Remarks on Prof. Williamson's New Classification of the Vascular Cryptogams

Abstract

IN discussing the points at issue between Prof. Williamson and myself, it will be necessary for me to say a few words on stems in general, because we evidently have very different views of the construction of stems; and until we thoroughly understand each other, it is impossible for us to come to any definite conclusions. In a young dicotyledonous stem (see Oliver's “Lessons,” p. 112, fig. 67) we find three things: a quantity of cellular tissue surrounded by an epidermis, and near the centre a series of young fibro-vascular bundles. As growth goes on, these separate bundles coalesce and form a central cylinder of united fibro-vascular bundles. These bundles leave a portion of the cellular tissue in the middle of the stem, which becomes the pith. Outside the fibro-vascular bundles we have also a small quantity of the cellular tissue, but it soon becomesto a great extent inseparable from the sub-epidermal cells. Other portions of the cellular tissue remain between the united fibro-vascular bundles, and form the medullary rays. In many stems and in most roots these rays are wanting, and the cellular tissue would therefore be divided into two portions by the united bundles. Each fibro-vascular bundle consists of two portions, which are separated by a layer of cells capable of division, the cambium. On the inner side of the cambium cells we have in general spiral vessels; porous vessels, and wood cells, while on the outer side we have the soft bast and bast fibres. The epidermis is soon thrown off in many cases, and is replaced by layers of cork-cells or peculiar thickened bast-fibre-like cells from underneath. The stem thus consists of three sets of tissues: (1) the limitary tissues, including epidermis, periderm, &c.; (2) the fibro-vascular bundles; and (3) the primitive tissue or Grundgewebe of Sachs (see “Mo. Mic. Journal,” vol. iii. p. 160). In an older dicotyledonous stem we find the limitary tissues becoming largely developed, cork-cambium and layers of cork being formed. The fibro-vascular bundles have also largely developed, the cambium cells by division, and the conversion of these new cells into permanent tissue has formed a number of annual rings of wood-cells.and vessels as well as layers of bast, while the primitive tissue only increases very slowly in the medullary rays, the pith not increasing, and the primitive tissue under the epidermis becoming lost in the rapidly-developing bark. Such is the structure of a dicotyledonous stem.