TREE FERNS OF NORTH AMERICA MAXON. 479 



sition contribute excellent diagnostic characters by which the species 

 may be distinguished. 



LEAVES OP THE DICKSONIE.E. 



Of the three genera which comprise the tribe Dicksonieae, two 

 (Dicksonia and Cibotium) are usually arborescent and have numerous 

 large leaves borne in erect-arching crowns, as in most species of the 

 Cyatheae, while the third (Culcita) is very much smaller, never develops 

 an upright trunk, and might readily be taken for a member of the 

 family Polypodiaceae. There is evident among them not only a 

 pronounced difference in size, but also in shape and cutting of the 

 blades, as mentioned hereafter. They have, however, one very dis- 

 tinct feature in common, namely, the development of great masses 

 of delicate, limp, threadlike scales, of a type not found in many 

 species of the tribe Cyatheae. These are produced in great abun- 

 dance upon the growing crown of the plant and extend freely along 

 the stipes and rachises, from which, however, they are readily 

 deciduous. They are usually yellowish or yellowish-brown and 

 glistening, either straight or matted together, fragile, each com- 

 posed of a single series of thin, elongate, flattish cells set on end. 

 Apparently scales of no other types are produced in this tribe. The 

 collection of this soft woollike substance from the crowns of several 

 species of Cibotium occurring in the Hawaiian Islands was long a 

 commercial industry, the material (there called "pulu") being used 

 mainly as a stuffing for small pillows. The wool of the Asiatic 

 Cibotium Baronetz, the fabled "Scythian lamb," and of a species of 

 Culcita has been similarily used, and hi a limited way also for stopping 

 bleeding of wounds in surgery. 



CLASSIFICATION. 



The recognition of the Cyatheaceae as a distinct family of ferns is 

 based for the most part upon minute microscopical characters of the 

 sporangia, the "spore-cases," which form the sori or "fruit dots." 

 The division of the family into tribes rests in part upon similar 

 characters, but also very largely upon more apparent differences in 

 the form of the indusia, special outer structures which more or less 

 completely inclose and protect the sporangia during their period of 

 development. As already stated the North American species are 

 associated in two tribes, the CyatheaB and the Dicksonieas, whose 

 main diagnostic characters may be readily recognized. 



The tribe Cyatheae is distinguished by having the sori borne 

 directly upon the ultimate veins of the segment and never at the ends 

 of the veins, which extend nearly to the margin. The sorus may in 

 different genera be either naked (without a special protective outer 

 structure) or either partially or wholly gurrounded by such an organ, 



