Fig. 96.—A, Lepidodendron Stem with Hollow Ring of Wood W and Zone of Secondary Wood S. B, Longitudinal View of the Narrow Pits of the Wood Elements.
To supply the vascular tissues of the leaf traces, simple strands come off from the outer part of the primary wood, where groups of small-celled protoxylem project (see px in [fig. 97]). The leaf strands lt move out through the cortex in considerable numbers to supply the many leaves, into each of which a single one enters.
Fig. 97.—Transverse Section of Outer Part of Primary Wood of Lepidodendron, showing px, projecting protoxylem groups; lt, leaf trace coming from the stele and passing (as lt1) through the cortex
As regards the fructifications of Lepidodendron much could be said were there space. The many genera of Lepidodendron bore several distinct types of cones of different degrees of complexity. In several of the genera the cones were simple in organization, directly comparable with those of the living Lycopods, though on a much larger scale (see [p. 67]). In some the spores were uniform, all developing equally in numerous tetrads. The sporophyll was radially extended, and along it the large sausage-shaped sporangia were attached (see [fig. 98]). The tips of the sporophylls overlapped and afforded protection to the sporangia. The axis of the cone had a central stele with wood elements like those in the stem. The appearance of a transverse section of an actual cone is shown in [fig. 99]. Here the sporangia are irregular in shape, owing to their contraction after ripeness and during fossilization. Other cones had sporangia similar in size and shape, but which produced spores of two kinds, large ones resulting from the ripening of only two or three tetrads in the lower sporangia, and numerous small ones in the sporangia above.
Fig. 98.—Longitudinal Diagram, showing the arrangement of the elongated sporangia on the sporophylls
a, Main axis, round which the sporophylls are inserted; S, sporangium; s, leaflike end of sporophyll.