CORDAITALES.
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.
[Illustration: Fig. 486. _Pitys antiqua._ Central region of stem,
showing primary xylem strands as black dots. (Kidston Collection, 598
A.)]
_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.
[Illustration: Fig. 487. _Pitys antiqua._ A, radial section of
secondary xylem; B, transverse section showing the inner edge of the
secondary wood and two primary-xylem strands with protoxylem, _px_.
(Kidston Coll.)]
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.
[Illustration: Fig. 488. _Pitys primaeva._ A, transverse section of
the secondary xylem; B, tangential section of the secondary xylem.
(From sections of the type-specimen in the Edinburgh University
Botanical Department.)]
_Pitys Withami_ (Lindley and Hutton). The Craigleith Tree.
1831. _Pinites Withami_ Lindley and Hutton, Foss. Flora, Vol. +i.+
Pl. +ii.+
1831. _Pinites medullaris_, Ibid. Pl. +iii.+
1902. _Pitys Withami_ Scott, Trans. R. Soc. Edinb. Vol. +xl.+ p. 354,
Pl. +ii.+ fig. 10: Pl. +vi.+ fig. 21.
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.
[Illustration: Fig. 489. _Callixylon Trifilievi._ A, C, the inner
margin of the secondary xylem showing strands of primary xylem,
_a_, _b_, and C. B, tracheids of the secondary xylem. (A, C, from
photographs supplied by Prof. Zalessky; B, Cambridge Botany School.)]
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.
[Illustration: Fig. 490. _Mesopitys Tchihatcheffi._ A, transverse
section showing rings of growth in the wood. B, primary xylem with
protoxylem, _px_, and the edge of the secondary xylem, _x²_. (Kidston
collection, 2156.)]
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.
[Illustration: Fig. 491. A–F, _Antarcticoxylon Priestleyi._ A, B,
transverse sections of the xylem; _a_, small tracheids simulating
protoxylem; _x_, xylem at the edge of the pith. C, transverse section
showing leaf-traces, _l_, _a_, _b_. D, leaf-trace with short tracheids,
_t_, in the pith; _s_, sclerous cells. E, tangential section. F, pith
in longitudinal section. G, _Pityosporites antarcticus_; see Vol. +iv.+
(All the sections are in the British Museum.)]
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.