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.
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.)
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.