portions of the green branches recalls that of the _Sphenophyllum_ stele, except that the axial xylem elements of the Palaeozoic genus are usually represented in _Psilotum_ by mechanical tissue. The cortex consists of three regions (fig. 119, A), an outer zone of chlorophyllous tissue (_a_) rich in intercellular spaces succeeded by a band of mechanical tissue (_b_) which gradually passes into an inner region of larger and thinner-walled cells (_c_). [Illustration: FIG. 119. A. Diagram of transverse section of aerial shoot of _Psilotum triquetrum_. _a_—_c_, cortex; _p_, phloem; _e_, endodermis; _s_, stereome; _x_, xylem; _a_, gap in xylem. B. Enlarged view of one of the angles of the xylem shown in A. _px_, protoxylem. C. Part of transverse section of an approximately triangular rhizome stele showing a portion of the metaxylem _x_¹; _px_, protoxylem elements; _x_², secondary xylem.] [Sidenote: TMESIPTERIS] The genus _Tmesipteris_[46] agrees with _Psilotum_ in general habit and in its epiphytic and probably in some degree saprophytic mode of life. Its brown rootless rhizome, which grows among the roots of tree-ferns or rarely in the ground, gives off pendulous or erect shoots reaching a length of two feet and bearing lanceolate mucronate leaves 2–3 cm. long (fig. 120, A) attached by decurrent leaf-bases. The sporophylls, replacing the upper leaves or occurring in more or less well-defined zones alternating with the foliage leaves, consist of a short axis terminating in a pair of lanceolate lobes and bearing on its adaxial surface an elongated bilocular synangium attached to a very short stalk (fig. 120, B). Reference has already been made to the divergent opinions as to the morphological nature of the sporophylls or sporangiophores, but recent investigations distinctly favour the view that a sporophyll is best interpreted as a stalked leaf with two sterile laminae and an almost sessile, or in some cases a more obviously stalked, synangium; the whole sporophyll is characterised by the possession of a ventral and a dorsal lobe[47]. The drawings reproduced in fig. 120, D and F, illustrate some of the frequent variations described by Thomas in plants which he observed in the New Zealand forests. The sporophyll shown in fig. 120, D and F, has branched twice and bears three synangia. [Illustration: FIG. 120. _Tmesipteris._ A. Foliage leaves. B. Sporophyll and bilocular synangium. C. Diagram of transverse section of stele. _px_, protoxylem. D, F. Abnormal sporophylls. (From drawings made by Prof. Thomas and generously placed at my disposal. A.C.S.) E. Portion of C enlarged.]