.
First, into an nuale, which are those that spring up from the seed, the first season and die; such plants are composed mostly of fibrous roots, which act a powerful part in absorption, but are good for. nothing else. These fibres, usually proceed from the sides, of the tap-roots, or else the whole plant divides itself at once into numerous, blanching fibres, such as the grasses. The food which such a plant absorbs, after having, been digested and elaborated in the leaves, is all expended in the production of branches and flowers. The flowering process and the maturation of the fruit greatly exhaust the. resources, of the plant, con-auming all the nourishment which it contains, or in storing it up for the future offspring, and, hawing no accumulation, of sap, the root, is unable to supply; the increasing demand, and the consequence, is, it dies aft soon as the growing season is over, or whenever the seeds are fully ripe. The second class compose the biennials, or such as lite (wo years; these do not flower until the second season, when the; die as the annuals; in this case the root serves as a reservoir for nourishing the assimilated matter, such as starch, vegetable jelly and sugar, (that is, its cells become gorged with these articles). - also, such roots receive the general appellation of fleshy, but have received different names according to whatever shape they assume.
For instance, if the enlargement takes place in the trunk or tap-root, it becomes conical, as in the carrot. When it regularly tapers from the crown to the apex, it becomes fusiform, or spindle-shaped. But if it leaves the middle the largest, in which instance it tapers from both ends, it becomes spheriform, or turncp-shaped. If some or the branches are thickened, instead of the main root, they are said to be clustered. Such plants do consume much of the supply of sap in the production of leaves and branches, but they form a large tuft of leaves just at the surface of the ground, which supply the roots with nearly the whole summer's supply of nourishment. In the following spring, when reaction is commenced, it shoots up a large thrifty stem, with leaves and flowers, which is wholly supported by the nourishment of the previous year; and the plant, in the mean time, neglecting to form roots anew, gradually perishes from the immense absorption of the external part, (or stem.) This class includes a very large proportion of our most useful vegetables.* Augustus A.Fahnkstock.
* Gang's Teak Book.