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M. Dutrochet calls by the name of embryo-buds (fig. 24.) those nodules which are so well known in the bark of the Beech, and some other trees, and which are externally indicated by small tumours of the bark. According to this author such bodies are at first very small and globular, in the tissue of the bark, near its surface; he has found some not larger than a pin's head, and thinks they are born in the parenchymatous tissue. They are at first completely free, and isolated in the bark, have a peculiar bark of their own, which is united with that of the parent tree, but which may in the Cedar be easily distinguished by the direction of its fibres.
The form of such nodules is variable ; sometimes they are rounded, sometimes conical, &c. When in the progress of developement, the woody nodules born in the thickness of the bark, bring their wood in contact with that of the tree which bears them, the intermediate bark disappears, being destroyed by the pressure to which it is subjected, and then the wood of the nodule becomes adherent to the wood of the tree. This adhesion sometimes does not take place for several years. The wood of the nodules is arranged in concentric zones around a common centre, and has both pith and medullary rays; and, however irregular, the form is evidently in all cases a genuine sphere; it has all the elements of organisation found in the trunk of the tree, but arranged differently. The side next the wood of the parent tree is thicker than the opposite side, which Dutrochet attributes to its being more immediately in contact with the cambium which nourishes it. In the Cedar of Lebanon the nodules have been seen producing a small branch from the summit. M. Dutrochet regards these nodules as adventitious buds arrested in their formation, and he compares them to the internode of Tamus communis, which forms a tuberous root-like body in that species.
A circumstance to which this physiologist attaches great importance is, that these nodules have an abundance of cambium in the spring, and yet they are not, he says, in communication with the alburnum of the tree; whence he concludes that cambium is elaborated by the bark exclusively. I am not, however, able to reconcile this statement (Memoires, i. 311.) with another (p. 304.), that the base of the nodule is “certainement" in adhesion with the wood of the tree.
2. Of its External Modifications.
It has already been stated, that the first direction taken by the stem immediately upon its developement is upwards into the air. While this ascending tendency is by many plants maintained during the whole period of their existence, by others it is departed from at an early age, and a horizontal course is taken instead ; while also free communication with light and air is essential to most stems, others remain during all their lives buried under ground, and shun rather than seek the light. From these and other causes, the stems of plants assume a number of different states, to which botanists attach particular terms. It will be most convenient to divide the subject into the varieties of —
1. The subterranean stem; and 2. The aerial stem.
The SUBTERRANEAN stem was confounded by all the older botanists, as it still is by the vulgar, with the root, to which it bears an external resemblance, but from which it is positively
distinguished both by its ascending origin, and by its anatomical structure. (See Root.)
The following are the varieties which have been distinguished :
The Cormus, fig. 25. (Lecus of Du Petit Thouars, Plateau of De Candolle), is the dilated base of the stem of Monocotyledonous plants, intervening, between the roots and the first buds; and forming the reproductive portion of the stem of such plants when they are not caulescent. It is composed of cellular tissue, traversed by bundles of vessels and pleurenchyma, and has often the form of a flattened disk. The fleshy “root” of the Arum, that of the Crocus and the Colchicum, are all different forms of the cormus.
It has been called bulbotuber by Ker, and bulbus solidus by many others; the last is a contradiction in terms. (See Bulb.)
The stems of Palms have by some writers been considered as an extended cormus, and not a true stem, but this seems an extravagant application of the term; or rather an application which reduces the signification of the term to nothing. A cormus is a depressed subterranean stem of a particular kind; the trunk of a Palm is, as far as its external character is concerned, as much a stem as that of an Oak. De Candolle applies the name cormus only to the stems of Cryptogamous plants, and refers to it the Anabices of Necker.
The Tuber, fig. 26. (Tuberculum if very small), is an annual thickened subterranean stem, provided at the sides with latent buds, from which new plants are produced the succeeding year, as in the Potato and Arrow-root. A tuber is, in reality, a part of a subterranean stem, excessively enlarged by the developement to an unusual degree of cellular tissue. The usual consequences attendant upon such a state take place; the regular and symmetrical arrangement of the buds is disturbed; the buds themselves are sunk beneath the surface, or half obliterated, and the whole becomes a shapeless
Such is not, however, always the case; the enlargement sometimes occurs without being accompanied by much distortion, and the true nature of the tuber stands revealed; this is remarkably the case in the Asparagus Potato. In most, perhaps all tubers, a great quantity of amylaceous matter is deposited, on which account they are frequently found to possess highly nutritive properties.
The Creeping stem, fig. 27. (soboles), is a slender stem, which creeps along horizontally below the surface of the earth, emitting roots and new plants at intervals, as in the Triticum repens. It differs in nothing whatever from the rhizoma, except in being subterranean. This is what many botanists call a creeping root. It is one of those provisions of nature by which the barren sands that bound the sea are confined within their limits; most of the plants which cover such soils being provided with subterranean stems of this kind. It is also extremely tenacious of life, the buds at every node being capable of renewing the existence of the individual; hence the almost indestructible properties of the Couch grass, Triticum repens, by the ordinary operations of husbandry: divi*sions of its creeping stem, by cutting and tearing, producing no other effect than that of calling new individuals into existence as fast as others are destroyed. The term soboles is applied by Link and De Candolle to the sucker of trees and shrubs. (See Surculus.)
Of the AERIAL stem, the mos remarkable forms are the following:
The Runner, fig. 28. (sarmentum of Fuchs and Linnæus), is a prostrate filiform stem, forming at its extremity roots and a young plant, which itself gives birth to new runners, as in the Strawberry. Rightly considered, it is a prostrate viviparous scape, that is to say, a scape which produces roots and leaves instead of flowers. It has been called flagellum by some modern botanists, but that term properly applies to the trailing shoots of the vine.
The Sucker, fig. 30. (surculus), is a branch which proceeds from the neck of a plant beneath the surface, and becomes erect as soon as it emerges from the earth, immediately producing leaves and branches, and subsequently roots from its base, as in Rosa spinosissima, and many other plants. Link applies the term soboles to this form of stem. From this has been distinguished by some botanists the Stole (stolo), which may be considered the reverse of the sucker, differing in proceeding from the stem above the surface of the earth, into which it afterwards descends and takes root, as in Aster junceus ; but there does not appear to be any material distinction between them. Willdenow confines the term surculus to the creeping stems of Mosses. By the older botanists a sucker was always understood by the word stolo, and surculus indicated a vigorous young shoot without branches.
The shoots thrown up from the subterranean part of the stem of Monocotyledonous plants, as the Pineapple for example (the Adnata, Adnascentia, or Appendices of Fuchsius), are of the nature of suckers.