Sphenophyllostachys Dawsoni (Will.). Figs. 107, A and G, 108.
Probably the strobilus of Sphenophyllum cuneifolium (Sternb.).
Fig. 108. Diagrammatic longitudinal section of a Sphenophyllum strobilus.
The upper figure represents a portion of a whorl of bracts. (The smaller figure, after Zeiller.)
The cone consists of a central axis bearing a number of verticils of bracts coherent in their lower portions in the form of a widely open funnel-shaped disc, which splits up peripherally into 14–20 linear-lanceolate segments. The free segments of each verticil have an obliquely ascending or almost vertical position, and extend upwards for a distance of about six internodes. The smaller drawing in fig. 108 shows the appearance in side view of the narrow bracts of a single whorl. A transverse section of a strobilus would include, therefore, sections of several concentric series of ascending bracts. The verticils of Sphenophyllostachys Dawsoni are probably superposed, but this point has not been definitely settled. From the upper surface of the coherent basal portion of each verticil, there are given off twice as many sporangiophores as there are free segments, and these are attached close to the line of junction of the axis of the cone and the funnel-shaped disc. Each sporangiophore has the form of a slender stalk which bends inwards at its distal end and bears a single sporangium (cf. fig. 107, D). The sporangiophores given off from the same verticil of bracts vary in length. All the sporangiophores are attached to the coherent bracts at the same distance from the axis of the cone; but as the sporangia between each verticil of bracts are arranged in two or three concentric series, it follows that the length of the sporangiophores varies considerably. The diagrammatic longitudinal section of a strobilus in fig. 108 shows three concentric series of sporangia between successive bract-verticils. A similar diagram was published by Williamson in 1892[884], and afterwards copied by Potonié[885], but in Williamson’s restoration the sporangiophores of the three series of sporangia are erroneously represented as arising from different points on the surface of the bracts. There is little doubt, as regards the strobilus of S. cuneifolium, that the sporangiophores were given off in a single series close to the axils of the bracts, as is partially shown in fig. 108.
The central part of the axis of the cone is occupied by a single triangular stele like that of the stem, except that each ray of the xylem strand has a comparatively broad blunt termination, and is not tapered to a narrow arm as in fig. 105, A and B. The wood consists of pitted tracheae, with two groups of protoxylem elements at each of the truncated angles of the solid strand of xylem. From the angles of the stele branches of vascular tissue pass out through the cortex to supply the sterile and fertile segments of each verticil. One of the transverse sections of the Sphenophyllum cone in the British Museum Collection (no. 1898 E) affords a good example of the misleading appearance occasionally presented by an intruded ‘rootlet’ of Stigmaria; the vascular tissue of the cone has disappeared, and a Stigmarian appendage with its vascular bundle occupies the position of the stelar tissues.
The bracts consist of parenchymatous tissue limited externally by an epidermis containing stomata. A single stoma with subsidiary cells is represented in fig. 107, A. The sporangiophores are composed internally of thin-walled cells with stronger cells towards the surface. The longer sporangiophores in a series may be more or less coherent for part of their length to the upper surface of the verticil of bracts. In fig. 108 the slender sporangiophores do not appear to come off always from the same portion of the bracts, but this is due to some of them lying on the surface of the latter during part of their course to support the external circle of sporangia. The hook-like distal end of a sporangiophore, towards the point of attachment of the sporangium, is characterised by the larger size and greater prominence of the surface cells; these larger cells, which pass over the upper surface of a sporangium base, probably constitute a kind of annulus which determines the dehiscence of the sporangial wall[886].
Fig. 107, G, represents a sporangiophore and its sporangium cut through transversely just below the point of attachment of the latter to the end of the hook-like termination of the former. The spores are characterised by an irregularly reticulate thickening of the outer coat or exospore, as seen in the figure.
One of the chief points of interest suggested by a Sphenophyllum cone is the exact morphological nature of the sporangiophores. Are they branches borne in the axils of bracts, or may we regard each sporangiophore as a modified leaf, which has become coherent with the whorls of sterile leaves? Or is a sporangiophore merely a stalk of a sporangium; or a ventral lobe of a leaf, of which the sterile bracts represent the dorsal lobes? Although it is impossible without the evidence of development to decide with certainty between these alternatives, it would seem most probable that a sporangiophore may be looked upon as a ventral lobe of a leaf, the sterile lobes forming the bracts or members of the sterile whorls of the cone. This question is discussed by Zeiller[887] and Williamson and Scott[888], also more recently by Scott[889] in his memoir on Cheirostrobus.
Sphenophyllostachys Römeri (Solms-Laubach)[890]. Fig. 107, C and D.