82 THE MORPHOLOGY OF PTERIDOPHYTES 



its leaves are potential sporophylls. It differs from Isoetes, 

 however, in having hmited powers of vertical growth and in 

 being able to branch, both dichotomously and adventitiously, 

 so as to form the characteristic tussocks. Two plants are 

 illustrated in longitudinal section, one young and un- 

 branched (Fig. 13C), the other older and branching (Fig. 

 1 3D). Perhaps the most remarkable feature is the way in 

 which the roots are borne up one side only and receive their 

 vascular supply from a rod of primary wood which is quite 

 distinct from that supplying the sporophylls; the two run 

 side by side within the axis (Fig. 13E). The nature of the 

 axis is, therefore, even harder to interpret than in Isoetes. 

 Rauh and Falk draw a comparison with the Cretaceous 

 Nathorstiana (Fig. 13B) in which the roots arise from a 

 number of vertical ridges round the base of the stem. This 

 in turn may be compared with Pleuromeia and ultimately, 

 therefore, with the Lepidodendrales. 



Selaginellales 



This group contains two genera, one living (Selaginella) and 

 one fossiUzed {Selaginellites). More than 700 species of 

 Selaginella are known, of which some occur in temperate 

 regions, but the vast majority are confined to the tropics and 

 subtropics, where they grow in humid and poorly illuminated 

 habitats, such as the floor of rain-forests. Some, however, 

 are markedly xerophytic, inhabiting desert regions, and are 

 sometimes called 'resurrection plants' because of their 

 extraordinary powers of recovery after prolonged drought. 

 Relatively few are epiphytes, unlike Lycopodium. Some 

 form delicate green mossy cushions, others are vine-Uke, 

 with stems growing to a height of several metres, while 

 many have creeping axes, from which arise leafy branch 

 systems that bear a striking superficial resemblance to a 

 frond. 



Hieronymus^2 divided the genus into the following 



sections and subsections : 



