PTEROPSIDA 



153 



to distinguish stems from leaves. When it occurs in fronds, 

 as in this case, it is, therefore, taken as evidence that they 

 have evolved from stem structures (or are still in the process 

 of doing so). Further evidence that the frond of Lygodium 



Fig. 24 



Sporangia of Filicales: a, Anemia; b, Schizaea; c, Lygodium; 

 D, Mohria; E, F, G, Gleichenia; H, i, Matonia; J, k, l, Hymeno- 

 phyllum; M, N, Cibotium; o, p, Hemitelia; Q, R, s, Dipteris; 

 T, u, Adiantum 



(a-d, after Prantl; e-s, Bower; t, u, Miiller) 



is very primitive is provided by the structure of the leaf 

 trace, which shows only slight departures from radial 

 symmetry. The other three genera have leaf traces which are 

 clearly dorsiventral and 'gutter-shaped'. Their stem struc- 

 tures, too, are more advanced for, whereas Lygodium has a 

 creeping protostelic rhizome, Schizaea has an oblique 

 rhizome with a medullated protostele, Anemia has a creep- 

 ing or oblique rhizome which is either solenostelic or dictyo- 

 steHc, while Mohria is dictyosteHc. It is interesting to note, 



