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 phenomenafor 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.

9. The name of Lenticular glands has been given to a peculiar kind of spots observed on the bark of trees. These spots are in the first instance oval lengthwise, then round, and afterwards transversely elongated. They present a remarkable and very smooth surface, as if the cuticle were dried up: they often become swollen, and end by splitting asunder. Below the cuticle is a substance, sometimes green, sometimes white, which appears to be composed of detached cells, in the form of egg-shaped bladders. It is from these organs that such roots are put forth, as shoot from branches, whether spontaneously, or when plunged in earth; they may with truth be called root buds. They differ from the ordinary buds which produce leaves or flowers, both by their form and position: they absorb nothing from without, as the spongioles do, nor do they appear at all to serve the purpose of evaporation, like the stomata.

10. Glands, in the animal economy, signify those organs which have the power of elaborating some peculiar fluid from the nutritive juices of the body. The word preserves the same meaning when applied to vegetable anatomy.

11. Hairs (pili, villi).-Vegetable hairs are prolongations of one or many cells, which by their length rise above the surface: they are principally glandular and lymphatic; the former being the supporters of separate little glands, and the channels by which the fluid secreted by a gland passes off. It is worthy of remark, that in all glands furnished with excretory hairs, the juice secreted is of an acrid nature, and is only directed towards the exit prepared for it, when the gland, pressed on by some exterior force, suffers the fluid to escape; the juice then flows through the excretory canal, which by its pointed

extremity punctures the skin of the animal which has incautiously touched the plant, and deposits its fluid beneath it. This defensive organization closely resembles the structure of the venom bag and tooth of serpents, and is well illustrated in the common nettle. Lymphatic hairs are much more abundant than the preceding; they are of various forms, and are only found on those parts of plants which are exposed to the air. Their office is probably that of preventing evaporation in certain portions of the plant, and of protecting the more delicate organs against cold, moisture, insects, &c.; and in support of this view of their use, it will be found that the tender bud is often defended by these hairs, which, when the shoot approaches to maturity, either drop off entirely, or become thin and widely scattered.

12. Air cavities.-The cellular tissue is often distended in such a manner as to form cavities filled with air. They are sometimes composed of large cells regularly arranged, in which case they are essential to the species, as in water-plants; in other instances they are merely occasioned by the distension of the cellular tissue.

13. Articulations and Dehiscences.-At certain parts of a plant, the cells or vessels instead of being, as usual, dovetailed together, so as to afford the greatest strength, are all arranged in one plane, and consequently easily disunited; at these points, called articulations, all parts of plants which naturally fall off, as the leaves of deciduous trees for example, separate; where these articulations do not exist, the parts may perish, dry up, and be destroyed by degrees, but are never detached entire. The surface left exposed by the fall of the organ which was attached to the plant by such an articulation, is called a cicatrice or scar. Dehiscence consists in a deter