« PreviousContinue »
which by its spiral convolutions forms a continuous tube. These vessels are very apparent in the young shoots of plants, particularly those which can be readily broken without tearing, such as the rose, &c. They are formed in the medullary sheath (27) in the nervures of leaves, &c.; but are usually wanting in wood and bark, and are never seen in any of the lower tribes of plants. Their diameter varies from the three-hundredth to the three-thousandth of an inch. * B. Ducts are transparent tubes, the sides of which are marked with rings, bars, or transverse streaks. They differ essentially from Tracheae by being inelastic, and incapable of unrolling. They are found in the wood of phaenogamous plants, and of Ferns and Lycopodiae. 4. “The office of all the ducts is the same—that of conveying fluid. It is only in the true spiral vessel that we find air.”* And even here at certain
* Carpenter’s Elem. Weg. Phys., p. 66.
“The functions of the Ducts” have not been accurately determined. It is probable that they act as spiral vessels when young; but it is certain that they become filled with fluid as soon as their spires are separated.” (Lindley’s Elements of Botany,
“There are some large Ducts which appear to have originated from cells, which have been placed together end to end, and . whose partitions have been so broken down as to form one continuous tube. These are the largest vessels (if they may be truly so considered) in the whole vegetable fabric, and are of the class called ‘ dotted ducts :’—through them the sap principally rises.” See Dr. Carpenter’s Vegetable Physiology—S 84, et seq.-The line of demarkation between the form of the true spiral vessel, and some of the ducts, is sometimes difficult to find ; in some vessels there are obscure traces of spiral form, interrupted in places, and covered by membrane.—“ In Ferns (which have no irue spiral vessels), we find Ducts, which very closely approach the spiral vessel in character, having an unbroken coil of spiral fibro throughout their whole extent; but besides the important difference that these ducts are long, continuous tubes, they are
periods of the existence of a plant, fluid has also been found by recent observers; though if a branch be cut asunder whilst in a soft state, no juice is ever seen to issue from the orifice of a spiral vessel; and though, as the lymph is found to ascend in the stalks of mosses, &c., which do not possess these vessels, we may probably conclude that they are not requisite to the transmission of fluid, though occasionally so employed. The Laticiferous Tissue consists of very delicate and anastomosing tubes, principally occurring in the young bark, and on the under sides of young leaves. They convey the fluid called Latea, or proper juice; which constitutes the nourishment of the young organs, and in which a curious oscillation of globules is visible in the bright sunshine, with a powerful microscope.* 5. Fibres and Layers. When a branch of a vascular plant is cut transversely, a certain number of points are observed, which are of a more compact character than the rest of the structure. If the branch be divided lengthwise, we shall perceive that these points are the ends of so many longitudinal threads, which will separate from the rest of the tissue more readily than they will themselves break. These threads are called fibres. With a microscope we can see that each fibre is composed of bundles of vessels, bound up and intermixed with cellular tissue. If we macerate the branch in water, after some time the fibres separate of themselves, as in the case of hemp, flax, &c. This separation in reality disorganizes the vegetable struc
further distinguished by the brittleness of the spire, which snaps when we attempt to unrol it.” Ibid., § 82.
* For a further account of this and other local circulations, see Appendix A.
ture; the water first dissolves the softer parts, i.e., the true cellular tissue, and so releases the fibres which it held together, and if the process be continued, the disorganization proceeds still farther, and a homogeneous pulp alone remains, as is seen in the manufacture of paper, where the fibres which had formed the thread are artificially torn and reduced to a pulp, in which, however, a good microscope will still show us the remains of a fibrous structure. This description of the structure of fibres explains why they are more difficult to break across than to rend asunder lengthwise; this is what workmen call following the grain of the wood. These fibres constitute what is termed Woody Tissue, or Pleurenchyma. It is also found in the young bark, and in the nervures of leaves, “and gives strength to the vegetable fabric.” When many fibres are
* “A peculiar form of woody fibre is found in the stems of resinous woods, especially the Pine and Fir tribe. The diameter of its tubes is much greater than that of any other woody
tissue . . . . . . it is by a peculiar set of dots, seen along their course, that these woody tubes may be readily distinguished from all others . . . . . Whatever be their character, they are
of great interest as tending to establish the true nature of coal.
“That this substance had a vegetable origin has long been generally admitted; but from the comparative frequency and perfection with which the remains of Ferns occur in it, it has been supposed to have been produced by the decay of vast forests of this tribe of plants. As Ferns do not form resins, however, this hypothesis would not account for the large quantity of bituminous matter which coal contains; and hence it was supposed that coal must have been formed from resinous woods, even though the remains of such were very scanty and imperfect. Now on applying the microscope to transparent sections of such fragments of coal as most distinctly exhibit the fibrous structure, it is seen that they present the character which has been described as peculiar to the resinous woods—the glandular form of woody fibre, as it has been technically termed, and hence it is established beyond doubt that the immense masses of coal which now contribute so much in every way to the comfort and
distributed circularly round an axis, whether real or imaginary, the whole together is called a Layer. It is thus that the annual rings of Dicotyledonous trees are formed. 6. Skin, called also Cuticle, or Epidermis. The whole surface of the plant, wherever it is exposed to the air, with the single exception of the stigma, is covered by this membrane, which may generally be separated from the rest of the tissue, and is seen under the microscope to be formed of a range of flattened cells, distinct from those of the Parenchyma. 7. Stomata, or Pores, are exceedingly minute oval-shaped orifices, capable of expansion and contraction, which are easily visible with the assistance of the microscope on the cuticle of the herbaceous surfaces of plants. They exist more or less in all the leafy surfaces of vascular plants, but are wanting in all roots, in old stems, in fleshy fruits, and in all the organs of cellular vegetables, with the exception of certain mosses, in which recent observers have detected them,-and are rarely found in seeds. These stomata are distributed at nearly equal distances; their principal use appears to be that of effecting the aqueous transpiration, a view of their office which is strongly confirmed by the facts that they are very abundant in those plants with membranous leaves which transpire freely, and wanting in those which transpire little; and that they are closed during darkness, when no transpiration takes place, and open in sunshine, when it is most copious. It is probable that, in addition to these visible stomata,
social improvement of the human race, are but the remains of vast forests, probably the growth of many successive centuries, which chiefly, if not entirely, consisted of trees of the Pine and Fir kind.” (Carpenter’s Veget. Physiology, pp. 65, 66.)
the superficies of plants may be studded with other pores, too small to be detected by the highest powers of the microscope, and whose existence is only suspected in consequence of physiological phenomena— for instance, if a portion of a plant, known to be devoid of visible stomata, is exposed to the air, it gradually loses weight; and consequently the liquid it contained must have found some exit.
8. Spongioles are certain exterior portions of vegetable tissue, which, without offering under the microscope any appearance of a peculiar organization, have a very strong disposition to imbibe moisture, and seem to act like small and very absorbent sponges. The radical spongioles are situated on the fibrous extremities of the roots, and it is by these extremities, only that the absorption of juices by the roots takes place.* Senebier placed two roots in such a manner that in the one the extremity alone touched the water, while the whole surface of the other root was covered by it, except the point, which was out of the fluid: the former took up water in the ordinary manner, the other imbibed no sensible quantity. The root fibre and its spongiole may be well observed in the common duckweed, in which it hangs from the under surface of every leaf. Spongioles are found on the stigmas and on the seeds of plants.
* Dr. Carpenter, in his Vegetable Physiology, (§ 106), mentions a strong instance of the practical value of an acquaintance with the nature and structure of the spongioles, in the removal of some vines from Shropshire into Norfolk, which was effected without the smallest injury to the plants by first digging a trench round them at such a distance as included all their roots, and then removing the earth “not with spades and trowels, but with the fingers; every fibril being thus uncovered without injury.” The vines bore an abundant crop in the following season.