In many plants is found a system of channels (milk-ducts) independent of the vascular bundles, which contain an opaque, white, or yellow juice. This liquid is seen to exude from the broken stem of the milk-weed (Asclepias) of lettuce, or of celandine (Clie/idonium), and may be noticed to gather in drops upon a fresh-cut slice of the sweet potato. The milky juice often differs not more strikingly in appearance than it does in taste, from the transparent sap of the cell-tissue and vascular bundles. The former is commonly acrid and bitter, while the latter is swcet or simply insipid to the tongue. In relation to the motion of the nutrient matters of the plant, the occasional rapid passage of a current of water upwards through the plant must not be con founded with the normal, necessary, and often contrary motion of the nutrient matters out of which new growth is organized, but is an inde pendent or highly subordinate process by which the plant adapts itself to the constant changes that are taking place in the soil and atmosphere as regards their contents of moisture A plant supplied with enough moisture to k'eep its tis sues turgid is in a normal state, no matter whether the water within it is nearly free from upward flow or ascends rapidly to compensate the waste bx evaporation. In both cases the motion of the matters disolved in the sap is nearly the same. In both cases the plant develops nearly alike. In both cases the nutritive matters gathered at the root-tips ascend, and those gathered by the leaves descend, being distributed to every growing cell; and these motions are comparatively independent of, and but little influenced by, the motion of the water in which they are dissolved. The upward flow of sap in the plant is confined to the vascular bundles, whether these are arranged symmetrically and compactly, as in the exogenous plants, or distrib uted singly through the stem, as in the endogens. This is not only seen upon a bleeding stump, but is made evident by the oft-observed fact that colored liquids, when absorbed into a plant or cutting, quite visibly- follow the course of the vessels, though they do not commonly penetrate the spiral ducts, but ascend in the sieve-cells of the cambium. The rapid supply of water to the foliage of a plant, either from the roots or from a vessel in which the cut stem is immersed, goes on when the cellular tissues of the bark and pith are removed and interrupted, but is at once checked by severing the vascular bundles. The proper motion of the nutritive matters in the plant—of the salts dissolved from the soil and of the organic principles compounded from carbonic acid, water, and nitric acid or ammonia in the leaves—is one of slow diffusion mostly through the walls of imperforate cells, and goes on in all directions. New growth is the for mation and expansion of new cells into which nutritive substances are imbibed, but not poured through visible passages. When closed cells are converted into ducts or visibly communicate with each other by pores, their expansion has ceased. Henceforth they merely become thick ened by interior deposition. Prof. Johnson adds: When a cutting, from one of our commou trees, is girdled at its middle and then placed in cir cumstances favorable for growth, as in moist, warm air, with its lower extremity in water, roots form chiefly at the edge of the bark just above the removed ring. The twisting, or half breaking, as well as ringing of a layer, promotes, the development of roots. Latent buds are often called forth on the stems of fruit trees, and branches grow more vigorously, by making a transverse incision through the bark just below the point of their issue. Girdling a fruit-bearing branch of the vine near its junction with the older wood has the effect of greatly enlarging the grapes. It is well known that a wide wound made on the stem of a tree heals up by the for mation of new wood, and commonly the growth is most rapid and abundant above the cut. From these facts it was concluded that sap descends in. the bark and, not being able to pass below a wound, leads to the organization of new roots or wood just above it. Nearly all the organic substances (carbohydrates, albuminoids, lignin, etc.), that are formed in a plant are produced in the leaves, and must necessarily find their way down to nourish.the stem and roots. The facts just mentioned demonstrate, indeed, that they do go down in the bark. We have, however, no proof that there is a downward flow of sap_ Such a flow is not indicated by a single fact,. for, as we have before seen, the only current of water in the uninjured plant is the upward one which results from root-action and evaporation, and that is variable and mainly independent of• the distribution of nutritive matters. Closer investigation has shown that the most abundant downward movement of the nutrient matters. generated in the leaves proceeds in the thin walled sieve-cells of the cambium, which, in. exogens, is young tissue common to the outer wood and the inner bark—which, in fact, unites bark and wood. The tissues of the leaves com municate directly with, and are a continuation of the cambium, and hence matters formed by the leaves must move most rapidly in the cambium. If they pass with greatest freedom. through the sieve-cells, the fact is simply a demon stration that the latter communicate most directly with those parts of the leaf in which the mat ters they conduct are organized. In endogenous, plants and in some exogens (Piper medium, Anzaranthus sanguineus) the vascular bundles containing sieve-cells pass into the pith and are.
not confined to the exterior of the stem. Gird ling such plants does not give the result above, described. With them, roots are formed chiefly or entirely at the base of the cutting, according to Hanstein, and not above the girdled place. In all cases, without exception, the matters organized in the leaves, though most readily and abundantly moving downwards in the vascular tissues, are not confined to them exclusively. When a ring of bark is removed from a tree, the new cell-tissues, as well as the vascular, are interrupted. Notwithstanding, matters are trans mitted downwards, through the older wood. When but a narrow ring of bark is removed. from a cutting, roots often appear below the. incision, though in less number, and the new growth at the edges of a wound on the trunk of a tree, though most copious above, is still decided' below—goes on, in fact, all around the gash. Both the cell-tissue and the vascular thus admit' of the transport of the nutritive matters down wards. In the former, the carbohydrates- starch, sugar, inulin—the fats, and acids, chiefly occur and move. In the large ducts, air is con tained, except when by vigorous root-action the. stem is surcharged with water. In the sieve ducts (cambium) are found the albuminoids, though not unmixed with carbohydrates. If a tree have a deep gash cut into its stem, but not reaching to the colored heart-wood, growth is not suppressed on either side of the cut, but the nutritive matters of all kinds pass out of a vertical direction around the incision, to nourish the new wood above and below. Girdlin& a tree is not necessarily fatal if done in the spring or early summer when growth is rapid, provided the young cells, which form externally, are protected from dryness and other destructive influences. An artificial bark, that is, a cover ing of cloth or clay to keep the exposed wood moist and away froni air, saves the tree until the -wound heals over. In these cases it is obvious that the substances which commonly prepon derate in the sieve-ducts must pass through the oell-tissue in order to reach the point where they nourish the growing organs. Evidence that nutrient matters also pass upwards in the bark is furnished, not only by tracing the course of colored liquids in the stem, but also by the fact that undeveloped buds perish in most cases when the stem is girdled between them and active leaves. In the exceptions to this rule, the vas cular bundles penetrate the pith, and thereby demonstrate that they are the direct channels of this movement. A minority of these excep tions again makes evident that the sieve-cells are the path of transfer, for, as Hanstein has shown, in certain plants (Solanacece, Asclepiadacca, etc.), sieve-cells penetrate the pith unaccompanied by any other elements of the vascular bundle, and girdled twigs of these plants grow above as well as beneath the wound, although all leaves above the girdled place be cut off, so that the nutri ment of the buds must come from below the incision. The substances which are organized in the foliage of a plant, as well as those which are imbibed by the roots, move to any point where they can supply a want. Carbohydrates pass from the leaves; not only downwards to nourish new roots, but upwards, to feed the buds. flowers and fruit. In case of cereals, the power of the leaves to gather and organize atmospheric food nearly or altogether ceases as tlaey approach maturity. The seed grows at the expense of matters previously stored in the foliage and stems to Buch an extent that it may ripen quite perfectly although the plant be cut when the kernel is in the milk, or even earlier, while the juice of the seeds is still watery and before starch-grains have begun to form. In biennnial root-crops, the root is the focus of motion for the matters organized by growth during the first year; but in the second year the stores of the root are completely exhausted for the support of flowers and seed, so that the direction of the movement of these organized matters is reversed. In both years the motion of water is always the same, viz., from the soil upwards to the leaves and this because the soil always contains more water than the air. If a plant were so situated that its roots should steadily lack water while its foliage had an excess of this liquid, it cannot be doubted that then the sap would pass down iu a regular flow. In this case, nevertheless, the nutrient matters would take their nornial course. The summing up of the whole 'natter is that the nutrient substances in the plant are not absolutely confined to any path, and may move in any direction. The fact that they chiefly follow certain channels, and move in this or that direction, is plainly dependent upon the structure and arrangement of the tissues, on the sources of nutrineent, and on the seat of growth or other action. From what has been shown, it will be seen that the circulation of the sap, like all other forces in nature, is guided by simple and yet perfect laws. The study of vegetable physiology is a most intricate and, at the same thne, most interesting study. Scientists are con stantly conducting experiments, bringing out something new. Nevertheless, there are very many questions still wrapped in obscurity, and probably always will be, since it is not likely that all the processes of nature will ever be laid open to the human mind, and for one reason, if for no other—the processes of nature are ever changing, according to the conditions present.