« AnteriorContinuar »
peduncle, no longer a passage for fluids, dries up and becomes unequal to supporting the fruit, which at last falls to the earth. Here, if not destroyed by animals, it lies and decays: in the succeeding spring its seeds are stimulated into life, strike root in the mass of decayed matter which surrounds them, and spring forth as new plants to undergo all the vicissitudes of their parent.
Such are the progressive phenomena in the vegetation, not only of the apple, but of all trees which are natives of northern climates, and of a large part of the herbage of the same countries, modified, of course, by peculiarities of structure and constitution; as in annual and herbaceous plants, and in those the leaves of which are opposite and not alternate: but all the more essential circumstances of their growth are the same as those of the apple tree.
If we reflect upon these phenomena, our minds can scarcely fail to be deeply impressed with admiration at the perfect simplicity and, at the same time, faultless skill, with which all the machinery is contrived upon which vegetable life depends. A few forms of tissue, interwoven horizontally and perpendicularly, constitute a stem; the developement, by the first shoot that the seed produces, of buds which grow upon the same plan as the first shoot itself, and a constant repetition of the same formation, cause an increase in the length and breadth of the plant; an expansion of the bark into a leaf, within which ramify veins proceeding from the seat of nutritive matter in the new shoot, with a provision of air-passages in its substance, and of pores on its surface, enables the crude fluid sent from the root to be elaborated and digested until it becomes the peculiar secretion of the species; the contraction of a branch and its leaves forms a flower; the disintegration of the internal tissue of a petal forms pollen; the folding inwards of a leaf is sufficient to constitute a pistil; and, finally, the gorging of the pistil with fluid which it cannot part with causes the production of a fruit.
In hot latitudes there exists another race of trees, of which Palms are the representatives; and in the north there are many herbs, in which growth, by addition to the outside, is wholly departed from, the reverse taking place; that is to say, their diameter increasing by addition to the inside. As the seeds of such plants are formed with only one cotyledon, they are called monocotyledonous; and their growth being from the inside, they are also named endogens. In these plants the functions of the leaves, flowers, and fruit are in nowise different from those of the apple; their peculiarity consisting only in the mode of forming their stems. When a monocotyledonous seed has vegetated, it usually does not disentangle its cotyledon from the testa, but simply protrudes the collum and the radicle; the cotyledon swelling, and remaining firmly encased in the seminal integuments. The radicle shoots downwards to become root; and a leaf is emitted from the side of the collum. This first leaf is succeeded by another half-facing it, and arising from its axil; the second produces a third half-facing it, and arising also from its axil; and, in this manner, the spiral production of leaves continues, until the plant, if caulescent, is ready to produce its stem. Up to this period, no stem having been formed, it has necessarily happened that the bases of the leaves hitherto produced have been all upon nearly the same plane: and, as each has been produced from the bosom of the other without any such intervening space as occurs in dicotyledonous plants, it would be impossible for the matter of wood, if any were formed, to be sent downwards around the circumference of the plant; it would, on the contrary, have been necessarily deposited in the centre. In point of fact, however, no deposit of wood like that of dicotyledons takes place, either now or hereafter. The union of the bases of the leaves has formed a fleshy stock, cormus, or plate, which, if examined, will be found to consist of a mass of cellular tissue, traversed by perpendicular and horizontal bundles of vascular and woody tissue, taking their origin in the veins of the leaves, of which they are manifest prolongations downwards; and there is no trace of separable bark, medullary rays, or central pith: the whole body being a mass of pith, woody and vascular tissue, mixed together. To understand this formation yet more clearly, consider for a moment the internal structure of the petiole of a dicotyledon : it is composed of a bundle or bundles of vascular tissue encased in pleurenchyma, surrounded on all sides with pith, or, which is the same thing, parenchyma. Now suppose a number of these petioles to be separated from their blades, and to be tied in a bunch parallel with each other, and, by lateral pressure, to be squeezed so closely together that their surfaces touch each other accurately, except at the circumference of the bunch; if a transverse section of these be made, it will exhibit the same mixture of bundles of woody tissue and parenchyma, and the same absence of distinction between pith, wood, and bark, which has been noticed in the cormus, or first plate, of monocotyledons.
As soon as the plate has arrived at the necessary diameter, it begins to lengthen upwards, leaving at its base those leaves which were before at its circumference, and carrying upwards with it such as occupied its centre; at the same time, new leaves continue to be generated at the centre, or, as it must now be called, at the apex of the shoot.
As fresh leaves are developed, they thrust aside to the circumference those which preceded them, and a stem is by degrees produced. Since it has not been formed by additions made to its circumference by each successive leaf, it is not conical, as in dicotyledons; but, on the contrary, as its increase has been at the centre, which has no power to extend its limits, being confined by the circumference which, when once formed, does not afterwards materially alter in dimensions, it is, of necessity, cylindrical: and this is one of the marks by which a monocotyledon is often to be known, in the absence of other evidence. The centre, being but little acted upon by lateral pressure, remains loose in texture, and, until it becomes very old, does not vary much from the density acquired by it shortly after its formation; but the tissue of the circumference being continually jammed together by the pressure outwards of the new matter formed in the centre, in course of time becomes a solid mass of woody matter, the cellular tissue once intermingled with it being almost obliterated, and appearing among the bundles it formerly surrounded, like the interstices around the minute pebbles of a mosaic gem.
Such is the mode of growth of Palms, and of a great proportion of arborescent monocotyledons. But there are others
in which this is in some measure departed from. In the common Asparagus the shoots produce a number of lateral buds, which all develope and influence its form, as the buds of dicotyledons; so that the cylindrical figure of monocotyledons is exchanged for the conical: its internal structure is strictly endogenous. In Grasses a similar conical figure prevails, and for the same reason ; but they have this additional peculiarity, that their stem, in consequence of the great rapidity of its growth, is fistular, with transvere phragmata at its nodes. The phragmata are formed by the crossing of woody bundles from one side of a stem to the other; and are, perhaps, contrivances to enable the thin cylinder of the stem to resist pressure from without inwards.
In such herbaceous plants as Colchicum, the stem, after a time, is a small tuber with two buds; one at the apex, which becomes the flowering stem and leaves; the other at the base, directed downwards at an obtuse angle. Such a tuber is multiplied by the latter bud, which pushes forward obliquely, and turning upwards, throws up a new flowering stem in the autumn; the base of the flowering stem thickens, enlarges, and assumes the appearance of a new cormus; in the spring, leaves sprout forth, and elaborate matter enough to fill the cells of the new cormus with fæcula, and to organise another oblique bud at the base, and then the growth of a new individual is accomplished. In the mean while, the original cormus is exhausted of all its organisable contents, which are consumed in the support of the young cormus produced from its base; and, by the time that the growth of the latter is completed, the mother is shrivelled up, and dies. It is easy to conceive many modifications of this.
Upon one or other of the two plans now explained are all flowering plants developed; but in flowerless plants it is different. In arborescent Ferns the stem consists of a cylinder of hard sinuous plates, connected by parenchyma, and surrounding an axis, hollow, or filled up with solid matter. It would seem, in these plants, as if the stem consisted of a mere adhesion of the petioles of the leaves in a single row; and that the stem simply lengthens at the point, without transmitting woody matter downwards. Some valuable observations upon this point have been made by Mohl, who has, however, been able only to investigate the anatomical condition of Tree Fern stems, without studying their mode of growth. Lycopodiaceae equally increase by simple addition to the point; and, as this seems also to be the plan upon which developement takes place in other cryptogamic plants, I have proposed the term Acrogens, to distinguish the latter from Exogens and Endogens.
When leaves are no longer formed, but growth takes place by an irregular expansion of cellular tissue in various directions, the preceding rules are departed from, and nothing being left of the vegetable fabric except the horizontal order of growth, a stem ceases to appear, and a plant becomes an unsymmetrical body, either consisting of solid masses increasing in all directions, or of filamentous matter multiplying itself by internal septation at the elongating apex.