The relation of the parichnos to the oval scars below a Lepidodendron leaf-cushion has been worked out in detail by Weiss who shows that, at least in some species, the two arms do not bend downwards as shown in the diagram, [fig. 144], B, but pursue a straight gradually ascending course as seen in [fig. 145], A. Just below the leaf-scar region of the cushion each arm comes into association with a group of lacunar, aerenchymatous tissue, such as occurs in the roots of certain Mangrove plants, and it is this aerenchyma which is exposed on the two oval depressions below the leaf-scar. The structure of this aerenchyma is shown in [fig. 145], B; it consists in this species (L. Hickii Wats.) of stellate cells which would constitute an efficient aerating system. Probably, as Weiss suggests, these patches of aerenchyma were originally covered by an epidermis provided with stomata, and it is owing to the destruction of this superficial layer that the two oval scars often form a prominent feature on Lepidodendron leaf-bases[244]. The diagram reproduced in [fig. 144], B, may be taken as practically correct, as the patches of aerenchyma described by Weiss do not differ essentially from the parichnos tissue.

Fig. 144. Lepidodendron Veltheimianum Sternb.

  1. Leaf-cushion and leaf-scar seen in surface-view at a; on the rest of the specimen a slightly lower surface is exposed. (After Stur.)
  2. Diagrammatic longitudinal section to explain the differences between its two surfaces a and b shown in fig. A.
  3. The shaded portion c represents the rock matrix, the surfaces ab, ed, mark the outer and inner edge of the outer portion of the bark of the Lepidodendron stem.
  4. lt, leaf-trace; p, p′, parichnos.

Fig. 145.

  1. Diagrammatic surface-view and longitudinal section of a Lepidodendron leaf-cushion.
  2. Aerenchyma below the leaf-scar. (After F. E. Weiss.)

The parichnos scars are shown on the leaf-scar and cushion in [fig. 146], C. In the lower leaf-cushion shown in [fig. 146], E, the infra-foliar parichnos scars, p, are clearly seen, but the preservation of the leaf-scar is not sufficiently good to show them on that part of the fossil. In the upper cushion ([fig. 146], E) the position of the parichnos arms is shown on the leaf-scar, but the infra-foliar parichnos scars are hidden by two small spiral shells. The genus Spirorbis, to which these shells are referred, appears to have persisted from the Silurian epoch to the present day. The comparatively frequent occurrence of Spirorbis shells on the leaves and other parts of Palaeozoic plants, has recently been dealt with in a paper by Barrois[245] who discusses in detail the habitats of these small animals from the point of view of the conditions under which the plants were preserved. In a note by Malaquin appended to Barrois’ paper the belief is expressed that Spirorbis lived on pieces of Palaeozoic plants which lay under water.

The fact that with one exception all the Spirorbis shells on the specimen of Lepidodendron, of which two leaf-cushions are shown in [fig. 146], E, occur on the large parichnos scars on the cheeks of the cushions, suggests the possibility that the escape of gases from the parichnos tissue may have rendered the position attractive to the Spirorbis. It can hardly be accidental that the shells occur on the parichnos strands. This fact recalls the view held by Binney[246] and accepted with favour by Darwin[247] that Lepidodendron and other coal-forest trees may have lived with the lower parts of the stems in sea water.

Above the leaf-scar is a fairly deep triangular or crescentic pit ([fig. 146], C, l) known as the ligular pit from the occurrence on younger shoots of a delicate organ like the ligule of Isoetes ([fig. 132]) embedded in a depression in the upper part of the leaf-cushion. The ligule was first figured in Lepidodendron by Solms-Laubach[248] and described in English material by Williamson under the name of the adenoid organ[249].