XVII] ARCHETYPE FOR FERNS 339 



Comparison of living Ferns has led definitely to the conclusion that the 

 superficial position of the sorus, so prevalent in modern Ferns, is a later and 

 derivative state, and that a distal or mm^ginal position of the sorus was 

 primitive. The latter is found in the Botryopterideae among the fossils, and 

 in the Ophioglossaceae, in Osnmnda, and the Schizaeaceae, and in many 

 other primitive types of living Ferns. The sporangia of the Simplices are 

 all formed simultaneously , and they are usually without indusial protections. 

 These are also indications of a primitive state. In many of the Simplices the 

 sporangia are grouped in sori : but sometimes the sporangia are solitary, as 

 in the Schizaeaceae, and each is seated on its own vein-ending, as in Botry- 

 chium. This is the condition styled by Prantl the " monangial sorus," and 

 it is probably the most general, primitive soral condition of all. It is well 

 represented by the fossil Stauropteris,\v\{\Q}ci is considered to be a very early 

 type of the Filicales. The primitive sporangium was a relatively large one, 

 of the Eusporangiate type, with a thick wall and a simple opening mechanism, 

 and with a large spore-output. All these characters are seen in the Ophio- 

 glossaceae, Marattiaceae,and Osmundaceae,as well as in the Botryopterideae, 

 and in Stauropteris. 



Using the archaic characters thus recognised by comparison of the sporo- 

 phyte in living and fossil Ferns, it is possible to visualise a primitive type 

 which would represent them all, and so to sketch an archetype for the Class. 

 It would consist of a simple upright shoot of radial symmetry, possibly root- 

 less, dichotomising if it branched at all, and with the distinction between 

 axis and leaf ill-defined. The leaf, where recognisable as such, long-stalked 

 with distal dichotomy, tending in advanced forms towards the sympodial 

 origin of a dichopodium. All the limbs of the dichotomy would be narrow, 

 and distinct from one another. The whole plant would be relatively robust 

 as regards cellular construction, and traversed by conducting strands with a 

 solid xylem-core. The surface might be glabrous, or invested with simple 

 hairs. The solitary sporangia would be relatively large, and distal in position, 

 with thick walls, and a simple method of dehiscence: and each would contain 

 numerous homosporous spores. Though all the chief primitive features are 

 concentrated into this description, it is not to be assumed that they will all 

 be represented in the same individual or type: but it is quite conceivable 

 that they should be. 



The specification thus given is entirely based upon comparisons of plants 

 recognised as belonging to the Filicales, living or fossil. But if it be checked by 

 reference to the form and structure of the fossil Psilophytales of the Devonian 

 Period, it is at once apparent that a real similarity exists between the verbal 

 specification and plants which have actually lived (Fig. 308). No attempt is 

 made to link this archetype of the Filicales definitely with any one of the 

 described fossils, or to point to any one of them as their actual ancestor. 



