By the activity of a zone of growing tissue encircling the cylinder of wood the main trunk and branches grew in thickness year by year: the general uniformity in size of the secondary conducting elements affords no indication of changing seasons. As the branches grew stouter and shed their leaves the surface of the bark resembled in some degree that of a Spruce Fir and other species of Picea, in which the leaf-scars form the upper limit of prominent peg-like projections, which, at first contiguous and regular in contour, afterwards become less regular and separated by grooves ([fig. 140]) and at a later stage lose their outline as the bark is stretched to the tearing point ([fig. 140], C). The leafless branches of Lepidodendron were covered with spirally disposed oval cushions less peg-like and larger than the decurrent leaf-bases of Picea, which show in the upper third of their length a clean-cut triangular area and swell out below into two prominent cheeks separated by a median groove and tapering with decreasing thickness to a pointed base, which in some forms (e.g. Lepidodendron Veltheimianum, [fig. 185], C, D), is prolonged as a curved ridge to the summit of a lower leaf-cushion.
Fig. 140. Picea excelsa. Shoots of different ages showing changes in the appearance of the leaf-cushions: a leaf attached to a cushion in fig. A. (Slightly enlarged.)
A portion of the cushion below the triangular leaf-scar often shows transverse gaping cracks or depressions ([fig. 185], C) such as occur on a smaller scale on the older cushions of a Fir twig ([fig. 140]). Secondary thickening, as in recent trees, is not confined to the vascular cylinder but at an early stage, frequently before there are any signs of secondary wood, the outer region of the broad cortex becomes the seat of active cell-formation which results in the addition of a considerable thickness to the bark. At a later stage of increase in girth, the leaf-cushions are stretched apart and the original surface-features become obliterated by vertical cracks and by the exfoliation of the superficial tissues[238].
Some species of Lepidodendron produced branches characterised by spiral or vertical series of scars; these in older shoots were replaced by depressions having a diameter of several inches and comparable in appearance, as also perhaps in manner of formation, with the scars left on the stem of a Kauri Pine (Agathis australis)[239] on the abscission of lateral branches by a natural process. These shoots, known as Ulodendron, are described in a subsequent section. ([page 128].)
A fully-grown Lepidodendron must have been an impressive tree, probably of sombre colour, relieved by the encircling felt of green needles on the young pendulous twigs. The leaves of some species were similar to those of a fir while in others they resembled the filiform needles of the Himalayan Pine (Pinus longifolia). The occasional presence of delicate hyphae in the tissues of Lepidodendron demonstrates susceptibility to fungal pests.
Architecturally, if one may use the term, Lepidodendron owed its power of resistance to the bending force of the wind to its stout outer bark formed of thick-walled elements produced by the activity of a cylinder of cortical meristem (figs. [148], [172], etc.). The vascular axis, of insignificant diameter in proportion to the size of the stem (figs. [152], [153], [172], [181], A), must have played a subordinate part, from a mechanical point of view, as compared with the solid mass of wood of a Pine or an Oak.
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Within the compass of a text-book it is impossible, even if it were desirable, to include an account of the majority of the species of the widely distributed Palaeozoic genus Lepidodendron. In spite of the great number of known species of this common member of Carboniferous floras, our knowledge of the type as a whole is deficient in many points, and such information as we possess needs systematising and extending by comparative treatment based on a re-examination of available data.
In order to appreciate the meaning of certain external features characteristic of Lepidodendron stems it is essential to have some knowledge of the internal structure.