VEGETABLE PHYSIOLOGY. 
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CROSS AND PERPENDICULAR SECTIONS OF THE STEM OF 
A DICOTYLEDON, THREE YEARS OLD. 
In the centre is seen the pith, a, composed of cellular 
tissue; surrounding it is the so-called medullary sheath, 
b ; and exterior to this are three rings of -wood, each 
consisting of porous ducts, c, c, and wood-cells or fibres, 
d, d. The outermost is partly in the delicate state called 
cambium, and joins the hark, the various layers of which 
are indistinct. 
To the elastic spiral structures succeed annular and reticulated tubes, more solid forms of the same 
structures; then comes a considerable mass of wood, properly so called, the component parts of which 
differ very much in different plants. Commonly the greater portion consists of the spindle-shaped 
wood-cells, which become more and more solid by internal 
deposits as they grow older; while, scattered here and 
there among the wood-cells, occur large ducts or porous 
tubes, the open ends of which are often visible to the 
naked eye in wood cut across the grain. In some plants 
the wood is wholly composed of these porous ducts or 
tubes; in others the wood-cells are large, and marked 
with pores, like the ducts. In some the wood-cells are very 
small, and closely packed, giving great density; and 
again, in others, loose cellular regions occur, scattered 
through the wood, giving to the stem a light and spongy 
consistence. Into the minutiae of these points it is not 
worth while to enter here, although they offer a most 
interesting field of observation to the microscopist. 
Toward the outer part of the wood we find the tissue 
of the fibro-vascular bundles become gradually more and 
more delicate, so that at last its cells are quite soft, and 
offer but verv little resistance to external violence : it is 
at this point that the rind peels off in stripping young 
twigs, or barking wood, and when the cells of this region 
are gorged with sap in spring, they present so little density 
of texture, that this region was supposed formerly to be 
a free space in which the thick sap or cambium flowed to 
form the new wood. It has been clearly shown, however, that the tissue extends uninterruptedly into 
the bark, and this delicate region is merely to be distinguished by the name of the cambium layer; its 
importance will be shown presently. As the wood becomes gradually more delicate towards the cam¬ 
bium layer, so this latter again, but more suddenly, changes outwards, and we arrive next at a new 
woody structure; this consists of the liber bundles, which are long bundles or strips of a firm stringy 
texture, lying opposite to and outside the fibro-vascular bundles of the wood, and forming the fibrous 
region of the bark. These bundles are composed of cells resembling those of the wood, except that 
they are exceedingly long and slender; and, as they are not collected into such masses as the cells of 
the internal regions of the stem, they form tough and stringy textures instead of solid wood. They 
are well seen on the surface of the stem of the Vine or Clematis, when the outer layers of the bark 
decay so as to expose its liber in stringy shreds. The liber bundles are surrounded and imbedded in 
a layer of cellular tissue, into which the ends of the medullary rays pass out between them ; the cells 
of this layer are usually filled with green matter, and this region of the bark is the seat of an active 
vegetation, performing in young stems, and probably to some extent in old ones, similar functions to 
those of the leaves. The outside of the stem is formed at first of a 
layer of green epidermis, the “ skin ” of the plant, which will be 
spoken of hereafter; as it grows older, and in most plants by the 
end of the first year, it has acquired a brown colour, from the 
development of the corky layer, a layer of cells having thin but 
strong walls, and constituting a light, but fine and tough tissue, 
destined to protect the more delicate and more actively occupied 
structures beneath. Such is the structure of a Dicotyledonous 
stem at the close of the first year of its growth; but its develop¬ 
ment does not cease here. The stem thus formed is capable of 
increasing, by a regular and gradual mode of growth, to an almost 
unlimited extent, at all events to an extent which is limited by 
external agencies alone, since the fibro-vascular region is endowed 
with a perennial vitality, which causes it to produce new layers 
year after year so long as the other organs of the plant furnish it 
with nutriment. 
In the second spring, the cells of the cambium layer begin to multiply once more; they divide, 
subdivide, and expand, pushing the bark outwards, and thus form a new layer of wood all over the 
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A DICOTYLEDONOUS STEM, ONE YEAR OLD. 
A DICOTYLEDONOUS STEM, TWO YEARS OLD. 
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