« 上一頁繼續 »
of trees and shrubs ; when cut across, it is found to consist of numerous concentric layers. It is supposed that one of these circular layers is formed every year.
To prove that the wood is deposited externally from the cambium, pieces of metal have been introduced under the barks of trees that were growing, the wounds carefully bound up, and after some years on cutting them across, the layers of new wood have been found on the outside of the metal.
The strength and hardness of wood, is owing to woody fibres extending longitudinally; these fibres are chiefly of vascular texture, and contain sap, and the various secreted juices ; some contain only air.
For illustration of the formation of wood, see Fig. 93, B, which represents a section of a woody stem of three years' growth; i h, is a layer of the first year's growth, and the hardest part of the wood; h g, is a layer of the second year's growth; and gb, of the third ; the latter is the sap wood recently formed from the cambium.
Pith. The pith (see Fig. 93, B, at k andi,) is situated in the centre of the trunk and branches of plants, and is a soft
spongy substance, analogous to the marrow of animals. It is composed of cellular texture. The cells, which are very large in the elder, are filled with fluids when young, but in old branches the fluids disappear, and the cells are filled with air. In general, herbs and shrubs have a greater proportion of pith than trees. It is also more abundant in young than old vegetables; it extends from the root to the summit of the trunk or stem of the plant.
The medullary* rays are lines which diverge from the pith towards the circumference; they are fibrous textures interwo. ven in the wood, the alburnum, and the different layers of the bark. The new buds seem to originate from the points at which they terminate.
The pith has been compared to the spinal marrow in animals; it appears to be an important part of the vegetable substance, though its offices are perhaps less understood than those of the other parts. The letters e, Fig. 93, represent the me. dullary rays as proceeding from the pith and terminating in the cellular integument.
You are not to expect that every stem or branch of a dico. tyledonous plant will present you with all the various parts
* So called from medulla, marrow, a name often given to the pith.
How has it been proved that wood is deposited externally ? Strength and hardness of wood-Pith—Medullary reys-Pith, to what compared— Various parts not always distinct in different plants.
which we have described as constituting the vegetable body; neither when they exist are they always distinct, for as there is a chain of connexion between them, so they often pass
into each other in such a manner as to leave their boundaries diffi. cult to define. Many species of plants have no distinct layers of bark, and in many there is such a similarity between the alburnum and the perfect wood, as to render a distinction very difficult.
Growth of a Plant. Let us now take a rapid view of the growth of a woody plant. Before germination, the substance of the plume or ascending part of the embryo, exhibits a delicate and regular cellular texture ; where the liber and medullary rays are to be formed, traces of cambium appear.
When the germination commences, the vascular system begins to organize around the pith, and to form the medullary rays; the extremities of these rays exhibit cellular texture, which is soon converted into liber. See f, Fig. 93, which shows the extremities of the medullary rays, and the points where the liber is formed. This liber at first expands, then hardens, and is at length converted into a layer of alburnum ; the alburnum gradually acquires tenacity; the cells appear merged into vessels of a firmer kind, and it is no longer a layer of alburnum, but of perfect wood. While this change is taking place, the cambium, which may almost be termed a fluid, cel. lular texture, flowing between the bark and the wood, reproduces a new layer of liber, which in its turn becomes alburnum and then perfect wood; to this succeeds a third and fourth layer, and thus the growth of the vegetable goes on until death completes its term of existence.
Each layer of wood is generally the product of one year's growth ; but it is only near the base of the trunk, that the number of layers of wood is a criterion of the age of the tree ; for in trees where one hundred layers may be counted near the base, no more than one can be found at the extremity of the branches. These layers, then, do not extend through the length of the tree; but while the base exhibits all the layers which have been formed, the extremity of the branches contains under the bark only the continuation of an annual layer. The age of branches
may be determined by the number of layers of wood at the base of each branch.
Appearance of a woody plant before germination, or while in embryoChange at the commencement of germination-Process in the formation of perfect wood Number of layers of wood a criterion of the age of a tree.
We will now consider the manner in which the tree increases in height. A seed germinates; the plume rises ; the liber, by the vegetative power is urged upward; but in developing, it gradually becomes less capable of extension; at length, when it is converted into wood, its growth ceases. The layer of wood then exhibits the form of an elongated cone; at the summit of the cone a bud is formed, from which a new shoot issues ; a new liber organizes upon the surface of the cone; this new liber in turn, becomes a woody cone covering the one first formed; and thus the tree goes on increasing in height and in diameter. The terminal bud is formed each successive year. After a hundred years of vegetation a hundred cones might be found boxed into each other inthe manner first described; the spaces comprised between the summits of the cones would show the succession and elongation of the annual shoots.
As the wood is formed by the conversion of cambium into alburnum, so from the same liquid, the inner layers of bark are formed, to renew the waste occasioned by the destruction of the epidermis. W While the wood is growing externally, that is, at an increasing distance from the centre, the bark is forming internally, and the new layers are pressing outward.
The growth of trunks, which we have hitherto considered, has relation only to woody plants, but there is a marked difference in the growth of plants, which seems to originate in the peculiar formation of their seeds.
Between plants which grow from seeds with one cotyledon, and such as grow from seeds with two cotyledons, there is a great difference as to the mode of organization.
The first kind of plants are called monocotyledonous ; the second, dicotyledonous. Their stems, on account of their different modes of growth, have been distinguished into endogenous, signifying to grow inwardly; and exogenous, signifying to grow outwardly. The discovery of the different modes of growth in these two great divisions of plants, is of recent origin, and constitutes an important era in vegetable physiology.
The stems of monocotyledons, or endogenous plants have seldom a bark distinct from the other texture ; they have no liber, or alburnum disposed in concentric layers; they have no medullary rays ; and their pith, instead of being confined to the centre of the stem, extends almost to the circumference.
Advance of the tree in height-Difference in the growth of wood and barkRemarks on the different organization of plants-Monocotyledonous plants Why called endogenous-Exogenous plants - Describe the stem of a monocotyledonous plant.
The wood is divided into fibres run
ning longitudinally through the A
stem ; (see 94, where the dots represent the fibres ;) each of these fibres seems to vegetate separately; they are ranged around a cen. tral support; and are so disposed that the oldest are crowded out. wardly by the developement of new fibres in the centre of the stem ; this pressure causes the external layers to be very close and
compact. This mode of increase, little favourable to growth in diameter, produces long and straight stems, which are nearly uniform in their size through. out their whole extent ; as the palms and sugar-canes of the tropics, and the Indian corn of our climate. Most of these plants present us with roots of the fibrous kind.
In the part
Fig. 95, at A, represents a section of the stipe or stem of a palm tree; at B is the same magnified; a b, a part of the stipe in which the woody fibres are most dense and hard ; b c, shews the fibres less numerous, less compact, and less hard; c d, the woody fibres, tender and scattered; we here see the orifices of tubes, which have disappeared, at c a. c d, the cellular tissue occupies a greater space than at c, b, and much more than at b a, where the woody fibre or vascu. lar texture predominates. The fibres at e, are of new forma. tion ; at f they are older, and at g still more ancient; thus the developement of the wood proceeds inversely to that of dicotyledonous plants.
Endogenous plants continue to increase in height, long after they cease to grow in diameter; the stem is gradually extend. ed upwards by new terminal shoots, which are formed annually. The epidermis is formed of the foot stalks of leaves, which annually sprout from the rim of a new layer of wood; the leaves falling in autumn, their foot-stalks become indurated, and incorporate themselves into the outer surface of the plant.
We have now taken a brief view of the most important facts and principles which constitute the science of vegetable physiology, including anatomy. That you may have found the subject somewhat tedious, need give you no apprehension that your minds are not fitted for minute investigations. The most scientific observer of nature, cannot but feel, with yourselves, that after all his scrutiny, organic life is shrouded in mystery. Although the vegetable structure is less complicated than the animal, there are many analogies between them; and many parts of the former have been named, and various phenomena explained, by a reference to names and principles common to animal anatomy and physiology. You cannot therefore expect, at the first glance, to comprehend explanations which presuppose some knowledge of those intricate subjects. By attention to the vegetable structure, you will, doubtless, be in. duced to think more upon the wonderful mechanism of your own material frames, and upon the great resemblance, yet infinite difference, between yourselves and the trees of the forest, and the lilies of the field.
You will, in the language of the Psalmist, be led to exclaim, “Oh Lord, how manifold are thy works, in wisdom hast thou made them all!” You are nourished by the same elements as the grass that perisheth; the flowers have a much more refined corporeal substance than belongs to you; and yet how much more precious are you in the sight of the Almighty than the vegetable creation.
Do you ask, why you are of more value than the lilies of the field,” or than “ many sparrows ?" It is the very princi. ple within you which enables you to make this inquiry, that renders
you thus precious ; it is your soul that raises you above the inanimate and brute creation. Your body is sister to the crawling worm and noxious weed; but your soul may aspire to the fellowship of angels, and to be an inhabitant of heaven. Oh then let me entreat you, suffer not your chief thought to be given to the decoration of the perishable part, the mere temporary dwelling place of the immortal mind! but seek to prepare this mind for admission into "the glorious company of the spirits of the just, now made perfect in heaven.”
The wisdom of the Creator manifest in his works-Reflection.