Fig. 114. Sphenophyllostachys fertilis (Scott). (After Scott.) Diagram of a node in longitudinal section, showing one sporophyll and the base of the opposite one. v.l. ventral lobe of sporophyll; v.s. one of the segments into which it divides; v.s′. stump of another segment; d.l. dorsal lobe; d.s., d.s′. segments of dorsal lobe.
A more recent addition to our knowledge of the fertile shoots of Sphenophyllum is due to Scott who has described a new type of cone under the name Sphenophyllum fertile[11]. The petrified specimen on which the species was founded was discovered by Mr James Lomax in the Lower Coal Measures of Lancashire; it represents a portion of a cone 6 cm. long and approximately 12 mm. broad. The axis contains a single vascular cylinder agreeing in essentials with the type of stem structure known as Sphenophyllum plurifoliatum. The nodal regions, which exhibit the slight swelling characteristic of the genus, bear several (probably twelve) appendages connate at the base and forming a narrow flange encircling the axis. Each bract, the base of which forms part of the narrow collar surrounding the axis, consists of two lobes, ventral and dorsal, divided palmately into several (sometimes four) segments or sporangiophores ([fig. 115]). Each sporangiophore terminates distally in an oblong or oval lamina bearing two sporangia on its adaxial face ([fig. 114]). The space between the axis and the periphery of the cone is thus occupied by crowded peltate laminae, each with its pair of sporangia. A single vascular bundle supplies each sporangiophore and bifurcates in the distal lamina into two branches which extend to the bases of the sporangia. The sporangia agree in structure with those of other species of Sphenophyllum: the spores are of one size and elliptical, characterised by the presence of several sharp ridges or flanges encircling the spore-wall in the direction of the major-axis. Sphenophyllostachys fertilis differs from all previously recorded types in the absence of sterile bracts. The appendages of the cone-axis are all fertile, a striking contrast to the differentiation into protective and sporangia-bearing bracts which constitutes a constant feature in the cones of Sphenophyllum and Calamites. It is possible, as Scott suggests, that the absence of sterile segments is the result of modification of the more usual type of strobilus; instead of the dorsal and ventral lobes of the bracts sharing between them the duties of protection and spore-production, the whole of each bract is constructed on the plan of the maximum spore-output, the laminar terminations of the sporangiophores serving the purpose of protection. The cone may be described as more specialised than the normal type of strobilus for reproductive purposes[12].
Fig. 115. Sphenophyllostachys fertilis (Scott). (After Scott.) Diagram of a single sporophyll as it would appear in a transverse section of the cone; showing one lobe (dorsal or ventral). ax, part of axis to which the sporophylls are attached.
Fig. 116. Sphenophyllostachys Dawsoni. (After Thoday.) A. Larger spores; B, abortive spores; C, mature spores showing the characteristic spines.
It has been stated, on evidence which is unsatisfactory, that Sphenophyllum possesses two kinds of spores. While regarding the genus as homosporous on the evidence before us, it is interesting to find that cases occur in which the spores in the same sporangium exhibit a marked difference in size. Attention has been called by Williamson and Scott[13] to variation in the dimensions of spores: a more pronounced difference in size has been recorded by Mr Thoday[14] who gives 120μ as the maximum and 90μ as the minimum diameter of the spores in a cone of Sphenophyllostachys Dawsoni. The presence of several abortive spores in the sporangium ([fig. 116]) containing the larger spores favours the view that this difference in size may be the first step towards the development of heterospory.
It is clear that the types of strobilus designated Sphenophyllostachys ([figs. 112–114]) present a divergence of characters too great to be comprised under one genus; but in the absence of fuller information, we cannot do otherwise than follow the only logical custom of grouping them together as examples of strobili borne by plants which, in the present state of our knowledge, are most conveniently referred to the genus Sphenophyllum.