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Rh that of the amphiphloic haplostele, is maintained throughout the adult stem (Lindsaya). 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 endodermis, appears in the centre. These successive new tissues, appearing in the centre of the stele, as the stem of a higher fern is traced upwards from its first formed parts, are all in continuity with the respective corresponding external tissues at the point of origin of each leaf trace (see below). 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 mesocycle—these two layers, together with the outer mesocycle and pericycle, constituting the conjunctive tissue of the now hollow cylindrical stele. (The conjunctive frequently forms a connected whole with bands of starchy xylem-parenchyma, which, when the xylem is bulky, usually appear among the tracheids, the phloem also often being penetrated by similar bands of phloem-parenchyma.) . 3-15—Types of Stele in Vascular Plants. 3.—Diarch stele of root of a Fern. 4.—Haplostele of stem of young Fern. 5.—Amphiphloic haplostele of stem of young Fern. 6.—Solenostele of stem of Fern showing detachment of leaf-trace and leaf-gap. 7.—Dictyostele of Fern. 8.—Tricyclic solenostele of Matonia. 9.—Tricyclic dictyostele of Danæa. 10.—Diarch haplostele of Selaginella. 11.—Tristelic stem of Selaginella. 12.—Modified haplostele of Lycopodium. 13.—Typical siphonostele of dicotyledon. 14.—Stele of monocotyledon. 15.—Polyarch root of Veratrum (a monocotyledon).

Explanation of Lettering: st. stele; mst. meristele; l.t. leaf-trace; l.g. leaf-gap; cor. cortex; p.t. peristelar tissue; p.l. peristelar lacuna; end. endodermis; p.c. passage cell; per. pericycle; ph. phloem; mes. mesocycle, x. xylem; px. protoxylem; mx. metaxylem; p. pith; ''scl. p. sclerised pith; c. cambium; p.m.r.'' primary, medullary, ray. In the other groups of Pteridophytes internal phloem is not found and an internal endodermis but rarely. The centre of the stele is however often occupied by a large-celled pith resembling the cortex in structure, the cortex and pith together being classed as ground tissue. To this type of

stele having a “ground-tissue pith,” whether with or without internal phloem, is given the name siphonostele to distinguish it from the solid haplostele characteristic of the root, the first-formed portion of the stem, and in the more primitive Pteridophytes, of the whole of the

axis. 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. The solenostele of the ferns is broken by the departure of each leaf-bundle, the outer and inner endodermis joining so that the stele becomes horseshoe-shaped and the cortex continuous with the pith (fig. 6). Such a break is known as a leaf-gap. A little above the departure of the leaf-bundle the stele again closings up, only to be again broken by the departure of the next leaf-bundle. Where the leaves are crowded, a given leaf-gap is not closed before the next ones appear, and the solenostele thus becomes split up into a number of segments, sometimes band-shaped or semilunar, sometimes isodiametric

in cross-section (fig. 7). In the after case each segment of the solenostele frequently resembles a haplostele, the segments of inner endodermis, pericycle, phloem and mesocycle joining with the corresponding outer segments to form a nearly concentric structure. For this reason a stem in which the vascular system has this type of structure used to be spoken of as polystelic, the term “stele” being transferred from the primary central cylinder of the axis and applied to the vascular strands just described. In this use the term loses, of course, its morphological value, and it is better to call such a segment of a broken-up stele a meristele, the whole solenostele with overlapping leaf-gaps being called a dictyostele. The splitting up of the vascular tube into separate strands does not depend wholly upon the occurrence of leaf-gaps. In some forms other gaps (perforations) appear in the vascular tube placing the pith and cortex in communication.