The Origin of the Pteridophyta. 33 
sporangiferous thallus became specialised for assimilatory functions 
much as we see in the gametophyte of such forms as are found in 
the “ erect ” sections of the liverwort genera Blyttia, Symphyogyna 
and Hyinenophyton. In such forms a sympodial rhizome bearing 
apparently lateral assimilating fronds is produced, but while in the 
Hepatic genera the development of such a type of organisation is 
strictly limited by the necessities of aquatic fertilisation, the 
potentiality of development of a similar spore-bearing plant would 
be practically unlimited. Transitions from such a form to an upright 
radially organised type, with more or less crowded leaves, is easy 
enough, as may be seen in modern Ferns, and probably both types 
of structure existed from an early period. Roots may be regarded 
as branches of the thallus modified in relation to underground life. 
That such modification can occur is shown by the underground 
parts of Polytrichaceae, where a scale-bearing rhizome takes on 
very nearly complete root-structure in the arrangement of its 
tissues. 
From such forms the Ferns, Sphenophyllales and Equisetales 
originated, the first group retaining the characters of the primitive 
type, the other two exhibiting progressive reduction-series in 
correspondence with the acquirement of a verticillate leaf-insertion. 
The Lycopods may, as we have already pointed out, perhaps be 
regarded as an extreme case of leaf-reduction in one of these lines. 
On the other hand they may be independently derived from the 
primitive Protopteridophytes by foliar specialisation of ultimate 
branchlets of the thallus, instead of whole branch systems as in the 
Filicinean type. 1 
Thus it seems possible to bring the Lycopods into line with the 
other Pteridophytes without either hypothecating extreme reduction 
or abandoning an explanation of the morphological nature of the 
leaf which seems by far the most rational and convincing that has 
ever been suggested. 
The dichotomising thallus with its generalised functions, its 
power of unlimited growth, and the possibility of easy transition to 
“ monopodial ” branching, seems to me by far the most natural 
type of plant-body from which to derive the sporophyte of the 
higher plants. It is simply a question of processes of subordination 
1 It is also conceivable that microphyllous forms, such as the 
Lycopods, might be derived from thalloid types with lobed 
margins, the lobes eventually becoming definite appendicular 
structures, as we see in the Blasia-Fossombronia-Calobryum 
series of Liverworts. 
