Xlll] PSILOTUM 19 



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^ 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 abnormalites in 

 Psilotum and Tmesipteris shows a decided tendency in the 

 Psilotales to a repeated dichotomy of the sporophylls I 



A single stele' 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 occasion- 

 ally 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, Bjpx). 

 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 paren- 



1 Thomas (02) p. 349. 



2 Another form of abnormality in the sporophylls of Psilotum has recently 

 been described by Miss Sykes. Sykes (08^). 



» Bertrand, C. E. (81) ; Ford (04). 



