Plant anatomists lay particular stress on the vascular system, which, in comparison with animal anatomy, holds an even more important position than does the skeleton. To understand the essential characters of stems, both living and fossil, and to appreciate their points of likeness or difference, it is necessary to have some knowledge of the general facts of anatomy; hence the main points on which stress is laid will be given now in brief outline.
Leaving aside consideration of the more rudimentary and less defined structure of the algæ and mosses, all plants may be said to possess a “vascular system”. This is typically composed of elongated wood (or xylem) with accessory cells (see [p. 57], table), and bast (phloem), also with accessory cells. These specialized conducting elements lie in the ground tissue, and in nearly all cases are cut off from direct contact with it by a definite sheath, called the endodermis (see [p. 55], [fig. 26]). Very often there are also groups or rings of hard thick-walled cells associated with the vascular tissues, which protect them and play an important part in the consolidation of the whole stem.
Fig. 36.—Diagram of Simplest Arrangement of Complete Stele in a Stem
W, Central solid wood; P, ring of bast; E, enclosing sheath of endodermis; C, ground tissue or cortex.
The simplest, and probably evolutionally the most primitive form which is taken by the vascular tissues, is that of a single central strand, with the wood in the middle, the bast round it, and a circular endodermis enclosing all, as in [fig. 36], which shows a diagram of this arrangement. Such a mass of wood and bast surrounded by an endodermis, is technically known as a stele, a very convenient term which is much used by anatomists. In its simplest form (as in [fig. 36]) it is called a protostele, and is to be found in both living and fossil plants. A number of plants which get more complex steles later on, have protosteles in the early stages of their development, as in Pteris aurita for example, a species allied to the bracken fern, which has a hollow ring stele when mature.
Fig. 37.—Diagram of a Stele with a few Cells of Pith p in the Middle of the Wood. Lettering as in [fig. 36]
Fig. 38.—Diagram showing Extensive Pith p in the Wood. Lettering as in [fig. 36]