VII] 



THE PROTOSTELE 



123 



F"ig. 116. Transverse section of the stele of 

 Trichomanes scandens. px — protoxy- 

 lem; 5 =endodermis. (After Boodle.) 



Lygodium there is a creeping rhizome, with straggling and cHmbing leaves 

 of great length. Their leaf-stalks are thin, with a single contracted vascular 

 strand. Thus the stele is not liable to be 

 influenced towards expansion by a broad 

 insertion of the leaf-trace. The case of 

 CheiropleiLria is less easily understood, for 

 in certain features it is a more advanced 

 type than these relatively primitive Ferns 

 (Fig. 1 1 5). Structurally it is very like Glei- 

 ckenia; and its creeping rhizome and at- 

 tenuated leaf-stalks may also be held as 

 favouring the retention of the primitive 

 state. These genera may all be regarded as 

 perpetuating in the adult shoot the proto- 

 stelic structure of the young plant. But 

 they all share the peculiarity of having 

 plentiful parenchyma scattered among the 

 tracheides of the bulky cylinder of wood. This is probably a derivative state, 

 for in Botryopteris and other very primitive Vascular Plants the xylem con- 

 sists only of tracheides. 



The protostelic state 

 with differences of detail 

 holds for the Hymeno- 

 phyllaceae. In these Ferns 

 with their filmy, hygrophil- 

 ous habit, small size, and 

 delicate leaf-stalks, the pro- 

 tostele of the axis is as a 

 rule minute (Fig. 116). But 

 in the larger species it often 

 shows an internal differen- 

 tiation very like that seen 

 in the Palaeozoic fossil, 

 Ankyropteris{¥\g. i I7),and 

 other Zygopterideae. Here 

 there is a central region 

 of the xylem, associated 

 with parenchymatous cells, 

 which is surrounded by an 

 outer xylem of tracheides 

 only. The similarity be- 

 tween the ancient fossil and the modern Filmy Ferns, in this as well as in 



Fig. 117. A)ikyropteris Grayi; stele, from a section in 

 Dr Kidston's collection. ( x 18.) (After Seward.) 



