Hymenophyllaceae , Schizaeaceae and Gleicheniaceae. 529 
3. The structure of nearly allied species should be carefully 
compared with that of the species dealt with. 
4. The structure of the young plant should be examined, 
chiefly to see whether it gives evidence of reduction not 
indicated by the mature plant, in the form of tissues not 
represented in a corresponding position in the mature stem. 
5. In interpreting all such data obtained, both internal 
evidence and also independent clues should be sought as tests 
of advance or reduction. 
General Theory. 
In the previous paper on the Schizaeaceae, as the forms 
included in that Order were found to possess features of 
special interest in relation to the stele, a discussion of some of 
the points at issue was given (Boodle, ’ 01 , p. 403 et seq.). 
It will be well to put together some further considerations on 
this subject and to restate others. 
Tansley and Chick (’01) deduce, from their researches 
on some of the Bryophyta and from the probability of simi- 
larity of physiological requirements in the unknown primitive 
ancestors of the Pteridophyta, that in the latter the stem 
possessed a solid central strand of conducting tissue of the 
protostelic type and having acropetal development, that leaf- 
traces were developed independently of this protostele, and 
that their connexion with it was only a secondary pheno- 
menon. This view appears well founded on theoretical 
grounds, and receives a certain amount of support from the 
fact that most cases of protostelic stems are found among 
the more primitive Ferns, and that as one passes from the 
lower to the higher forms the leaf-trace appears to exert more 
and more influence on the structure and development of 
the stele (cf. G Wynne- V aughan , ’ 01 , p. 87). If one adopts 
this view, the tissues of the stele and leaf-trace are not strictly 
homologous h 
1 Hence the writer prefers to retain the terms 1 leaf-trace ’ and ‘ petiolar bundle,’ 
rather than replace them by the word ‘ meristele/ suggested by Brebner (’ 02 , 
p. 523) for use in an extended sense. 
