240 THE SPORE-PRODUCING ORGANS [CH. 



sporangia themselves, as in Polypodmni crassifoliiun and Dryopteris Poiteana. 

 Such devices, which are well illustrated in Hooker's Genera Filicum (vi), 

 may be held to serve functionally as a sort of diffused indusium. 



General conclusions regarding the Sorus, 



AND ITS PLACE IN PhYLETIC MORPHOLOGY 

 In the systematic treatment of Ferns, as in that of the Flowering Plants, 

 priority should be given to the propagative over the vegetative system. But 

 the latter cannot be discarded. It is upon the sum of all the characters that 

 a final conclusion must rest. In the preceding pages the discussion of the 

 propagative organs has been taken up, and the sorus has been examined as 

 to its constitution, position, individuality, and pi'-otection. Diversity has been 

 found on all these points. Three types of constitution of the sorus have 

 been recognised, the Simple, the Gradate, and the Mixed. The first is 

 characteristic of those Ferns which comparison and the fossil history show 

 to be most primitive. The earliest of all was probably the "monangial 

 sorus," where the sporangium is solitary and distal. The last is that state 

 which prevails at the present day. The Gradate condition is found to take an 

 intermediate place in many though not in all evolutionary lines. As regards 

 position, the marginal, which is found to be characteristic of most of the 

 Simplices,is shown to have passed in several distinct series into the superficial, 

 as the area of the leaf-blade increased : and this makes it seem probable that 

 the latter is a derivatiye state wherever it occurs. The belief is entertained 

 that in the course of evolution the transition occurred early in some phyla 

 and later in others. The individuality of the sorus, where it is already 

 established, is liable to be lost in many sequences. This may be brought 

 about phyletically by fusions to form coenosori, as well as by fissions by 

 which means the number of sori may be increased in the course of Descent. 

 In many progressive lines the individuality of the sorus is apt to be ob- 

 literated by spreading of the sporangia generally over the surface of the 

 sporophyll. In Ferns at large the protection of the sporangia while young 

 is effected in various ways. Primarily it is by the circinate vernation, aided 

 it may be by hairs and scales, and incidentally by the massing of numerous 

 sporangia in the crowded sori. The most primitive Ferns relied mainly on 

 these simple devices for protection of the young sporangia. Where special 

 protective growths, designated by the general term " indusium," are present 

 they may be traced to very various origins. For instance, they may be special 

 outgrowths from the leaf-surface, independently originated in a plurality of 

 phyletic lines : or they may be products of the actual margin of the leaf or 

 pinna, which curls over the sori. Such developments, which are plainly not 

 homogenetic throughout, are specially characteristic of Ferns occupying a 

 middle position phyletically. Many of the most advanced types, belonging 



