242 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 Cyaihea, Alsophila, and Hemitelia include the 

 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 cup of Cyathea and the small indusoid scale of some 

 species ofAlsophila, 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 



