(a.) The Pith.—This is somewhat imperfectly preserved, and appears to have shrunk before fossilization. It consisted mainly of thin-walled parenchyma, but occasionally some of it is sclerenchymatous, though such strands are quite irregularly placed. No trace of xylem tracheids could be seen anywhere in the pith, and no internal phloem could be seen. In the cross-sections shown in the figures the structure is not well shown. Longitudinal sections are far more satisfactory, but none of these are figured.
(b.) Xylem Ring.—This consists of about forty to fifty strands of xylem, forming a most complete ring, and separated by extremly narrow bands of parenchyma often only one cell thick. The thin separating bands of parenchyma are always connected with the xylem sheath and are clearly continuous with it. In about one-third of the bands the parenchyma does not extend inwards to the pith. The xylem ring is ten or eleven elements wide, those near the periphery being of smaller diameter than those near the inner border of the ring. The xylem ring thus has about as many strands as that of O. skidegatensis and O. Kolbei, but
they are much more numerous than in other species, excepting O. Dunlopi, which is described as having a continuous xylem ring. On the other hand, the thickness of the ring is less than in O. skidegatensis and O. Kolbei, but greater than in the other species. It is doubtful whether these characters in themselves are of any value for specific distinction. In longitudinal section the walls of the tracheids are clearly reticulately pitted as described by Kidston and Gywnne-Vaughan. In the larger vessels the pits are multiserial, but they are seldom in vertical rows. (cf. Zalesskya gracilis Kidston and Gwynne-Vaughan, pt. 2, pp. 221, 222). In transverse section the structure is rather peculiar, and more resembles that of Zalesskya than any described species of Osmundites or Todea. As seen in transverse section, an empty space extends along the centre of the cell-wall, but there is a thin black line which appears to extend along the whole length of this empty space. The empty space is most noticeable at the corners where three or more tiacheids meet. At the angle there is an empty three-cornered space with a three-or four-rayed black star within it. In Plate 51, fig. 2, it is clearly seen that the ray of the star forks where the three-cornered space passes into the linear space between the adjacent cell-walls. In Zalesskya Kidston and Gwynne-Vaughan show no empty spaces, and in Osmundites and Todea the black line is not seen, while the empty three-cornered space at the angles is not present, and again the linear space between the adjacent cell-walls is interrupted where the buttresses between the vertical rows of pits are situated. It is thus evident that the structure of the xylem consists of tracheids almost separated from one another. Wherever the tissue is fractured the crevice runs along the centre of the tracheid walls. The present species, therefore, has a more continuous central black line than Zalesskya, and a more pronounced development of the vacant spaces in the tracheid wall than Osmundites and Todea.
(c.) The Xylem Sheath.—This is about three layers of cells thick. The cells are of relatively smaller diameter than the tracheids and have much thinner walls. The cells are elongated and pointed. As previously mentioned, the xylem sheath is always connected with the parenchyma that separates adjacent xylem bundles. In both longitudinal and transverse sections the structure of the xylem sheath and the separating parenchyma appears to be identical. Conspicuous differences distinguish this parenchyma from the short-celled tissue of the pith. It is therefore suggested that the parenchyma bands are derived from and are a part of the tissue of the xylem sheath, and not of the pith. The idea is supported by the apparently similar nature of the granular matter that fills the cells of both of them. That the parenchyma bands are derived from the xylem sheath has been proved by Gwynne-Vaughan (1911) in the case of Osmunda regalis.
(d.) The Phloem.—This forms a continuous layer of nearly uniform thickness outside the xylem sheath. It consists almost entirely of sieve-tubes, some of which have a diameter little inferior to that of the majority of the xylem tracheids. In longitudinal section the sieve-plate structure can be distinctly seen. The exterior layer of the phloem is composed of tangentially elongated cells, but they do not show very distinctly in any of my preparations.
(e.) The Inner Cortex.—This consists of thin-walled parenchymatous tissue, but, like the pith, it appears to have shrunk greatly before fossiliza-tion took place. There are two rows of leaf-traces in this inner cortex. Each of the rows has about twelve leaf-traces.
Fig. 1.—Transverse section of stem of Osmundites aucklandicus; × 2.
Fig. 2.—Transverse section showing pith, xylem ring, and part of inner cortex; × 15. a, protuberance on ring in preparation for formation of leaf-trace; b, leaf-trace just departing from xylem ring; c, leaf-trace further developed, showing one root; d, completely formed departing leaf-trace, showing two roots developing laterally. × 15.
Fig. 1.—Transverse section of Osmundites aucklandicus showing portion of xylem ring and of inner and outer cortex with two roots; × 10. At a and b two leaf-traces departing without leaving any leaf-gap or band of parenchyma in the xylem.
Fig. 2.—Portion of outer part of xylem ring showing leaf-gap (l.g.), xylem sheath (x.s.), part of xylem ring (x.), sieve-tubes of phloem (s.t.), endodermis (e.), inner cortex (i.c.); × 100.
Fig. 1.—Portion of outer part of xylem ring showing leaf-gap (l.g.). Lettering as in Plate 49, fig. 2. × 80.
Fig. 2.—Leaf-trace about to depart from xylem ring; × 50. A small rock-fracture is seen to traverse the tissue along the middle line of the tracheid walls.
Fig. 1.—Leaf-trace departing from outer cortex; × 50.
Fig. 2.—Leaf-base with xylem assuming a horseshoe shape; × 50.
Fig. 3.—Leaf-base with xylem further developed; × 50.
Fig. 4.—Leaf-base showing the formation of sclerenchyma strand within the bend of the horseshoe; × 50.
Fig. 5.—Leaf-base with sclerenchyma more developed; × 50.
Fig. 6.—Final form of the leaf-base near the margin of the specimen; × 30.
(f.) The Outer Cortex.— There are four rows of leaf-traces in this region of the stem, each of which consists of about twelve leaf-traces. The tissue of the outer cortex consists of thick-walled sclerenchyma that is of a uniform nature throughout. The roots traverse this tissue as well as that of the inner cortex. The outer cortex appears to be much thicker than in other described species, though O. Dunlopi approaches it in this respect. The outer part of the specimen consists of closely adherent stipular leaf-bases. In these the parenchyma strands have a distribution which approaches most closely to that in Todea. superba, though there are considerable differences. The leaf-traces are given off in a manner very similar to those of Todea. The pocket of xylem sheath starts just above the actual leaf-trace and gradually extends across the xylem ring, but does not always reach the pith. The protoxylem is at first almost mesarch in a few instances, but soon becomes exarch in all. The leaf-trace passes through the inner and outer cortex in the usual manner, and acquires a ring of each of these tissues during its passage. The form of the leaf-trace is reniform until it has passed through the outer cortex, and it then rapidly becomes horseshoe-shaped. In passing through the outer cortex the xylem of the leaf-trace becomes somewhat smaller, and the number of tracheids is reduced. This smaller number is retained until the horseshoe shape is well developed. At about the sixth ring of leaf-traces outside the outer cortex a small amount of sclerenchyma begins to appear within the bend of the horseshoe of xylem; this soon divides into two strands of sclerenchyma, and the leaf-base develops an appearance somewhat similar to that of Todea barbara.
This species of Osmundites appears to resemble O. Kolbei more closely than any other species that has been described. Its woody structure, however, appears to connect Osmundites with Zalesskya. Though a relatively early species, it has a complete wide pith without any tracheids; the xylem ring also is narrow with reference to the diameter of the pith. The breaks in the xylem ring are very numerous, a fact that is obviously related to the large number of leaf-bases. It is clearly seen that the parenchyma that breaks the xylem ring is derived from the xylem sheath.
The type and microscopic preparations are deposited in the botanical department, Otago University.
I am much indebted to Dr. Holloway for pointing out some inaccuracies in the proof.