IX] IMPORTANCE OF THE LEAF-TRACE 175 



which are of value for phyletic seriation. They run substantially parallel to 

 those of the axis, and there is reason to believe that the advancing complexity 

 of structure of the blade has had its influence downwards, in promoting an 

 increased complexity in the structure of the leaf-base and of the axis. The 

 most archaic region of the leaf is believed to be the base, and particular 

 comparative importance attaches to the leaf -trace itself. In the most ancient 

 types it resembled the axial protostele. Progressive steps, such as are seen 

 in the ontogeny of the leaf-supply in Thaimiopteris, are believed to indicate 

 correctly the phyletic dilatation of the cylindrical trace to the horse-shoe 

 curve so characteristic of Ferns (Fig. 155). Further steps led to the gradual 

 disintegration of the curved trace, and the phyletic primitiveness or advance 

 may be judged according to the degree of its disintegration. The curve may, 

 however, be contracted secondarily into a compact form in such Ferns as 

 Gleichenia, Lygodium, or Odontosoria, in accordance with their climbing 

 habit. The apparently similar simplicity of their leaf-traces is probably 

 homoplastic, and does not give valid ground for phyletic argument. 



The origin of the pinna-trace from the vascular supply of the phyllo- 

 podium is a less reliable feature than that of the leaf-trace from the stele. 

 The marginal origin of the piniia-trace is held to be relatively primitive, the 

 extra-marginal being derivative. But the latter may arise homoplastically, as 

 a convenient adjustment in large leaves, and may even be departed from in 

 the apical region of an individual leaf where it is present lower down. 

 Nevertheless, in general terms the marginal origin has priority in evolu- 

 tion, and is characteristic for certain alliances, especially where the leaves 

 are relatively small. The value of the venation for comparison lies rather 

 in the study of it in surface view than in the anatomical details. // is thus 

 seen that in the anatomy of the leaf the most valuable features for comparison 

 are found in the basal region, and in tJie leaf -trace itself 



The Root 

 The vascular system of the root in Ferns shows greater uniformity than 

 that of the shoot. The roots are as a rule fibrous, and of small size ; and 

 their stele is correspondingly simple in structure. There is a broad cortex 

 often densely sclerotic surrounding the relatively small stele, which is bounded 

 on all sides by endodermis. Internally to this is the pericycle, sometimes 

 consisting of only a single layer of parenchymatous cells, but often more, 

 especially in xerophytic Ferns (Fig. 9, p. 8). The xylem is commonly diarch, 

 as in Pellaea ; but it may be monarch in the roots of OpJiioglossum, while in 

 the large roots of the Marattiaceae and some Ophioglossaceae the number 

 may be as high as six {Helminthostachys), or more {Angiopteris). Beyond 

 such variations in number, which appear in general to be correlated with 

 size, the construction of the root is along lines familiar in other vascular 



