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. [Illustration: 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. [Illustration: Fig. 37.—Diagram of a Stele with a few Cells of Pith _p_ in the Middle of the Wood. Lettering as in fig. 36] [Illustration: Fig. 38.—Diagram showing Extensive Pith _p_ in the Wood. Lettering as in fig. 36] The next type of stele is quite similar to the protostele, but with the addition of a few large unspecialized cells in the middle of the wood (_p_, fig. 37); these are the commencement of the hollowing process which goes on in the wood, resulting later in the formation of a considerable pith, as is seen in fig. 38, where the wood is now a hollow cylinder, as the phloem has been from the first. When this is the case, a second sheath or endodermis generally develops on the inner side of the wood, outside the pith, and cuts the vascular tissues off from the inner parenchyma. A further step is the development of an inner cylinder of bast so that the vascular ring is completely double, with endodermis on both sides of the cylinder, as is seen in fig. 39. [Illustration: Fig. 39.—A Cylindrical Stele, with _e_, inner endodermis, and _ph_, inner phloem; W, wood; P, outer phloem; E, outer endodermis. L, part of the stele going out to supply a large leaf, thus breaking what would otherwise appear as a closed ring stele]