The leaves are borne in verticils at the nodes, those in the same whorl being usually of the same size, but in some forms two of the leaves are distinctly smaller than the others. Each verticil contains normally 6, 9, 12, 18 or more leaves, which are separate to the base and not fused into a sheath; the number of leaves in a verticil is not always a multiple of six. They vary in form from cuneiform with a narrow tapered base, and a lamina traversed by several forked veins, to narrow uninerved leaves and leaves with a lamina dissected into dichotomously branched linear segments. The leaves of successive whorls are superposed.
The strobili are long and narrow in form, having a length in some cases of 12 cm., and a diameter of 12 mm.; they occur as shortly stalked lateral branches, or terminate long leaf-bearing shoots. The axis of the cone bears whorls of numerous linear lanceolate bracts fused basally into a coherent funnel-shaped disc, bearing on its upper surface sporangiophores and sporangia.
The strobili are usually isosporous, but possibly heterosporous in some forms.
The stem is monostelic, with a triarch or hexarch triangular strand of centripetally developed primary xylem, consisting of reticulate, scalariform and spiral tracheae; the protoxylem elements being situated at the blunt corners of the xylem-strand. Foliar bundles are given off, either singly or in pairs, from each angle of the central primary strand. The secondary xylem consists of radially disposed reticulate or scalariform tracheae, developed from a cambium-layer. The phloem is made up of thin-walled elements, including sieve-tubes and parenchyma. Both xylem and phloem include secondary medullary rays of parenchymatous cells. The cortex consists in part of fairly thick-walled elements; in older stems the greater part of the cortical region is cut off by the development of deep-seated layers of periderm.
The roots are apparently diarch in structure, with a lacunar and smooth cortex.
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The branch of Sphenophyllum emarginatum Brongn. given in fig. 109 shows the characteristic appearance of the genus as represented by this well-known species which Brongniart figured in 1822. The Indian species shown in fig. 111 illustrates the occurrence of unequal leaves in the same whorl, and in fig. 110, B, we have a form of verticil in which the leaves are deeply divided into filiform segments. A larger-leaved form is represented by S. Thoni, Mahr. (fig. 110, A), a species occasionally met with in Permian rocks.
No specimens of Sphenophyllum have so far been found attached to a thick stem; they always occur as slender shoots, which sometimes reach a considerable length. One of the longest examples known is in the collection of the Austrian Geological Survey; the axis is 4 mm. in breadth and 85 cm. long, bearing a slender branch 61 cm. in length. The manner of occurrence of the specimen as a curved slender stem on the surface of the rock suggests a weak plant, which must have depended for support on some external aid, either water or another plant. The anatomical structure and other features do not favour the suggestion of some writers that Sphenophyllum was a water-plant[867], but there would seem to be no serious obstacle in the way of regarding it as possibly a slender plant which flung itself on the branches and stems of stronger forest trees for support.
A. The anatomy of Sphenophyllum.
The following account of the structural features of the stem and root is based on the work of Renault[868], Williamson[869] and Williamson and Scott[870]. We may first consider such characters as have been recognised in different examples of the genus, and then notice briefly the distinguishing peculiarities of two well-marked specific types.