In their anatomy all the members of the family have monostelic structure (see p. 62). This is noteworthy, for at the present time though a number of genera are monostelic, no family whose members reach any considerable size or steady growth is exclusively monostelic. In the shape of the single stele, there is much variety in the different genera, some having it so deeply lobed that only a careful examination enables one to recognize its essentially monostelic nature. In fig. 86 a radiating star-shaped type is illustrated. Between this elaborate type of protostele in _Asterochlaena_, and the simple solid circular mass seen in _Botryopteris_ itself (fig. 88) are all possible gradations of structure. [Illustration: Fig. 87.—The Stele of a Botryopteridean Stem, showing soft tissue in the centre of the solid wood of the protostele. (Microphoto.)] In several of the genera the centre of the wood is not entirely solid, but has cells of soft tissue, an incipient pith, mixed with scattered tracheids, as in fig. 87. In most of the genera numerous petioles are given off from the main axis, and these are often of a large size compared with it, and may sometimes be thicker than the axis itself. Together with the petiole usually come off adventitious roots, as is seen in fig. 88, which shows the main axis of a _Botryopteris_. The petioles of the group show much variety in their structure, and some are extremely complex. A few of the shapes assumed by the steles of the petioles are seen in fig. 89; they are not divided into separate bundles in any of the known forms, as are many of the petiole steles of other families. [Illustration: Fig. 88.—Main Axis of _Botryopteris_ with simple solid Protostele _x_. A petiole about to detach itself _p_ and the strand going out to an adventitious root _r_ are also seen. (Micro-photograph.)] [Illustration: Fig. 89.—Diagrams showing the Shapes of the Steles in some of the Petioles of different Genera of Botryopterideæ A, _Zygopteris_; B, _Botryopteris_; C, _Tubicaulis_; D, _Asterochlaena_.] In one genus of the family secondary wood has been observed. This is highly suggestive of the condition of the stele in _Heterangium_, where the large mass of the primary wood is surrounded by a relatively small quantity of secondary thickening, developed in normal radial rows from a cambium. Another noteworthy point in the wood of these plants is the thickening of the walls of the wood cells. Many of them have several rows of bordered pits, and are, individually, practically indistinguishable from those of the Pteridosperms, cf. fig. 81 and fig. 90. These are unlike the characteristic wood cells of modern ferns and of the other family of Palæozoic ferns.