The genus Tmesipteris ([fig. 120], A) is represented by a single species T. tannensis Bertr.[39] which usually occurs as an epiphyte on the stems of tree-ferns in Australia, New Zealand, and Polynesia. Psilotum, with two species P. triquetrum Sw. ([fig. 118]) and P. complanatum Sw., flourishes in moist tropical regions of both hemispheres, growing either on soil rich in organic substances or as an epiphyte. Both genera are considered to be more or less saprophytic.

Fig. 118. Psilotum triquetrum (½ natural size).

  1. Synangium.
  2. Sporophyll after removal of the synangium. (M.S.)

Psilotum. The common tropical species P. triquetrum ([fig. 118]) is characterised by an underground rhizome which forms a confused mass of dark brown branches covered with filamentous hairs as substitutes for roots and gives off erect repeatedly forked aerial shoots. In P. complanatum[40] the habit is similar to that of the more abundant and better-known species, but the pendulous shoots are characterised by their broader and flatter form. In both species the function of carbon-assimilation is performed by the outer cortex of the green branches, as the small size of the widely-separated foliage leaves renders them practically useless as assimilating organs.

The sporophylls consist of a short axis terminating in two small divergent forks and bearing on its adaxial surface a trilocular or in rare cases a bilocular synangium ([fig. 118], A and B). The walls of the loculi are composed of several layers of cells and dehiscence takes place along three lines radiating from the centre of the synangium. Professor Thomas[41] has recorded “fairly numerous instances in Psilotum of a second dichotomy of one branch of the first fork, or, less frequently, of both branches”: instead of one synangium subtended by the two slender leaflets of the forked sporophyll-axis, there may be two synangia and three leaf-lobes or three synangia and four leaf-lobes. The occurrence of both these abnormalities in Psilotum and Tmesipteris shows a decided tendency in the Psilotales to a repeated dichotomy of the sporophylls[42].

A single stele[43] with a fluted surface occupies the axis of an aerial shoot ([fig. 119], A); the axial region is occupied by a core of elongated mechanical elements (s), which may occasionally extend to the periphery of the xylem and break the continuity of the band of scalariform tracheae ([fig. 119], A, a). The tracheae form the arms of an irregularly stellate stele and each arm is terminated by protoxylem elements ([fig. 119], B, px). The rays of the xylem cylinder, which may be as many as six or eight in the upper part of the aerial shoots, become reduced in number as the rhizome is approached, assuming a diarch structure near the junction. In the rhizome the xylem forms an approximately triangular group of tracheae without any core of mechanical elements. Three to four layers of parenchyma succeeded externally by an ill-defined phloem ([fig. 119], A, p) surround the xylem and a fairly distinct endodermis ([fig. 119], A and B, e) encloses the whole. To Mr Boodle[44] is due the 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 x2 associated with radially disposed thin-walled cells abutting on the primary xylem, x1. 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[45]. 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 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).

Fig. 119.

  1. 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.
  2. Enlarged view of one of the angles of the xylem shown in A. px, protoxylem.
  3. Part of transverse section of an approximately triangular rhizome stele showing a portion of the metaxylem x1; px, protoxylem elements; x2, secondary xylem.