Stems. 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 to the several parts of the plant, the leaves, the flowers, etc.
The stems of plants are divided into two classes; those which grow internally, hence called endogenous - they are also called mo-nocotyledons, from their seed having only one cotyledon, or lobe: and those which grow externally, called exogenous, or dico-tyUdons, from their seed having two lobes.
There is a third class, denominated acoty-ledons, which have no cotyledons, and no vascular system, such as fungi, lichens, etc.
The date, the palm, and the cocoa-nut tree, the sugar-cane, and most of the trees of tropica) climates, belong to the monocotyledons, or endogenous plants. Their stems are cylindrical, being of the same thickness from the to]> 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 this 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 lard, 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.
These plants have usually no branches. Corn, ami all gramineous plants, the lila-ceous 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 stern. The sugar-cane, which grows in this manner, 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 cylindrical form. This soft. pulpy body, does not grow or increase in size with the tree, but retains the 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 thickness. This mode of growing renders the form of the stem conical, the 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 alburnnum, 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. (See Roots, p. 187).