The single xylem bundle consists of primary tracheae only, at least in such laminae as have been identified as Lepidodendroid. Surrounding the xylem strand occur delicate parenchymatous cells in some cases accompanied by darker and thicker-walled elements. As in Sigillaria, the leaves of which are more fully described on [page 210], a fairly broad sheath of wider and shorter scalariform or spiral transfusion tracheids surrounds the conducting strand (figs. [142], t; [143], B, C, t). As Renault shows in the case of Lepidodendron esnostense[242], the small leaves of which are 1·5–2 mm. broad at the base and several centimetres long, the stomatal grooves and keel die out towards the apex when the lamina assumes a more nearly circular form ([fig. 143], C).

Fig. 143.

The area of the cushion excluding the leaf-scar is spoken of by some writers as the field. Below the leaf-scar the kite-shaped cushion tapers to a gradually narrowing basal position: in Lepidodendron Veltheimianum, a species characteristic of Lower Carboniferous strata, it is seen to be continuous, as a ridge with sloping sides, with a lower cushion ([fig. 185]).

Below a leaf-scar the cushion frequently shows a pair of oval areas on which a fine pitting may be detected in well-preserved impressions, these oval scars, as seen in [fig. 185], D, are practically continuous at the upper end with the parichnos scars on the leaf-scar area; this is explained by the fact that these infra-foliar scars also owe their existence to patches of lacunar, aerenchymatous tissue in close connexion with the parichnos[243].

Shortly before entering the base of the leaf-lamina the parichnos divides into two arms which diverge in the outer cortical region right and left of the vascular bundle, and passing obliquely upwards they come close to the surface of the leaf-cushion just below the leaf-scar. The diagram—[fig. 144], B—shows a leaf-trace, lt, in the leaf-cushion, as seen in a diagrammatic drawing of a vertical radial section of a stem, the dotted lines, p, p′, show the two parichnos arms which are represented as impinging on the surface of the leaf-cushion at p′, and then bending upwards to pass into the leaf-base right and left of the vascular bundle or leaf-trace. For convenience the arms of the parichnos are represented in one plane though actually in different vertical planes.

Fig. 144, A, shows the difference between a view of the original surface of a Lepidodendron, as at a, where a leaf-cushion with a leaf-scar is seen, and a view of an impression representing the outer cortex, b, a short distance below the surface. The surface b, in [fig. 144], A, corresponds to the face d-e in the diagrammatic longitudinal section [fig. 144], B: the outline of each cushion is clearly visible and in the centre is seen the leaf-trace, lt, with its parichnos.

The surface-features, a ([fig. 144], A), have been impressed on the rock, c, ([fig. 144], B) in which the specimen was entombed and by the removal of the cast of the stem, that is the thickness b to e in [fig. 144], B, the form of the leaf-cushion is revealed. The presence of the two infra-foliar parichnos scars at p′ ([fig. 144], A) is explained by the diagram, [fig. 144], B, p′.