This section is from the book "The Gardener V1", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
The Stem or Trunk is that portion of the tree which has by some been termed the axis, supplied with pipes, cells, and filters, and through which the sap rises in its progress to the leaves. Part of the stem displays a vascular, and the other portion exhibits a cellular, structure. The stem is produced by the successive development of leaf-buds, which cause a corresponding horizontal growth between them, and varies in structure in four principal ways, - viz., Exogens, with 2 cotyledons, the radicle itself usually elongating; leaves net-veined; perianth (petals) in 4's or 5's; wood in a continuous ring, formed by successive additions to the outside. Endogens, with 1 cotyledon, the radicle usually remaining undeveloped; leaves straight - veined; perianth in 3's or multiples of 3's; wood in isolated bundles, formed by successive additions to its centre. Acrogens, when the stems are formed by the union of the bases of the leaves, and the extension of the point of the axis; or by simple elongation or dilatation, where no leaves or buds exist, as among Thallogens. In altitude or length, and diameter, stems present the most varied and contrasted features. According to some travellers, there is a Palm that grows 15 feet high, with a trunk not thicker than a finger.
A comparison, indeed, between the stems of various plants would in some cases afford examples of widely divergent extremes. The Scirpus capillaris is not thicker than a hair, and some are as fine as a gossamer thread; while the trunk of the Baobab is nearly 100 feet in circumference. The stem of Exogens may be distinguished into the Pith, the Medullary Sheath, the Wood, the Bark, and the Medullary Rays.
The Pith consists of cellular tissue, occupying the centre of the stem. It is always solid when first organised; but in some cases it separates into regular cavities, as in the Walnut, when it is disciform; or it tears into irregular spaces, as in the Uinbelliferae (Parsley, Carrot, and Parsnip, for instance).
The Medullary Sheath consists of spiral vessels; it immediately surrounds the pith, projections of which pass through it into the medullary rays, and is in direct communication with the leaf-buds and the veins of the leaves; and by it oxygen is carried upwards, liberated by the decomposition of carbonic acid or of water, and conducts it into the leaves.
The Wood lies upon the medullary sheath, and consists of concentric layers; is formed by the successive deposit of organised matter descending from the buds, and by the interposition of the medullary system connecting the pith and the bark. New wood is formed annually and deposited between the external surface of the woody skeleton and the inner surface of the liber. Therefore the central wood is the oldest and firmest, and necessarily the most mature and permanent. Our forest-trees supply examples of this kind of growth, and the age of a tree may be known by the number of concentric layers. A layer is the produce of one year's growth in countries having a winter and summer; but this rule is of uncertain application, as in some tropical countries, owing to the very small and sometimes no period of rest, more than one is formed.
The Medullary Rays consist of cylinders or compressed parallelograms of cellular tissue belonging to the medullary system : they connect together the tissue of the trunk, maintaining a communication between the centre and circumference; act as braces to the woody and vasiform tissue of the wood; convey secreted matter horizontally from the bark to the heart-wood; and generate adventitious leaf-buds.
The stem of Endogens offers no absolute distinction of pith, medullary rays, wood, and bark. Endogenous plants deposit a layer of wood internally and towards the centre. The external cylinder is consequently the oldest and first formed, and therefore the exterior is the hardest or most indurated, as is also the lowest part of the trunk; and, it may be repeated, the stem is thus gradually built up by a prolongation of the fibres of the leaves that are deflected at a specific angle towards the centre. From this view of the case it must appear that the extension in length of Endogens bears no ratio of proportion to their diameter, as is exemplified in the case of Bamboos or the Palm.
In what are called Dictyogens, the stem has the structure of Exogens, the root that of Endogens - Smilax is an example.