The external conjunctive is usually a living comparatively small-celled tissue, whose cells are consider ably elongated in the direction of the stem-axis and frequently contain abundant starch.
In the majority of ferns, at a higher level, after the stele has increased greatly in diameter, a large-celled true pith or medulla, resembling the cortex in its characters, and quite distinct from conjunctive, from which it is separated by an internal endodernlis, appears in the centre.
Sometimes, however, the centre of a bulky root stale has strands of metaxylem (to which may be added strands of metaphioem) scattered through it, the interstices being filled with conjunctive.
The external conjunctive tissue is often arranged in relation to each bundle separately, the pericyclic fibres for instance, already referred to, being cften confined to the bands of pericyclic tissue abutting on the phloem of each bundle, while the Cortex and pith frequently form rays in the intervals between the adjacent bundles.
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.