Outside this are three arcs of large cells showing characters typical of the endodermis in a vascular plan.t; these are interrupted by strands ofnarrow, elongated, thick-walled cells, which send branches into the little brown scales borne by the rhizome.
Besides this there is usually a living conducting tissue, sometimes differentiated as leptom, forming a mantle round the hydrom, and bounded externally by a more or less well-differentiated endodermis, abutting on an irregularly cylindrical lacuna; the latter separates the central conducting cylinder from the cortex of the seta, which, like the cortex of the gametophyte stem, is usually differentiated into an outer thick-walled stereom and an inner starchy parenchyma.
Rarely in the leaf, frequently in the stem (particularly in Pteridophytes), and universally in the root, the phloeoterma is developed as an endodermis (see below).
This consists of a few xylem elements, e a a segment of phloem, pericycle, and usually an arc of h~s endodermis, which closes round the bundle as it detaches ~
Where internal phloem is present this is separated from the internal endodermis by an endocycle or internal pericycle, as it is sometimes called, and from the xylem by an internal mesocyclethese two layers, together with the outer mesocycle and pericycle, constituting the conjunctive tissue of the now hollow cylindrical stele.
In the other groups of Pteridophytes internal phloem is not found and an internal endodermis but rarely.
The type of siphonostele characteristic of many ferns, in which are found internal phloem, and an internal endodermis separating the vascular conjunctive from the pith is known as a solenostele.
Haplostele, the segments of inner endodermis, pericycle, phloem and ~ Pig.
The whole stele may be surrounded by a common external endodermis; sometimes there is an internal endodermis in addition, separating the bundles from the pith; while in other cases each bundle possesses a separate endodermis surrounding it.
At the nodes the relation of the endodermis to the bundles undergoes rather complex but definite changes.
The inner layer of the cortex (phloeoterma) may form a well-marked endodermis, or differ in other ways from the rest of the cortex.
In other cases the reduction goes much further, till the endodermis eventually comes to surround nothing but an intercellular channel formed in place of the stelar tissue.
A pine needle grown iji continuous light differs from one grown under ordinary conditions in the absence of hypodermal fibres, in the absence of the characteristic infoldings of the mesophyll cell-walls, in the smaller size of the resin-canals, &c. The endodermis in Pinus, Picea and many other genera is usually a well-defined layer of cells enclosing the vascular bundles, and separated from them by a tissue consisting in part of ordinary parenchyma and to some extent of isodiametric tracheids; but this tissue, usually spoken of as the pericycle, is in direct continuity with other stem-tissues as well as the pericycle.