20 DAVALLIOID FERNS [ch. xxxvii 



for not only will their similarities confirm the relation to some central type, 

 but also their divergent features may illuminate the problem of their descent. 

 The order in which the genera of the Davallieae are arranged by Diels 

 appears to be quite arbitrary (^Nat. PJianzenfam. I, 4, p. 204). But if regard 

 be paid to certain features of advance which they show, a natural grouping 

 may follow. Such features are: (i) the relation of the sori to the margin or 

 surface of the blade: (ii) the elimination of the lower indusium : (iii) the 

 linkage of sori laterally to form coenosori. Parallel with comparisons ac- 

 cording to these soral characters, those derived from the dermal appendages 

 and the vascular anatomy may be held as adding grounds for criticism or 

 confirmation. 



(i) Position of the sorus relative to the leaf-margin. 



HUMATA Cav. 



This genus was merged by Sir W. Hooker as a section of the genus 

 Davallia. It comprises 14 species, and is characterised by more or less deeply 

 intra-marginal sori, terminal on the veins, with a circular or kidney-shaped 

 lower indusium having free margins. The rhizomes bear scales, and alternate, 

 coriaceous, and sometimes simple leaves: these may be regarded as small 

 and condensed derivatives from a more highly divided type, such as is seen 

 in the Dennstaedtiinae. Anatomically H. heteropJiylla (Sm.) Desv., and 

 H. repens (L. fil.) Diels, present in transverse section of the rhizome the 

 characteristic vascular system of Davallia, but with a leaf-trace of only two 

 equal strands. Beyond the suggestion that Htiniata bears a condensed leaf- 

 structure, which is frequently associated with a deeply intra-marginal adult 

 position of the sori, the genus calls for no special remark (compare Diels, 

 Nat. Pflanzenfam. I, 4, Fig. 1 12). 



Nephrolepis Schott 



This genus comprises 17 species of Ferns with upright, slightly scaly 

 stock, giving rise to runners which are often tuberous. The leaves are simply 

 pinnate, with open venation : they may often show continued apical growth. 

 The stolons are leafless, and protostelic : but when enlarged into tubers they 

 illustrate how disintegration into meristeles follows on increase of transverse 

 section, even in the absence of foliar gaps (Vol. i, Fig. 182, p. 190). The 

 normal stock is dictyostelic, with foliar gaps, and the leaf-traces consist 

 usually of three strands. When a stolon turning its apex upwards enlarges 

 into a leafy stock, the ontogenetic progression from protostely to dictyostely, 

 as it is seen in the sporeling, is repeated in its vascular system: in fact, the 

 stolon may be regarded as corresponding to a prolongation of the protostelic 

 stage of the embryo (G.-V. MSS.). 



