XIIl] TMESIPTERIS 21 



interesting discovery that in some parts of the rhizome the 

 parenchymatous zone surrounding the scalariform tracheae 

 may become the seat of meristematic activity which results 

 in the production of secondary tracheae often characterised 

 by a sinuous longitudinal course. There is no definite 

 cambium, but the radially disposed tracheae and the adjacent 

 parenchymatous elements clearly demonstrate the secondary 

 nature of the tissue immediately external to the group of 

 primary xylem. Fig. 119, C, drawn from a section kindly 

 supplied by Mr Boodle, shows the secondary xylem elements at 

 a? associated with radially disposed thin-walled cells abutting 

 on the primary xylem, x^. It is probable that this added tissue 

 may be a remnant of a more extensive secondary thickening 

 characteristic of the ancestors of the recent species. In their 

 manner of occurrence and sinuous course these secondary 

 tracheids bear a resemblance to the secondary xylem of 

 Lepidodendron fuliginosum^. The stele of the aerial shoot bears 

 a fairly close resemblance to the vascular axis of Cheirostrobus, 

 and its three-rayed form in the lower 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 chloro- 

 phyllous tissue (a) rich in intercellular spaces succeeded by a 

 band of mechanical tissue (6) which gradually passes into an 

 inner region of larger and thinner- walled cells (c). 



The genus Tmesipteris^ 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 



1 See p. 150. ^ Bertrand (81) ; Jennings and Hall (91). 



