480 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1911. 



the indusium. If an indusium of any sort be present, it is never formed 

 of any portion of the leaf margin, but is developed separately, being 

 attached at the base of the receptacle — the raised conical, globose, or 

 club-shaped structure upon which the sporangia are borne. 



The tribe Dicksonieae, on the other hand, has all the sori produced 

 upon the ends of the veins; that is, they are truly terminal and 

 marginal. In this tribe the sorus is invariably provided with a 

 bilobed or bivalvate ir.dusium of a wholly different character from 

 that of the Cyatheae, since it is formed in part by the more or less 

 modified and concave margin of small lobes of the leaf segment, 

 these facing inward. A similarly fashioned concave inner lip, facing 

 outward, arises from the base of the receptacle. Thus the two lips 

 together constitute the "double" (i. e., bilobed or bivalvate) indusium 

 mentioned above, within which the sporangia are contained. At 

 maturity the lips are thrust widely apart by the expanding sporangia. 



These are the more obvious technical characters by which the two 

 tribes may be distinguished. Without entering very far into the 

 subject it may be well to indicate briefly the grounds for recognizing 

 the several genera constituting these. Recourse will be had to 

 illustrations showing the parts upon a greatly enlarged scale, which 

 will serve far better than description alone to indicate the position 

 of the sori and to emphasize distinctions in form and structure of 

 the indusia. 



THE TRIBE CYATHE.E. 



The tribe Cyatheae was divided long ago into three genera, Cyathea, 

 Alsophila, and Hemitelia, mainly upon characters of the indusium; 

 and this conception has prevailed generally down to the present time. 

 It happens, however, that within the limits of this tribe there is found 

 nearly every intermediate stage between a complete hollow sphere 

 wholly inclosing the sporangia and the absence of any indusium 

 whatever. Because of this, the justification for laying great weight, 

 as a basis of generic classification, upon structures of such an ephem- 

 eral and variable nature has been seriously challenged by nearly 

 every recent writer upon ferns; and also because in many other 

 groups of ferns the indusia are now looked upon as really of such 

 secondary importance as often to preclude their use for more than 

 minor taxonomic distinctions, though they may be very serviceable 

 in distmguishing species or for associating them in minor groups 

 below the rank of a genus. The proposition again put forward, of late, 

 to merge Cyathea, Alsophila, and Hemitelia under a single generic 

 name (Cyathea) is thus not without merit. Yet, considering our 

 present incomplete knowledge and the extent of data still to be 

 derived from a thorough study of the group as a whole, it seems 

 desirable to hold to the established classification until a better one 



