CHAPTER XXXVIII 



PTEROID FERNS 



Under the heading VI Pterideae, as adopted by Diels in Engler and Prantl, 

 Naturl. Pfianzenfam. I, 4, p. 254, numerous Ferns are arranged of which the 

 collective characterisation is as follows: "Sori langlich bis lineal, langs der 

 Adern, an deren Enden oder einer Oueranastomose, Indusium meist fehlend. 

 Blattrand haufig umgeschlagen, oft modificirt, den Sorus liberdachend. 

 Blatter ungegliedert dem Rhizome angefiigt. Spreite seltener ungeteilt, meist 

 zusammengesetzt. Keine Spicularzellen. Bekleidung Spreuschuppen oder 

 Haare, letztere zuweilen Wachs ausscheidend." Then follows a list of some 

 thirty genera, grouped under the headings, I Gymnogramminae; II Cheilan- 

 thinae, III Adiantinae; and IV Pteridinae. In point of fact the Pterideae 

 of Diels includes the bulk of the Pterideae of Hooker, together with his 

 Grammitideae (see Synopsis Filiauii, 1873, pp. 9, 10). 



Diels' grouping is based upon a comprehensive rather than an exact 

 diagnosis. The adult sorus is naturally placed in the forefront; but its 

 objective features only are taken into account, without reference to their 

 evolutionary or even their ontogenetic origin. It is a systematic rather than 

 a morphological diagnosis, in fact convenient rather than scientific. If, how- 

 ever, the question be raised how the soral state that is used as a basis for 

 the grouping has been arrived at, it will be evident that at least two distinct 

 sources are possible for the conditions that are shown by the included 

 genera. One is from a fusion of originally isolated, two-lipped, marginal sori, 

 such as are characteristic of the Dicksonioid Ferns; the other is from some 

 types such as Todea or Plagiogyiia^ where the sporangia were already present 

 upon the leaf-surface, and were without any indusium. The immediate 

 question will then be how to discriminate in any individual case between 

 these two or other possible sources; for upon such decisions any more natural 

 grouping must depend. The reply is by comparison of allied forms, based 

 upon developmental study of the sori, and checked by comparison of other 

 characters of form and structure. The grouping, if it is to be natural, should 

 be according to induction founded upon a wide comparison and upon 

 development, rather than upon any arbitrary principle dictated by systematic 

 convenience. 



Those genera in which there appears to be the best chance of applying 

 consecutive comparative argument will be taken first. They are clearly those 

 few genera in which the sorus is actually marginal, with a double indusium, 

 such as is familiar in the Dicksonioid-Davallioid series. For reasons to be 



