TREE FERNS OF NORTH AMERICA MAXON. 487 



THE TRIBE DICKSONIE^E. 



Differences in habit and size of the plants and in the general shape 

 and dissection of the leaves offer more obvious marks for the recog- 

 nition of the three genera of Dicksoniese than do the indusia, which 

 are alike in position and of very similar structure. Thus, the several 

 species of Culcita are small plants, none of them ever treelike, having 

 short rootstocks only a few inches high and a spreading crown of 

 broad, skeletonlike, greatly dissected leaves rarely more than 3 or 4 feet 

 high, including the stipe; while Cibotium and Dicksonia are mostly 

 if not altogether arborescent, plants at least of huge growth. In dis- 

 tinguishing the latter genera it will be noted that the blades of Dick- 

 sonia are elongate and either lanceolate or oblanceolate in outline, 

 and that those of Cibotium are very much more ample and broadly 

 ovate or even triangular. Coupled with these differences, however, 

 are characters afforded by the sori, which, when once made out, are 

 unmistakable. The position of the sori and the general structure of 

 the double indusium has already been touched upon; the generic 

 distinctions follow. 



THE GENUS DICKSONIA. 



In Dicksonia the outer concave lip of the indusium consists of the 

 greenish leaf tissue of a small marginal lobule of the leaf segment 

 which is slightly if at all modified by its function as a partial covering 

 for the sporangia. The shape but not the texture of this outer lip is 

 shown in plate 13, figure B, representing Dicksonia navarrensis, of 

 Costa Rica and Panama. The iimer lip, or true indusium, is seen to 

 be of similar form, almost hemispherical, its hollow inner or under 

 surface embracing the sporangia upon the side opposite to the outer 

 lobe. It is yellowish and rigidly cartilaginous, thus conspicuously 

 different in texture from the outer lip, and is attached along the base 

 of the receptacle, upon which the sporangia are borne. 



But one other species of Dicksonia has been described from North 

 America, the closely related D. lobulata of Costa Rica. This and the 

 other members of the genus have like indusia. A section of a primary 

 pinna of D. navarrensis is shown in plate 14 at natural size. The leaf 

 tissue is rigidly coriaceous and of a yellowish green color. The trunks 

 of this species, as I have seen them, are about 9 to 12 feet high and 

 always roughly clothed with the very stout smooth bases of old 

 fronds, a feature which seems to be characteristic of the genus as a 

 whole. 



THE GENUS CULCITA. 



Although the indusia of Culcita are very similar to those of Dick- 

 sonia, the plants are so different in habit, in their smaller size, and in 

 the nondevelopment of an upright trunk, and bear quadripinnate 



