In a hundred parts of this air twenty-seven to thirty parts are of oxygen, which is in part lost during the day by the surface of plants under the direct influence of the solar rays."

With such evidence of the aeriferous functions of the spiral vessels it is difficult to contend; and, indeed, it seems probable that this question is settled as far as spiral vessels, properly so called, are concerned. But there are many vessels abounding in the wood, to which they give a porous appearance when cut across, and which I have called ducts, that, although perhaps mere modifications of the spiral vessel, are, nevertheless, so far distinct as to convey air at one period of their existence, and fluid at another. In the vine, for instance, the true spiral vessels of the medullary sheath and of the herbaceous parts, always filled with air, must be carefully distinguished from the ducts of the wood; which are, undoubtedly, filled during the principal flow of the sap with that fluid, although they finally become dry and empty. And it may be further remarked, that the dotted ducts of such plants as Phytocrene gigantea, or water-vine, so well represented by Mr. Griffith in Dr. Wallich's Planta Asiatica Rariores, are apparently the principal conduits in that curious plant, as they are in Gramineæ, and other monocotyledons, of the fluid absorbed from the earth.

So that, while true spiral vessels may be admitted as undoubted vehicles of air, ducts of all kinds, and especially dotted ducts, cannot be doubted to be the passages through which fluid is conveyed when great rapidity is required. I have already stated that, although all these vessels are, in the present state of our anatomical knowledge, considered as equally belonging to the vascular system, yet that the dotted will rather be referred eventually to the cellular, and then their lymphatic office will be unquestioned.

In regard to the functions of air-cells and lacunæ, it may be sufficient to remark, that in all cases in which they form a part of the vital system, as in water plants, they are cavities regularly built up of cellular tissue, and uniform in figure in the same species; while, on the other hand, where they are not essential to vitality, as in the pith of the walnut, the rice

paper plant, the stems of Umbelliferæ, and the like, they are ragged, irregular distensions of the tissue.

In the former case they are intended to enable plants to float in water; in the latter, they are caused by the growth of one part more rapidly than another.

CHAPTER II.

OF THE ROOT.

Ir 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 of all and 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 Ideath that often follows their destruction. This has been proved, in the following manner, by the celebrated 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 curvation, 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 shadow, 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 Mr. Knight, produced the same result. It is possible that this peculiarity may be universal in exogenous plants; but it certainly is not constant in endogenous plants; and I doubt very much whether it is not confined to roots with a woody structure. From the following experiments it will be seen that in Orchidea 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 and 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 1 foot 3 inches, the second 24 inches, the third 3 inches, and the fourth, or growing end, 1 inch and On the 19th September the first space measured 1 foot 3 inches, the second 2 inches, the third 34 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 state of anamorphosis.

By some botanists, and among them by M. 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 Mr. Knight, the energies of a variety artificially produced exist 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 some plants exude an acid fluid from their spongioles; and also by Mr. Macare, who has proved that to excrete superabundant matter from the roots is a general property of the vegetable kingdom. This, taken together with the fact that plants cannot digest their own secretions, explains why soil is so deteriorated by one species having long grown in it, that it will not support other individuals of the same species, until the fecal matter deposited in it shall have been decomposed. This is the solution of the necessity of the rotation of crops.