949 CHARACTERS OF TRIBES AND GENERA. 
It is difficult to arrive at any satisfactory conclusion as - 
to the number of species, Sir Wm. Hooker in his “ Species 
. . Filicum " enumerates about 100, which in the ** Synopsis” 
. is increased to 139, but that falls short of the number 
given by Mr. Moore in the “ Index Filicum," where under 
the genera Alsophila, Cyathea, and Amphicosma (Hemenos- 
tegia), 176 are enumerated, and when Hemitelia (not yet . 
indexed) is added, the total will be near 200. 
I am of opinion that important distinguishing characters 
are to be found in the formation of the stems. After the 
examination of a considerable number, I find that while | 
some are slender and never exceed 1 to 3 inches in thick- 
ness, others are a foot or more in diameter, and that the 
difference is consequent on the normal number of fronds, 
constituting the series or whorle of the progressing axis 
being few or many. In some species three complete the ` 
whorle, that being the lowest number capable of forming 
an erect arborescent stem, but in others the series consists 
of a greater number, and consequently a stem of larger 
diameter is formed, I must, however, observe that a great 
deal is yet to be done in the examination of Tree Fern ` 
stems, before the differences in their structure can be made ` 
practically available for determining species. 
The genera Cyathea, Alsophila, and Hemitelia include thie 
greater mass of species belonging to this tribe, and although 
the technical characters are in many sufficiently obvious to 
enable us to refer many of the species to their respective 
genera without much trouble, the differences are not always ` 
. well defined ; for instance, the transition between the com- 
plete cap of Cyathea and the small indusoid scale of some — 
species of Alsophila, is so gradual that upon taking a general 
view the whole would seem to form one genus; but for the | 
sake of convenience, and in order to show the principle. 
