Fossil plants, Vol. 3

C. PITYEAE.

Pitys. Witham.

This generic name in the form Pitus was first used by Witham[753] for some petrified pieces of large stems from Lower Carboniferous strata in the south of Scotland (Berwickshire). In an earlier paper Witham[754] gives an account of the beds on the banks of the Tweed from which the specimens of Pitys and other plants were obtained. The name was revived by Goeppert[755] as a substitute for Pissadendron[756] adopted by Endlicher and Unger: Scott[757] employed the older name in the account of his investigations, which have added greatly to our knowledge of Witham’s types. The distinguishing characters of Pitys are: the Araucarian pitting of the secondary xylem, the large pith, the occurrence of numerous small primary xylem strands of mesarch structure in the peripheral region of the pith, and the simple nature of the leaf-traces. Nothing is known of the extrastelar tissues, and beyond the fact that the species were arborescent we have no information with regard to the foliar[758] or reproductive organs.

Pitys antiqua Witham.

In 1899 Scott[759] published a preliminary description of some sections in the possession of Dr Kidston from Lennel Braes (Berwickshire: Calciferous series) under the name Araucarioxylon antiquum, but in the more complete account the name Pitys[760] is adopted. The following description is based on Scott’s paper and on the examination of the sections. Fig. 486 represents a diagrammatic sketch of a section (2·7 × 2·7 cm.) of the central region of a stem which had lost most of its secondary wood. In some cases the pith reaches a diameter of 3 cm.; it consists of large parenchymatous cells broader than deep, several of which contain a black substance and are probably secretory elements: the intercellular spaces are fairly large. Horizontally extended gaps, due to shrinkage and collapse, occur in the pith and give to it an appearance not unlike that of Cordaites. Numerous small elliptical strands of primary xylem, 0·15–0·3 mm. in diameter, are scattered in the peripheral region (fig. 486) at varying distances from the inner edge of the secondary wood and occasionally, when about to bend outwards as a leaf-trace, a primary xylem-strand abuts directly on the secondary xylem. The protoxylem occupies a more or less central position in each mesarch strand (fig. 487, B, px): the centrifugal tracheids have the Araucarian pitting while the centripetal elements are spiral. A few parenchymatous cells are associated with the more central tracheids. A leaf-trace about to bend outwards into the zone of secondary wood is double, but at a slightly higher level in its course it assumes the form of a single strand. The foliar bundles are thus single and not true double traces. Scott recognised, at the extreme edge of the pith, an association with outgoing leaf-traces of a single reparatory strand deeper in the medullary tissue. In this species there is no evidence that an outgoing trace is accompanied by an arc of secondary xylem, but that such was the case is rendered highly probable by the discovery of an arc of wood added by the cambium to a leaf-trace in Pitys Withami[761]. The radial disposition of the pith-cells, many of which appear to be secretory, around each primary xylem-strand (fig. 487, B) is a characteristic feature. A difficult problem, namely the relation of primary xylem and phloem, is raised by the fact that the primary strands are composed exclusively of xylem and are in most cases separated from the secondary wood by several parenchymatous cells. Scott favours the view that the primary phloem was originally at some distance from the primary xylem, the cambium being formed nearer to the phloem, an arrangement foreign to recent Gymnosperms. It is perhaps conceivable that the primary conducting strands included no true phloem.

The secondary xylem consists of tracheids with 4–5 alternate rows of hexagonal pits on the radial walls (fig. 487, A) and not infrequently on the tangential faces. Near the ends of the tracheids the pits are occasionally more scattered and separate and may be reduced to a single row[762], but on the whole the pitting is essentially Araucarian. The medullary rays are generally 4 cells in breadth, but may be 7 cells broad. In depth the rays extend to 70 cells. As seen in fig. 487, B, the inner ends of the rays are especially broad owing to the tangential dilatation of the cells. The innermost secondary tracheids are characterised by pitting intermediate between spiral and reticulate. There are no regular rings of growth in the wood.

Pitys primaeva Witham. This species[763], also founded on material from the Calciferous sandstone of Berwickshire, differs from P. antiqua in the broader and shorter form and greater breadth of the medullary rays (fig. 488, A, B), also in the larger tracheids and in the less crowded arrangement of the bordered pits in which the circular form sometimes replaces the hexagonal type. The structure of the pith is not known, but Scott was able to recognise in the partially preserved pith of a branch indications of primary-xylem groups and other features pointing to a close resemblance to P. antiqua[764]. A piece of stem originally described by Williamson[765] as Lyginodendron (?) anomalum from the Lower Carboniferous volcanic ash of the Isle of Arran would appear to be closely related to, if not specifically identical with, P. primaeva and, as Scott says, it should at least be included in the same genus.

This species was founded on some sections cut from an incomplete stem 36 feet long obtained in 1826 from the Calciferous sandstone of the Craigleith quarry near Edinburgh. The specimen named by Lindley and Hutton Pinites medullaris is no doubt specifically identical with the larger stem. In the same year (1831) Witham[766] published an account of a still larger stem from the same locality, 47 ft long and 5 ft in diameter in the lower part, and in his book the name Pinites Withami is adopted. A large specimen of this tree is erected in the grounds of the Natural History Museum, London, and other specimens are preserved in the Edinburgh Botanic Garden. Goeppert in 1850 referred the species to Dadoxylon and later to Pitys[767], while Brongniart[768] assigned it to his genus Palaeoxylon. Scott[769], who examined Witham’s sections placed it in Pitys. The pitting of the secondary tracheids is often multiseriate, but the medullary rays are narrower than in Pitys antiqua and P. primaeva, rarely exceeding 4 cells in breadth, though still of the manoxylic type. Primary-xylem strands occur in the peripheral region of the pith agreeing closely with those of P. antiqua. There are no regular and continuous annual rings though as Witham stated there are concentric markings on the wood which superficially resemble true rings. The leaf-traces are single and in their passage through the cylinder of wood an arc of secondary tracheids is added to the primary elements.

Archaeopitys. Scott and Jeffrey.

Archaeopitys Eastmanii Scott and Jeffrey. The genus Archaeopitys[770] has recently been instituted for a specimen of a stem from the Lower Carboniferous rocks of Kentucky which differs from Pitys antiqua, the type with which it is most closely allied, in the position and behaviour of the primary vascular strands in the pith. The type-specimen is a piece of stem 2·7 cm. in diameter including a solid parenchymatous pith 5·5 mm. broad and a cylinder of secondary wood. The wood is similar to that of Cordaites; the tracheids have 2–3 rows of pits but for the most part the details of structure are not preserved; the medullary rays are both uniseriate and multiseriate and not very deep; the structure is more Cordaitean than Cycadean. A characteristic feature is the occurrence of about 30 vascular bundles in the medullary region: these mesarch strands, with the protoxylem near the centre, are scattered through the pith and several lie on the outer edge in contact with the secondary xylem or are partially embedded in the xylem-cylinder. The examination of a series of sections demonstrated the fusion of perimedullary with medullary strands and the occasional union of the strands of both regions with one another. It appears that the perimedullary strands are the leaf-traces while the strands deeper in the pith are merely branches from the peripheral leaf-trace strands.

In Pitys antiqua the medullary xylem strands are confined to the outer zone of the pith and constitute the leaf-traces: in Archaeopitys, on the other hand, the medullary strands are scattered through the pith and the leaf-traces are restricted to the circummedullary region in actual contact with the secondary xylem. In the Devonian genus Callixylon there are similar strands but they are confined to the edge of the pith and are usually in contact with the wood as are the circummedullary strands in Archaeopitys. The grouping of the secondary xylem of Callixylon into wedge-shaped masses at the inner edge of the cylinder is a characteristic feature; this feature is less definite in Archaeopitys and absent from Pitys.

Callixylon. Zalessky.

Callixylon Trifilievi Zalessky. This genus is represented by a single species[771], originally referred to Dadoxylon, founded on material from Upper Devonian rocks in the Donetz basin, Russia, and based solely on the anatomical characters of the secondary wood and central region of an arborescent stem. The wood is pycnoxylic and of the Araucarian type; the inner portion of the xylem-cylinder is divided into groups, similar to the regular wedges in a Calamite stem, consisting of tracheids converging towards an obtuse apex occupied by a primary xylem strand, fig. 489, A, a, b. On the radial walls of the tracheids the bordered pits not infrequently form a single row of flattened ovals, but more usually there are two or sometimes three rows of alternate contiguous pits (fig. 489, B): circular and separate pits also occur. As Zalessky states, the pits do not always occupy the whole radial face; unpitted patches sometimes interrupt the continuity of the pitted areas[772]. Similar circular and more scattered pits are unusually abundant on the tangential walls. There are no complete rings of growth. The medullary rays are narrow and, except at their dilated inner ends, uniseriate; usually one or a few cells deep, they may reach a depth of 12 or more cells. The pits on the radial walls of the ray-cells are said to be 4–7 in the field. The pith consists of thin-walled flattened parenchyma frequently elongated in a radial direction. At the inner edge of the secondary xylem and generally in contact with it are several anastomosing strands of primary xylem, mesarch in structure but with the protoxylem nearer the inner edge. These bundles may be single (fig. 489, A, a) or double (fig. 489, A, b, and C) and closely resemble those of Pitys antiqua except in their closer relation to the secondary wood. The leaf-traces have not been described, but the occurrence of twin-bundles like those in fig. 489, C, suggests that they were double. The primary-xylem elements show particularly well transitional forms of pitting connecting the multiseriate and scalariform types.

Callixylon, though conveniently and justly regarded as a distinct genus, exhibits in its primary xylem a fairly close agreement with Pitys[773]. The above account is based in part on Zalessky’s description and partly on specimens in Dr Kidston’s possession.

Miss Elkins and W. Wieland[774] have recently described some Upper Devonian wood from Indiana characterised by a grouping of the circular or elliptical bordered pits in the radial walls of the tracheids similar to those in Callixylon Trifilievi which they include together with the Middle Devonian species Cordaites Newberryi[775] in Zalessky’s genus. Though these two American species are comparable in the discontinuous arrangement of the tracheal pits with the Russian type the latter is characterised by the presence of primary xylem-strands, a feature that has not been recognised in the American stems: it would seem, then, undesirable to adopt the designation Callixylon in preference to Dadoxylon unless there is evidence as to similar characters in the primary region of the xylem.

Coenoxylon. Zalessky.

Coenoxylon Scotti Zalessky. Prof. Zalessky[776] gave the name Coenoxylon to a small and incomplete piece of stem of doubtful provenance but possibly from the Ural Permian beds. The pith, 2 cm. broad, consists of parenchyma associated in the central region with numerous large sclerous cells. In one section a sinuous band of meristematic tissue was observed near the periphery of the pith: the appearance of this tissue in a photograph given to me by Prof. Zalessky suggests comparison with occasional strips of similar dividing cells in the pith of Lyginopteris. The secondary wood is composed of tracheids with 1–2 rows of flattened or hexagonal pits on the radial walls and narrow uniseriate medullary rays reaching a depth of 15 cells and with 2–7 oval pits in the field. As in the wood of Mesopitys Tchihatcheffi[777] there are distinct and apparently complete rings of growth.

It is on the ground of the arrangement and structure of the primary xylem that Zalessky instituted a new generic name. The primary xylem forms teeth of variable size which project into the pith from the edge of the secondary xylem: the prominent portions of the main mass of primary xylem give off branches, differing considerably in size and shape, some of which become separated by a comparatively broad band of parenchyma from the parent xylem-tissue. These bundles anastomose in their course through the pith and in doing so incorporate between them patches of parenchyma. The bundles of primary xylem are endarch. From the centrifugal strands at the periphery of the pith double leaf-traces are produced which pass almost horizontally through the secondary wood. As Zalessky points out, the leaf-traces in their dual nature and in the elongated and narrow form of the tracheal groups, as seen in tangential section of the secondary wood, bear a close resemblance to those of Ginkgo biloba.

This Russian genus agrees in its double leaf-trace with Mesoxylon, Mesopitys, and Antarcticoxylon: among recent plants Ginkgo would seem to be the most closely allied type.

Parapitys. Zalessky.

The designation Parapitys[778] has been proposed for a single Upper Carboniferous species characterised by the possession of secondary wood like that of Cordaites, double leaf-traces, and small mesarch primary xylem-strands. Nothing is known of the leaves or reproductive organs.

Parapitys Spenceri (Scott).

In 1880 Williamson[779] published a short account of a transverse section of a specimen found by Mr J. Spencer in Upper Carboniferous strata near Halifax in Yorkshire which afforded evidence of the occurrence of double leaf-traces. The following description is taken from Scott’s account[780] of the species, which he named Dadoxylon Spenceri. The parenchymatous pith, 5–6 mm. in diameter, is obtusely pentagonal, the prominent angles corresponding to the points of exit of paired leaf-traces like those of Mesoxylon, Ginkgo, and other genera. The secondary xylem consists of narrow tracheids with crowded multiseriate pits on the radial walls and narrow medullary rays one-cell broad and 1–8 cells deep. In contact with the inner margin of the secondary wood are a few small mesarch strands of primary xylem, the protoxylem and some parenchyma occupying a more or less central position. A leaf-trace about to enter the secondary xylem is represented by twin-bundles which retain their double nature as they traverse the stele, but at a lower level the two components fuse and appear as single bundles at the outer edge of the pith. The division of a leaf-trace into two before passing out, as in Poroxylon, constitutes a difference from Lyginopteris in which the division occurs later.

Zalessky’s generic name Parapitys is an appropriate substitute for Dadoxylon in view of the presence of separate primary xylem strands, a feature foreign to typical Dadoxylons which agree with recent Conifers and Cycads in the absence of vascular strands distinct from the endarch centrifugal wood. As Scott[781] says, Parapitys ‘is best regarded as a near ally of Mesoxylon.’

Mesopitys. Zalessky.

Mesopitys Tchihatcheffi (Goeppert). The genus Mesopitys was instituted by Zalessky for a Permian species founded by Goeppert[782] on a piece of decorticated stem from the Kousnetzk basin in Siberia and afterwards more fully described by Goeppert and Stenzel[783]. The structure of the secondary wood led Goeppert to adopt the name Araucarites for which Kraus[784] substituted Araucarioxylon. A recent investigation of additional material by Zalessky brought to light the existence of groups of primary xylem abutting on the secondary xylem and projecting into the pith, characterised by the occurrence of spiral protoxylem elements on the inner edge. The recognition of this important feature justified Zalessky in the adoption of a new generic term. In general anatomical characters Mesopitys agrees with Eristophyton Beinertianum (Goepp.) but is distinguished by the more feebly developed primary-xylem groups and by their endarch structure.

The examination of sections from some of Prof. Zalessky’s material lent to me by Dr Kidston enables me to confirm Zalessky’s description, though I am not convinced that the primary xylem-strands are exclusively endarch: in most of the primary groups the protoxylem is clearly on the inner edge, but in a few cases there may be a small amount of centripetal xylem present. The characters of Mesopitys Tchihatcheffi may be summarised as follows:

Annual rings well defined, varying considerably in breadth; the summer wood is represented by several rows of narrower tracheids (fig. 490, A). In the piece of stem shown in the figure the breadth of the wood from the flattened and crushed pith to the broken outer edge is 6 cm. The medullary rays are numerous, uniseriate, rarely 2 cells in breadth; the pits on the radial walls of the ray-cells, 7–10 in the field, are apparently simple, oval, and oblique: the rays are generally 3–4 cells in depth but may be deeper. There are 1–3 rows of hexagonal alternate rows of pits on the radial walls of the tracheids. The primary xylem consists of groups, varying in size and sometimes reduced to a very few elements, in contact with the secondary xylem, usually though probably not invariably endarch. In the two narrow radially elongated and partially destroyed primary strands shown in fig. 490, B, the protoxylem, px, is on the inner edge. The leaf-trace passes through the secondary wood as a single strand. In the section reproduced in fig. 490, A, the crushed and flattened pith measures 9 by 2 mm.; it consists of thin-walled parenchyma with a few scattered thicker-walled cells.

Nothing is known as to the structure of the cortical tissue or leaves.

Antarcticoxylon. Seward.

Antarcticoxylon Priestleyi Seward. The specimen on which this genus was founded was discovered by Mr Raymond Priestley[785] in a boulder, probably derived from the Upper Beacon sandstone, on the Priestley Glacier south of 74° S. lat. in the course of the journey of the Northern Party of Capt. Scott’s second Antarctic Expedition. The type-specimen is a piece of silicified stem 1 ft long and 3 ins. in diameter; there were no tissues preserved external to the secondary wood and it is impossible to say what proportion of the original thickness is represented. Annual rings are clearly marked at least macroscopically (fig. 491, C) though there is very little difference between the spring and summer tracheids: the centre of the compressed stem is occupied by a lighter coloured elliptical area 7 × 2 mm. which superficially resembles a pith, but in the peripheral region it consists of portions of a cylinder of spiral and scalariform tracheids, the actual pith being not more than 2–3 mm. in breadth. The pith consists of lacunar parenchyma separated by horizontal bands of dark cells containing some secreted substance (fig. 491, F): the preservation is, however, not sufficiently good to enable one to describe it in detail. The secondary xylem is of the pycnoxylic type; the tracheids have either a single row of contiguous and partially flattened pits on their radial walls or a double row of alternate polygonal pits; the medullary rays are nearly always uniseriate (fig. 491, E) and from 1 to 24 cells in depth. At the inner edge of the secondary wood there was a fairly broad zone of more delicate tracheids (fig. 491, A, x) characterised by spiral or scalariform bands and by their relatively small diameter. This innermost zone, which supplies the leaf-traces, is spoken of as the primary xylem; it appears to be endarch though this cannot be definitely determined. A characteristic feature of the primary xylem in the perimedullary region, as also in the leaf-traces on their way through the secondary wood, is the presence of short and broad tracheids (fig. 491, D, t) with reticulate thickening bands: these short elements may represent centripetal xylem and are similar to the short tracheids described by Scott[786] in the sheath of Mesoxylon Lomaxi and to the larger elements in the stem of Megaloxylon[787].

An interesting feature seen in transverse sections of the secondary wood is the occurrence of light bands concentric with the rings of growth which are broadest near the long axis of the stem (fig. 491, C). In their narrower parts these bands are clearly due to the partial destruction of the secondary tracheids, but in other places crushed parenchymatous tissue occurs which may be a traumatic phenomenon or possibly comparable with Nördlinger’s ‘medullary spots[788]’ formed by local hypertrophy of medullary tissue. Although the structure of the leaf-traces cannot be definitely determined, it would seem that each trace passed into the perimedullary region as a single bundle of relatively large size composed of spiral and scalariform tracheids narrower than the secondary elements. The traces during their outward course were accompanied by some parenchymatous tissue continuous with that in the pith, and the inner spiral tracheids of the trace were connected with isodiametric reticulate elements. The dimensions of the leaf-traces point to leaves of fairly large size.

In the structure of the secondary wood Antarcticoxylon agrees on the whole with an Araucarian stem: the broad zone of xylem composed of spiral and scalariform tracheids at the edge of the pith is a feature common to Mesoxylon, Cordaites, and Araucaria. There is no evidence of the occurrence of double leaf-traces such as characterise certain Cordaitalean genera and some existing members of the Araucarineae. In the single nature of the leaf-traces the Antarctic stem resembles Mesopitys Tchihatcheffi also in the presence of rings of growth[789], but in Antarcticoxylon the preservation of the primary xylem is too imperfect to admit of any satisfactory comparison as regards this important tissue with other types.

The precise age of the Beacon sandstone has not been determined, but the probability is that the upper beds from which the boulder containing Antarcticoxylon was derived are not older than the Rhaetic period. The chief interest of this imperfectly preserved stem with undoubted Araucarian affinities is its occurrence in the rocks of Antarctica in association with other remains of comparatively large stems.