418 PSARONIEAE [CH. 



the leaf-bases or leaf-scars farther out until it was exfoliated. 

 Farmer and HilF find it difficult to accept this explanation; but, 

 as Rudolph shows, the radial arrangement of the cortical cells 

 between the adventitious roots and their elongation in a radial 

 direction are arguments in support of the secondary nature of 

 the cortical zone. 



In sections of the adventitious roots of Psaronius Renaulti 

 figured by Williamson 2, the spaces between the cylindrical roots 

 are partially occupied by cell-filaments which, at first sight, 

 suggest root-hairs; it may well be, as Eudolph suggests, that 

 these felted hairs represent the outermost and looser part of 

 the growing secondary cortex which gradually passes into the 

 covering mass of free extra-cortical roots. 



As StenzeP has shown, slender stems of Zygopteris (= An- 

 kyropteris) are occasionally met with growing through the web 

 of Psaronius roots. 



Psaronius infarctus Unger. Fig. 296, A, B. 



This species, which Zeiller* has investigated from sections of 

 Unger's material, illustrates a type in which the vascular tissue 

 is very richly developed and forms crowded concentric series of 

 curved plates associated, in the more peripheral series, with 

 bands of mechanical tissue. The outermost part of the vascular 

 region consists of (i) a series of loops or variously curved bands 

 of conducting tissue representing leaf-traces at different stages 

 in their outward course, (ii) a series of similar vascular strands 

 (peripheral steles of Zeiller) confined to the stem (cauline) and 

 from which roots are given off, and (iii) bands of mechanical 

 tissue associated with the leaf-traces and peripheral steles. 

 The peripheral steles (fig. 296, A, B, p) form anastomoses with 

 the leaf-traces and contribute to their formation. 



The form of some of the vascular bands in the section 

 of Psaronius infarctus shown in fig. 296, A, illustrates the 

 occasional anastomosing of one dictyostele with another: the 



1 Parmer and Hill (02). 2 Williamson (76) PI. iii. 



3 Stenzel (89) PI. vi. 



•> Zeiller (90) p. 204, Pis. xvi. xvn.; see also Rudolph (05). 



