TREE FERNS OF NORTH AMERICA MAXON. 481 



can be devised, and to frankly recognize it as one merely of con- 

 venience. To the objection that the recognition only of the three 

 genera above mentioned is scarcely logical, since each of these is 

 susceptible of division into two or more fairly distinct groups, it may 

 be answered that at least the treatment here followed has the sanction 

 of long usage and on this account will incur neither confusion of old 

 names nor the substitution of new ones. The species constituting the 

 tribe Cyatheae may be grouped, then, under the three genera Cyathese, 

 Alsophila, and Hemitelia. 



THE GENUS CYATHEA. 



The North American representatives of the genus Cyathea fall 

 readily into two sections, based upon characters of the indusium; in 

 the first of these the indusium is cup-shaped or saucerlike; in the 

 second it is like a complete hollow sphere. Of the 50 species occurring 

 in North America about 30 fall under the latter section. 



An excellent example of the cup-shaped or hemispherical type of 

 indusium is seen in Cyathea elegans, of Jamaica, a species with large 

 tripinnatifid fronds. Plate 9, figure A represents the young son of 

 this species at a stage when the sporangia are just reaching maturity 

 and are beginning to crowd upward and outward, thus widening into 

 a broad mouth the opening above them which is first evident merely as 

 an apical pore. In plate 9, figure B is shown a later condition, in 

 which the sporangia have reached maturity and have mostly dis- 

 appeared. Another example of similar but less pronounced form is 

 shown in plate 10, figure A, representing the sori of Cyathea arborea. 

 In this the indusium is saucer shaped, and the sporangia are seen to 

 have been heaped high above its low even margins. In all species of 

 the section of which these two species are illustrative, the indusia are 

 firm, have perfectly entire margins, and almost invariably are per- 

 sistent long after the spores have been shed and the sporangia drop- 

 ped. The central position of the elevated receptacles may also be 

 noted in the figures cited, as well as details of venation and pubescence. 

 The sori are seen to be borne near the midrib (costule) of the segment, 

 being seated at or just below the forking of the slender lateral veins. 

 This section of the genus is known as Eucyathea or true Cyathea, 

 since it contains those species which in general form and structure of 

 the indusium agree with C. arborea, the type species of the genus. 

 Of the 20 North American species all are confined to the West Indies, 

 excepting only C. arborea, which occurs sparingly also on the continent 

 from Mexico southward. 



A very different type of indusium characterizes the second section 

 of Cyathea, which having once been regarded as generically distinct 

 38734°— sm 1911 31 



