creases through its whole length. The power of extensibility, which is inherent in vegetable tissue, especially when young, is now probably an agent in the growth; the ascending sap, which is partially decomposed in its upward course, supplies some nutritive matter to the young cells, and, it may be conjectured, stimulates them to that method of increase by the spontaneous formation of one cell on the surface of another, of which mention was made in describing the cellular tissue. The young leaves now begin to perform their office; they exhale water, decompose carbonic acid gas, and the formation of a descending current commences. This descending sap, depositing in its course such nutritive materials as are proper for the formation of wood, gradually solidifies the new shoot. If the ascent of the sap be augmented by placing the plant so that it may absorb a large quantity of water, or if the current of the descending sap be materially lessened, as will occur if it is in total darkness, then shoots are obtained extraordinarily long and herbaceous; as in the weeping willow, and in the blanched plants of flax, cultivated for the finest Flanders thread. On the contrary, if the quantity of water be diminished, and the plant exposed to the influence of such circumstances as will increase the fixation of carbon, we obtain shoots which are short, firm, and woody, as are seen in the dry and light situations of southern climates and high mountains. It appears from the above facts that the lengthening of the shoots depends on the influence of the ascending sap, while from the richness of the descending current, and consequent deposition of nutritive matter, arise its solidification and the diminution or cessation of vertical growth. Those plants which have the greatest tendency to form wood, attain proportionately the soonest to that state of hardness which arrests the lengthening of the shoot; thus it is seen that there is a sensible relation between the slowness of increase in height in each tree, and the quantity of carbon which it furnishes to combustion. In herbaceous perennials, the nourishment, which would in trees serve to form ligneous matter, is deposited in their roots, as gum, starch, or sugar, and serves to feed the young shoots of the following year. The newly-formed branches of exogenous trees do not grow much in diameter till they have attained their length. 48. It cannot be said that the ascent of the sap is absolutely null during the winter, but it is then much weaker than in the remainder of the year. In the early spring two phenomena occur; the heat of the sun begins to be felt on the bark, or cellular envelop, and the more strongly in proportion to the youth of the plant; the vital action is excited, and the sap begins to rise from the roots, whose spongioles, at this epoch of vernal vegetation, rouse from their lethargic state.* Besides this effect, a second occurs, less visible indeed, but highly important: during the depth of winter, the earth has been warmer than the air; this comparative warmth is felt by the roots, in which all the accumulated nourishment of the preceding year remains ;† their vitality is excited, and towards the end of winter radical fibres are formed; these being fresh and vigorous, begin to act, and pump up moisture from the soil: thus, the revival of vegetation is effected by the concurrence of two * Perhaps from the circumstance that during the winter the roots, being full of the sap, which has been there stored up, are incapable of imbibing more until that begins to rise, which it does as soon as the influence of the sun is felt on the bark. If a bad preceding year has rendered the quantity of nourishment small, the vegetation of spring is proportionably weak. causes the activity of the roots, and of the cellular envelop. The sap arriving at the leafy parts* promotes the development of the buds; it first reaches those at the summit of the branches, either because it moves more readily in a vertical than in a lateral direction, or because the wood and bark of the extremity of the branches, being young and herbaceous, the cells have there retained a stronger vital action. When the action of the leaves has furnished a certain quantity of nutritive juice, it descends through the laticiferous tissue, supplies the material from which the tissues and secretions of the plant are formed, and which "being poured out between the bark and the newest layer of wood, is the viscid substance called cambium; in which the rudiments of the cellular tissue that is to form part of the new layer of wood, after a time present themselves. Even if this cambium be drawn off from the stem, its particles show a tendency to arrange themselves in a form resembling that of cells and vessels; though no perfect tissues are produced by this kind of coagulation." When this cambium is formed, the tree is said to be in sap. The gradual solidification of the tissues then proceeds, but the leaves continue to take up nourishment, till, after some months of spring and early summer, they are loaded with earthy and carbonaceous particles, and then the buds which are situated at their axils become comparatively more active than the leaves themselves, and now absorb the sap, while the leaf wholly or in part ceases to do so. This effect, taking place before the year is sufficiently advanced to check the second vegetation (or * If the sap, as it rises, finds any fissure in the wood, it flows from it as from a fountain, as may be observed in what are called the tears of the vine when, pressed. + Carpenter's Veg. Phys., p. 208. midsummer shoot, as it is called), continues, and fresh branches are developed. At length the leaves in autumn, being too much encumbered with solid matter to retain any activity, cease to perform their functions, and finally die. Then if, as has been shown (13), they are articulated, they fall off; if not, they are destroyed by the inclemency of the air. The leaves of what are called evergreens form no exception, although they endure longer than those of deciduous plants, and instead of all falling off together in the autumn, are renewed at various intervals; yet each individual leaf undergoes the ordinary course of growth and decay. The change of color which withering leaves present is a very curious subject, and one which the recent experiments of Sir John Herschel and others have tended to explain. Mr. Hunt thus expresses his views of this phenomenon: "The change in the color of leaves appears to be entirely dependent upon the absorption of oxygen, which all the green parts of plants have the power of absorbing, particularly in the dark. This true case of chemical affinity, it would appear, goes on equally with the spring or the summer leaves, but during these periods the vital force, under the stimulus of light, is exerted in producing the assimilation of the oxygen for the formation of the volatile oils, the resins, and the acids. In the autumn the exciting power is weakened; the summer sun has brought the plant to a certain state, and it has no longer the vital energy necessary for continuing these processes: consequently, the oxygen now acts in the same manner on the living plant, as we find in experiment it acts upon the dried green leaves, when moistened and exposed to its action: they absorb gas and change color." (Researches on Light, p. 201.) To the fall of the leaf suceeeds the dormant wintry state; there is no absorption of moisture from the air, except through the cellular envelop; the roots have not yet formed the young radicles, and are in their least active state; and on account of these concurring circumstances, this is the most favorable period for transplanting. 49. Cellular plants have, as has been said before, no true vessels; their fibres, if they may be so called, are composed only of elongated cells, and are never identical with the ligneous fibre. The formation of the elongated cells, when such exist, determines the direction of the juices; thus in the mosses, for instance, the stem receives the water at its base, and by its radical fibrils, and transmits it in a longitudinal direction to the leaves, which direction is determined by the elongated cells. These plants are likewise nearly devoid of stomata, and can therefore only exhale the superabundant water slowly, and almost imperceptibly, and as a simple effect of the porous nature of the tissue. The nourishment of the cellular tribes appears then to be thus accomplished; the water which reaches them penetrates either at given points, or by the whole surface, and reaches the cells, where it is elaborated by each, separately, in its own cavity. 50. As the blood of animals performs two distinct offices, first, depositing throughout the whole body the materials necessary for the nourishment of each organ; and, secondly, undergoing, in certain particular organs, named glands, an operation which is called secretion, and from which results the formation of particular juices; so in the vegetable economy, the sap, besides affording the general sustenance which has been considered above, experiences a peculiar action in certain organs, and furnishes pecu |