This generic name was applied by Dr Scott[15] to a calcified cone obtained by Mr James Bennie in 1883 from the Lower Carboniferous plant-beds of Pettycur near Burntisland on the Firth of Forth. Cheirostrobus is distinguished from Sphenophyllostachys by its greater breadth (3.5 cm.); externally it agrees more closely with the fertile shoots of Lepidodendron than with those of Sphenophyllum. A single vascular cylinder having the form of a fluted Doric column ([fig. 117], B, x) occupies the axis of the cone: it consists for the most part of reticulate tracheae which tend to assume a short or isodiametric form in the central region; the smaller protoxylem tracheids with the spiral form of pitting constitute the sharp and prominent ridges at the periphery of the xylem-cylinder. In the outer part of the cylinder the metaxylem[16] consists exclusively of tracheae, but towards the centre of the axis these are associated with numerous parenchymatous cells.
The xylem is therefore centripetal in origin as in Sphenophyllum and in nearly all recent and fossil members of the Lycopodiales. In the type-specimen of Cheirostrobus the vascular cylinder of the cone consists entirely of primary xylem, but secondary xylem has been found in a more recently discovered specimen[17]. Secondary xylem occurs also in the peduncle of the cone. No appreciable remains of phloem have been found. The cortex consists of slightly elongated rather thick-walled tissue containing secretory sacs. Crowded superposed whorls of bracts (or sporophylls), usually twelve in each whorl, are borne on the axis and each sporophyll receives a single vascular bundle from one of the vertical ridges of the xylem column ([fig. 117], A, lt). The members of each whorl are connate at the base: from this narrow collar each sporophyll branches into an upper or dorsal and a lower or ventral limb ([fig. 117], A, f and s). Each limb divides palmately at a short distance from its origin into three slender segments, which extend in a horizontal direction and terminate in large laminar expansions ([fig. 117], B, s) to afford a protective covering to the surface of the cone. The upper set of three segments, constituting sporangiophores ([fig. 117], A, B, f) or fertile divisions of the sporophyll, expand distally into comparatively bulky laminae; each of these bears on its adaxial face four diagonally placed outgrowths which form the short pedicels of very long and narrow sporangia. The three lower segments—the sterile divisions of the sporophylls—([fig. 117], A, B, s) are similar to the upper set except in their greater length and in the kite-shaped form of their distal laminae which are provided with lateral lobes. The single vascular strand which supplies each sporophyll is represented at lt in [fig. 117], B; at lt′ the strand has divided into four, the three upper bundles in the figure supply the sterile segments and the single lower bundle ultimately divides into three which supply the fertile segments. A pair of blunt processes (fig. A, s) extend downwards over the ends of the underlying fertile lamina and two slender prolongations extend upwards through several internodes.
Fig. 117. A, B. Cheirostrobus pettycurensis Scott. (After Scott.)
C, D. Pseudobornia ursina Nath. (After Nathorst.)
- Diagrammatic radial longitudinal section of part of the cone-axis and two sporophylls. lt, bundle passing out to sporophyll; f, fertile segment of sporophyll showing two sporangia; s, sterile (lower) segment.
- Part of transverse section. x, stele; lt, lt′, bundles on their way to sporophylls; a, tips of sterile segments of lower sporophylls.
- Palmately branched leaf (½ natural size).
- Node of stem showing leaf-bases.
An economical arrangement of the long and narrow sporangia and of the sporophyll-segments between the axis and the periphery of the cone is rendered possible by the interlocking of the sterile and fertile segments by means of a groove in the upper face of the latter for the accommodation of the former. The sporangia are characterised by their unusually long and narrow form: the length of a sporangium may reach 1 centimetre. In the structure of the wall the sporangia of Cheirostrobus agree closely with those of Calamostachys[18] and Sphenophyllostachys. The spores are of one size only. The vascular cylinder of the peduncle, originally described by Williamson[19] as the peduncle of a large Lepidostrobus (the cone of Lepidodendron), is characterised by the presence of a short radially disposed zone of secondary tracheids, a feature, as Scott points out, which may extend into the axis of the cone. It is noteworthy that the protoxylem elements are not always external, but occasionally occur internal to one or two of the outermost metaxylem tracheae: the usual exarch[20] structure of the central cylinder is not therefore absolutely constant, but may be replaced by a mesarch arrangement.
The presence of a few sterile leaves on the peduncle below the fertile portion of the cone, which agree in their lobed laminae with the sporophylls, is the only fact which we possess as to the form of the vegetative characters of the genus.
The above description is sufficient to indicate the extraordinary complexity and high degree of specialisation of Cheirostrobus. The sporophylls, with their trilobed segments, and the crowded sporangia of exceptional length attached only by a narrow base constitute striking peculiarities of the genus.
It is unfortunate that we are still without any satisfactory evidence as to the nature of the plant the cones of which have been made the type of a new genus and a new family. Cheirostrobus affords an interesting example of a type of reproductive shoot constructed on a plan sui generis, and may be classed with some other extinct genera as instances of the production in the course of evolution of architectural schemes which appear to have been ill adapted for competition with equally efficient though much simpler types. But the discovery of these isolated forms of restricted geological range among the relics of the Palaeozoic vegetation frequently supplies a key to phylogenetic problems. Cheirostrobus by its complex combination of features characteristic of the Equisetales, the Lycopodiales and the genus Sphenophyllum throws a welcome light on the inter-relationships of groups which represent divergent series. The combination of morphological features in this generalised type led the author of the genus to describe it as a descendant of an old stock which existed prior to the divergence of the Equisetales and Lycopodiales.
The discovery of this new type of strobilus naturally led to a search among Lower Carboniferous plants for vegetative shoots exhibiting characters conformable with the whorled and branched leaves of Cheirostrobus. In Sphenophyllum we have a genus obviously comparable with Cheirostrobus as regards the form and disposition of the leaves, but the differences between the cones and the striking similarity of the vascular cylinder of the latter to that of Lepidodendron demonstrate conclusively that we must look elsewhere for the vegetative members of the plant which produced cones of the Cheirostrobus type.