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OF THE ROOT.
IT is the business of the root to absorb nutriment from the soil, and to transmit it upwards into the stem and leaves; and also to fix the plant firmly in the earth. Although moisture is, no doubt, absorbed by the leaves and bark of all, and by the stems of many, plants, yet it is certain that the greater part of the food of plants is taken up by the roots; which, hence, are not incorrectly considered vegetable mouths.
But it is not by the whole surface of the root that the absorption of nutriment takes place; it is the spongioles almost exclusively to which that office is confided: and hence their immense importance in vegetable economy, the absolute necessity of preserving them in transplantation, and the certain death that often follows their destruction. This has been proved in the following manner, by Senebier:- He took a radish, and placed it in such a position that the extremity only of the root was plunged in water: it remained fresh several days. He then bent back the root, so that its extremity was curved up to the leaves: he plunged the bent part in water, and the plant withered soon; but it recovered its former freshness upon relaxing the curvature, and again plunging the extremity of the root into the water.
This explains why forest trees, with very dense umbrageous heads, do not perish of drought in hot summers or dry situations, when the earth often becomes mere dust for a considerable distance from their trunk, in consequence of their foliage turning off the rain: the fact is obviously that the roots near the stem are inactive, and have little or nothing to do as preservatives of life except by acting as conduits, while the functions of absorption go on through the spongioles, which, being at the extremities of the roots, are placed beyond the influence of the branches, and extend wherever moisture is to be found.
This property prevents a plant from exhausting the earth in which it grows; for, as the roots are always spreading further and further from the main stem, they are continually entering new soil, the nutritious properties of which are unexhausted.
It is generally believed that roots increase only by their extremities, and that, once formed, they never undergo any subsequent elongation. This was first noticed by Du Hamel, who passed fine silver threads through young roots at different distances, marking on a glass vessel corresponding points with some varnish: all the threads, except those that were within two or three lines of the extremity, always continued to answer to the dots of varnish on the glass vessel, although the root itself increased considerably in length. Variations in this experiment, which has also been repeated in another way by Knight, produced the same result, and the whole phenomenon appears to be one of those beautiful evidences of design which are so common in the vegetable kingdom. If plants growing in a medium of unequal resistance lengthened by an extension of their whole surface, the nature of the medium in which they grow would be in most cases such as the mere force of their elongation would be unable to overcome; and the and the consequence would be, that they would have a twisted, knotted, unequal form, which would be eminently unfavourable to the rapid transmission of fluid, which is their peculiar office. Lengthening, however, only at the extremities, and this by the continual formation of new matter at their advancing point, they insinuate themselves with the greatest facility between the crevices of the soil; once insinuated, the force of horizontal expansion speedily enlarges the cavity; and if they encounter any obstacle which is absolutely insurmountable, they simply stop, cease growing in that particular direction, and follow the surface of the opposing matter, till they again find themselves in a soft medium.
It is curious, however, to remark that, although this property of lengthening only by the ends of their roots seems constant in most plants, yet that it is not impossible that it may be confined to roots growing in a resisting medium. From the following experiments it will be seen that in Or
chidaceae the root elongates independently of its extremity: -On the 5th of August I tied threads tightly round the root of a Vanilla, so that it was divided into three spaces, of which one was 7 inches long; another 4 inches; and the third, which was the free-growing extremity, 1 inch. On the 19th of September the first space measured 7 inches; the second, 4 inches; and the third, or growing extremity, 2 inches. A root of Aerides cornutum was, on the 5th of August, divided by ligatures into spaces, of which the first measured foot 3 inches; the second, 2 inches; the third, 3 inches; and the fourth, or growing end, 1 inch. On the 19th of September, the first space measured 1 foot 3 inches; the second, 2 inches; the third, 3 inches; and the fourth, 4 inches.
Occasionally roots appear destined to act as reservoirs of nutriment on which those of the succeeding year may feed when first developed, as is the case in the Orchis, the Dahlia, and others. But it must be remarked, that the popular notion extends this circumstance far beyond its real limits, by including among roots bulbs, tubers, and other forms of stem in a succulent state.
By some botanists, and among them by De Candolle, it has been thought that roots are developed from special organs, which are to them what leaf-buds are to branches; and this function has been assigned to those little glandular swellings so common on the Willow, called lenticular glands by Guettard, and lenticelles by De Candolle.
According to Knight, the energies of a variety artificially produced exists longer in the system of the root than in that of the stem; so that it is more advisable to propagate old varieties of fruit trees from cuttings of the root than from those of the stem.
The roots not only absorb fluid from the soil, but they return a portion of their peculiar secretions back again into it; as has been found by Brugmans, who ascertained that the Pansy exuded an acid fluid from its spongioles; and by others, who found that various Euphorbiaceous and Cichoraceous plants form little knobs at the extremity of their
Recently more important enquiries into this subject
have been made by Macaire, who, in a paper in the Transactions of the Physical Society of Geneva, has given an account of his important experiments, of which the following is an abstract: He found that Chondrilla muralis, and Cichoraceous plants in general, secreted a matter analogous to opium; Leguminous plants, a substance similar to gum, with a little carbonate of lime; Grasses, a minute quantity of matter consisting of alkaline and earthy muriates and carbonates, with very little gum; Papaveraceous plants, a matter analogous to opium; and Euphorbias, a whitish yellow gum, and resinous matter of an acrid taste.
He also found that plants actually possess the power of freeing themselves from matter that is deleterious to them, by means of their roots. Acetate of lead is a well-known active vegetable poison; he took two bottles, one of which, A, was filled with pure water, and the other, B, with water holding acetate of lead in solution. He placed a plant of Mercurialis annua with half its roots plunged in A, and the other half in B. After a short time the water in the bottle A contained a notable proportion of acetate of lead, which must have been carried into the system by the roots in bottle B, and thrown off again by those in bottle A. He also states that various plants which had lain several days in water charged with lime, or acetate of lead, or nitrate of silver, or common salt, in small quantity, having been carefully washed and placed in pure water, gave back from their roots the deleterious matter they had absorbed.
It is difficult to speculate upon the results to which this curious discovery may lead. It is perhaps an explanation of the necessity of the rotation of crops, of the action of what are called weeds, of the utility of changing the earth of plants growing in pots, and of other phenomena which could not previously be accounted for. It requires, however, a great deal of ulterior examination; but as the enquiry has been taken up by Dr. Daubeny, the learned Professor of Botany and Chemistry at Oxford, at the instance of the British Association, it is not to be doubted that a few years will throw much additional light upon the subject.
M. Payen has ascertained (Ann. des Sc., n. s., iii. 18.) that the
roots of plants contain a large proportion of azotised matter, which is so abundant in the spongioles, as immediately to give off ammoniacal vapours when decomposed by aid of heat. Aerial roots, especially those of many species of Pothos, contain more than such as are subterranean. This azotised matter is almost or entirely insoluble in water, and adheres inseparably to the cellular tissue: it is most abundant at the points of the spongioles, and gradually disappears in the interior of the root. It appears essential to the life of plants, and its large proportion at the extremities of the roots may help to explain why azotised manures are so peculiarly efficient. It also shows how the well known destructive effects of tannin upon roots take place, by precipitating the azotised matter, which is essential to the existence of roots.