It is surrounded by (I) a thin-walled, smaller-celled hydrom mantle; (2) an amylom sheath; (3) a leptom mantle, interrupted here and there by starch cells.
As the aerial stem is traced down into the underground rhizome portion, these three mantles die out almost entirelythe central hydrom strand forming the bulk of the cylinder and its elements becoming mixed with thick-walled stereids; at the same time this central hydromstereom strand becomes three-lobed, with deep furrows between the lobes in which the few remaining leptoids run, separated from the central mass by a few starchy cells, the remains of the amylom sheath.
In higher forms the conducting strands of the leaves are continued downwards into the stem, and eventually come into connection with the central hydrom cylinder, forming a complete cylindrical investment apparently distinct from the latter, and exhibiting a differentiation into hydrom, leptom and amylom which almost completely parallels that found among the true vascular plants.
The tracheids or vessels, indifferently called tracheal elements, together with the immediately associated cells (usually amylom in Pteridophytes) constitute the xylem of the plant.
The cylinder is surrounded by a mantle of one or more layers of parenchymatous cells, the pericycle, and the xylem is generally separated from the phloem in the stem by a similar layer, the mesocycle (corresponding with the amylom sheath in mosses).
In the simplest condition we have merely tracheae and sieve-tubes, respectively associated with parenchyma, which in the former case is usually amylom, i.e.
Among Gymnosperms the secondary xylem is similarly simple, consisting of tracheids which act as stereom as well as hydrom, and a little amylom; while the phloem-parenchyma sometimes undergoes a differentiation, part being developed as amylom, part as proteid cells immediately associated with the sieve-tube, in other cases the proteid cells of the secondary phloem do not form part of the phloem-parenchyma, but occupy the top and bottom cellrows of the medullary rays, the middle rows consisting of ordinary starchy cells.