thick-walled and more elongated cells. A noticeable feature in many Lepidodendron shoots is the occurrence of a circle of strands of secretory cells often surrounding fairly large ducts just internal to the edge of phelloderm: similar strands form irregularly concentric circles, as was pointed out in the case of _L. vasculare_, in the phelloderm itself. [Illustration: FIG. 165. _Lepidodendron fuliginosum._ Leaf-trace: _x_, xylem; _s_, secretory zone. (Binney Collection, Cambridge.)] [Illustration: FIG. 166. _Lepidodendron fuliginosum._ Leaf-trace: _p_, parichnos. (Binney Collection, Cambridge.)] Fig. 166 shows a leaf-trace in the outer cortex accompanied by its crescent-shaped parichnos, _p_, derived from the middle cortex and by means of which the outer cortex and the lamina of the leaves are connected with the inner region of the shoot. This lacunar middle cortex and parichnos doubtless constitute an aerating tissue-system which after leaf-fall is exposed directly to the air at the ends of the parichnos arms on the leaf-scars. Some of the sections in the Binney Collection (Sedgwick Museum, Cambridge) show early stages in the production of secondary xylem: in the section represented in fig. 167 the secretory zone is succeeded on its inner face by a zone of radially elongated cells, _m_, which are clearly in a meristematic condition. The same section shows also the more radially extended form of the xylem of a leaf-trace with its internal protoxylem, _px_, in contrast to the tangentially elongated form which is assumed during its passage through the cortex (cf. figs. 165, 166). [Illustration: FIG. 167. _Lepidodendron fuliginosum._ (Binney Collection, Cambridge.)] Some sections of _Lepidodendron fuliginosum_ in the Manchester University Collection are of special interest from the point of view of the method of secondary thickening. In the section reproduced in fig. 168, B, the meristematic zone is seen to consist in part of radially elongated elements, _m_, with parallel cross-walls evidently of recent origin. The same tissue is shown also in fig. 168, C, _a_, D, _a_, and in fig. 169, A, _a_ This band of meristem, which we may speak of as the cambium, occurs in the outer region of the meristematic zone immediately internal to the secretory zone, _sc_. [Illustration: FIG. 168. A. _Lepidodendron vasculare._ (Botany School, Cambridge.) B. _Lepidodendron fuliginosum._ (From a specimen from Shore, Lancashire, in the Cambridge Botany School Collection). C. _L. fuliginosum._ (“Biseriate Halonia” of Weiss No. 257, Manchester University Museum.) D. _L. fuliginosum._ (Manchester Univ. Museum.)]