420 GENERA OF FERNS. 
' affinity with other genera of this tribe than with any in 
Aspidiee. 
Dicksoniee contains about 200 described species, present- 
ing the utmost extremes of habit; a number having small, 
very membranous fronds, scarcely an inch high, rising from a 
slender creeping rhizoma ; while others attain the length of 
several feet, and are produced from a thick creeping or erect 
caudex, which in the latter case attains the height of 50 or 
more feet; the genera Trichomanes and Hymenophyllum 
being examples ofthe first extreme, as Dicksonia and its 
allies are of the other.—It may, therefore, appear unnatural 
that genera of such extreme habits should be placed in;the 
same tribe; but upon a due examination of the transitions 
of form exhibited by this tribe, I cannot otherwise naturally 
arrange them ; and a difficulty even arises in finding sufficient 
difference of structure to characterize the several groups that 
the difference of habit indicates as being distinct. 
The tribe presents, at least, four natural subdivisions, but 
which cannot, with any degree of precision, be characterized 
from each other by any definite character derived from the 
sori or venation ; for, with the exception of a few species, the 
venation is direct and free, and the difference of structure of 
the indusium presents a gradual transition from one group 
of species to another, differing chiefly in texture and its man- 
ner of connivence with the accessory indusium. I have 
therefore deemed it best to designate the sections by the 
names of the genera ùnder which the generality of the 
species formerly stood, viz., Lindsea, Davallia, Trichomanes, 
and Dicksonia. 
Sect. I. Linpsaa, J. Sm. 
Sporangia pedicellate, produced from an elongated trans- 
verse anastomose or rarely simple terminal receptacle ; in- 
dusia plane, bilabiate. 
