CHAPTER III.

OF SYMMETRY.

WHEN the elementary organs combine themselves into an organised structure endowed with life, they produce a body, 1. invariably bounded by curved lines, and, 2., having its parts balanced with great symmetry. In these respects they agree with the animal, and differ from the mineral, kingdom.

There is no such thing as an angle in vegetation: the points of the most acuminated leaves, the so-called angles of leaves and stems, the teeth of serrated leaves, are all in reality so many curves. Even the apparently flat surfaces of leaves and petals are only segments of large circles. This is a necessary consequence of the primitive spheroidal form of vegetable tissue, which, in whatever way it may develope, must always be bounded externally by the curved sides of the parenchyma.

In like manner, it will always be found, that every part of a plant is balanced by some other part. The stem is equipoised by the root; one leaf or pair of leaves is counterbalanced by the next leaf or pair; one side of a leaf answers to another; of the anthers, one lobe has its fellow on the opposite side of the connective; and this kind of comparison may be carried into the minutest part of vegetable structure. It is true, that it appears in some cases of irregularity to be departed from; as in Labiata and other irregular flowers, where the balance among the several parts seems to be destroyed; in such plants as Goldfussia anisophylla, in which one of every pair of opposite leaves is much smaller than the other; and in such leaves as those called by botanists oblique, in which one side of the leaf is much smaller than the other: but, even here, the symmetry is only destroyed in appearance. If in Labiatæ the force of developement preponderates in the anterior segments of the corolla, it is counterbalanced by the fuller

developement of the posterior stamens and sepals; and in Leguminosa, in which the posterior petal overbalances those in front, the anterior sepals and stamens restore the equipoise. If in Goldfussia it is the right hand-leaf of one pair which is the smaller, it is the left leaf of the next pair.

In the arrangement of the organs round the axis of a plant, the same careful provision of antagonistic developement is manifest. If one leaf grows on the right of a stem, the next appears on the left, the next on the right; if a pair of opposite leaves points N. and S., the next pair points E. and W., the next N. and S. again; and, if a whorl of 4 leaves is directed to the cardinal points, the next whorl is directed to the intermediate points of the compass, and so on. The same laws will be found to be observed, with little variation, through all the organs of a flower.

Messrs. Chatin and Moquin Tandon have been led, by such considerations as these, to assume that there are in the vegetable, as in the animal, kingdom, both a centripetal and centrifugal force of developement; the former appertaining to Exogens and Endogens, and the latter to Acrogens. I know nothing of the evidence upon which M. Chatin thinks this proposition may be maintained. M. Moquin Tandon has, however, given a sketch of his ideas upon the same subject, and I must confess they do not appear to me conclusive. (See Comptes rendus, v. 691.)

CHAPTER IV.

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 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 roots. Recently more important enquiries into this subject