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spongioles: the real roots are the small bundle of fibres which spring from it. Such a root is very tenacious of life, as any portion, in which there is an articulation, will grow. The ox-eye, whose strong penetrating roots strike deep into the earth, furnishes an example of the radix fusiformis, or spindle-shaped. It is also called the tap-root, from its tapering so considerably towards the end. The radix bulbosa, or bulbous-root, such as that of the lily, the hyacinth, or the onion, is improperly so called, for the tufts of fibres, pendant from the bulb, are the roots. The bulb constitutes the stem of the plant. The potato belongs to the class of tuberous, or knotted roots, which are of various kinds, comprehending all such as have fleshy knobs, or tumours. In all cases they are to be considered as reservoirs of nourishment, which enable the plant to sustain the casual privations of a barren or dry soil.
The root of the orchis is deserving of notice, from its singularity. It consists of two lobes, somewhat similar to the two parts into which a bean is divided. One of these perishes every year, and another shoots up on the opposite side of the remaining lobe. The stem rises every spring from between the two lobes, and since the new lobe does not occupy the same place as its predecessor, the orchis every year moves a little onwards.
The duration of roots is either annual, biennial, or perennial. To the first belong plants the existence of which is limited to one season, such as barley, and a vast number of garden and field flowers. The biennial root produces, the first season, only herbage, and. the following summer, flowers and fruits, or seed ; after which it perishes. To the perennial belong plants which live to an indefinite period, such as trees and shrubs.
Every plant has a stem through which the sap circulates, and from which the leaves and flowers spring. This stem is not always apparent: it is sometimes concealed under ground, sometimes disguised under an extraordinary form: the stem of the tulip, for instance, is contained within the bulb, which is commonly, but improperly, called its root ; that of the fern is subterraneous. The functions of the root and stem are totally different: the former merely sucks up nourishment from the soil, and transmits it to the leaves; the latter is supplied with organs to distribute it, variously modified, to the several parts of the plant, the leaves, the flowers, &c.
The stems of plants are divided into two classes ; those which grow internally, hence called endogenous ; they are also called monocotyledons, from their seed having only one cotyledon, or lobe; and those which grow externally, called exogenous, or dicotyledons.
There is a third class, denominated acotyledons, which have no cotyledons, and no vascular system, such as fungi, lichens, &c.
The date, the palm, and the cocoa-nut tree, the sugar-cane, and most of the trees of tropical climates, belong to the monocotyledons, or endogenous plants. Their stems are cylindrical, being of the same thickness from the top to the bottom. Their mode of growth is this : a hollow stem shoots up to a certain height, and there stops ; layer after layer grows in the interior of thiş hollow stem, till at length a period arrives when the outer coats are so hardened and distended, as to yield no longer, the stem has then attained its full growth in horizontal dimensions, and offers a broad, flat, circular surface to view, which has scarcely risen in height above the level of the ground. In this stage it resembles the stump of the trunk of a tree, which has been cut down. The following spring, there being no room for a new layer of wood to extend
itself horizontally, it shoots up from the centre of the stem vertically ; fresh layers every year successively perforate this central shoot, till it becomes hard, compact, and of the same horizontal dimensions as the base ; the second period of growth is then complete.
The leaves and fruit of this class of plants grow from the centre of the last shoot, and form a sort of cabbage at the top of the tree, on cutting off which, the tree perishes.
Endogenous plants have no real bark, the external coats of wood are so much hardened as to render such a preservation unnecessary. When an European woodcutter begins to fell a tree of this description, he is quite astonished at its hardness. “ If I have so much difficulty with the outside," says he “how shall I ever get through the heart of the wood ?”
But as he proceeds, he finds, that, contrary to what he has been accustomed to, it gets softer. This circumstance renders it very easy to perforate them, and makes them peculiarly appropriate for pipes, for the conveyance of water, and such like purposes.
These plants have usually no branches. Corn, and all gramineous plants, the lilaceous tribe of flowers, and bulbous roots, are all endogenous. Some of these send forth shoots, but they are not from the stem, but from a knot or ring upon the stem. The sugar-cane, which grows in this månner, is the largest of the gramineous plants.
The structure of the exogenous plants, or dicotyledons, to which the trees of our temperate climes belong, is much more complicated.
The stem is composed of two separate parts: the one ligneous, the other cortical, in other words, it is formed of wood and bark.
The wood consists, in the first place, of the pith, a soft medullary substance, which occupies the centre of the stem, and is almost always of a cylindical form. This soft, pulpy body, does not grow or increase in size with the tree, but retains the same dimensions it originally had in the young stem.
The first layer, surrounding the central pith, grows freely during a twelvemonth, but the following year it is enclosed by a new layer ; being, by the pressure
of this layer, prevented from extending laterally, it makes its way where there is no pressure; that is to say, vertically. When during the third year, a third layer surrounds and compresses the second, this, in its turn, escapes from the bondage by rising vertically. This process goes on year after year, so that the stem grows in height, at the same time that it increases in thick
This mode of growing renders the form of the stem conical, the number of layers diminishing as the stem rises.
These layers of wood attain a state of maturity, when they become so hard by continued pressure, as to be no longer susceptible of yielding to it. Previous to this period, the layers bear the name of alburnum, signifying white wood, for wood is always white, until it reaches this degree of consistency. The length of time requisite to convert the alburnum into perfect wood, varies from five to fifty years, according to the nature of the tree.
The vegetation of the bark is precisely the inverse of that of the wood; that is to say, it is endogenous, its layers growing internally: the new soft coat of bark, therefore, lies immediately in contact with the new soft layer of wood. The outer coats of bark, when they become too hard to be further distended by the pressure of the internal layers, crack, and becoming thus exposed to the injury of the weather, fall off in pieces : it is this which produces the ruggedness of the bark in some trees. The other layers, as they become external, and exposed to the same sources of injury, experience the same fate.
It has long been a disputed point, what part of the stem the sap rises through ; some have maintained the opinion, that it ascended through the pith : others, that it rose through the bark; but they have both been proved to be wrong. By colouring the water, with which the plant was watered, it has been traced within
the stem, and found to ascend almost wholly in the alburnum, or young wood, and particularly in the latest layers.
THE FUNCTIONS OF LEAVES.
If the leaves of a tree be stripped off, the fruit comes to nothing, which is exemplified every year in gooseberry bushes, the leaves of which have been devoured by caterpillars; and though the fruit-trees of warm climates, partly naturalized with us, peaches, for instance, ripen their fruit sooner, perhaps, if partially deprived of their leaves; yet if that practice be carried too far, the fruit perishes. The white mulberry, indeed, cultivated in the south of Europe, for the food of silkworms only, bears wonderfully the loss of its foliage three or four times a year.
These facts have led some to think, that leaves were merely a clothing, or a protection against cold and heats. Though this is undoubtedly true, still it is a very small part of the use of leaves.
That leaves give out moisture, or are organs of insensible perspiration, is proved, by the simple experiment of gathering the leafy branch of a tree, and immediately stopping the wound at its base, with wax, or any other fit substance, to prevent the effusion of moisture in that direction. In a very short time, the leaves droop, wither, and are dried up. If the same branch, partly faded, though not dead, be placed in a very damp cellar, or immersed in water, the leaves revive, by which their power of absorption is also proved.
The great annual sun-flower is said to have lost by perspiration, 1 lb. 14 oz. weight, in the course of twelve hours, in a hot dry day. In a dry night, it lost about three ounces; in a moist night, scarcely any alteration was observable; but in a rainy night it gained