An interesting discovery was made a few years ago which confirmed a statement by Renault which Williamson was unable to accept, namely that the xylem bundle of a rootlet occasionally gives off a delicate tracheal strand at right angles to the long axis of a rootlet. In some rootlets Weiss[605] found obliquely running delicate strands of xylem, surrounded by a layer of parenchymatous tissue, in the space between the vascular bundle and the outer cortical cylinder. It is clear that a few spiral tracheids are occasionally given off from the protoxylem of a rootlet bundle: these follow an oblique course to the outer cortex, where in some cases they have been traced into connexion with short and spirally marked cells resembling transfusion tracheae (fig. 210, A). This arrangement may serve as a means of facilitating the passage of water absorbed by the superficial cells into the xylem strand. It should be noticed that, like roots of recent water-plants, the rootlets of _Stigmaria_ had no root-hairs. Fig. 210, F, shows a transverse section of part of a rootlet in which the outer cortical cylinder, _c_³, is connected, as in the roots of _Isoetes_, with the sheath surrounding the vascular bundle. A few obliquely cut tracheae are seen in this section traversing the connecting band of parenchyma _t_, fig. 210, A. A point of biological interest in connexion with Stigmaria rootlets is the occasional presence of hypertrophied cells, the large size of which is due to the attacks of a fungus named by Weiss[606] _Urophlyctites stigmariae_. In addition to _Stigmaria ficoides_, which is by far the commonest form, a few other species have been founded on external characters. One of these is represented by _Stigmaria stellata_, Goepp.[607], characterised by the presence of radially disposed ridges and small tubercles surrounding each rootlet-scar. Kidston refers to Goeppert’s species as a Lower Carboniferous type. We have no evidence as to the meaning of the stellate ridges and tubercles, nor have we any reason to suppose that this form differed essentially in structure from _Stigmaria ficoides_. CHAPTER XVIII. =Bothrodendreae.= _Bothrodendron._ Figs. 211–216.