The Origin of the Pteridophyta. 29 
from what is strictly the continuation of the mother axis in size or 
in angle of divergence from the original axis. 
From our present point of view it is immaterial to discuss why 
strict developmental dichotomy is so rare, or which histogenetic 
type should be regarded as the more primitive. The term 
“ dichotomous ” is ordinarily applied to all cases in which an axis 
appears to fork into two more or less equivalent branches, without 
regard to the actual cell-divisions which take place, nor does it 
appear that any great phylogenetic importance should be attributed 
to the exact sequence of cell-divisions. It is, however, obvious 
that according to the relative rates of growth of two such “ forks ” 
or branches of an axis, either the two will remain “ equivalent ” in 
ultimate development, or the stronger and quicker growing member 
will tend to push the other aside and approximate to a continuation 
of the original axis. In this way effective “ monopodial ” branching, 
so far as the adult structure is concerned, may be attained. Such 
transitions from “ dichotomous ” to monopodial branching are in 
fact extremely common in the thallus of the Algae and of the 
Liverworts, as well as in the branching of the leaves of Ferns and 
in their ultimate venation. The ease with which such transitions 
occur in the individual plant is important from our present point of 
view. 
If we take a coup <T ceil of the methods of branching in the 
plant-body throughout the plant-kingdom, we find that dichotomy 
as applied to the actual form of adult structures is abundantly 
present in the lower groups, but becomes rarer as we ascend, till in 
the Gymnosperms and Angiosperms it has disappeared entirely 
from the axes and is found in the leaves only of a few primitive 
Gymnosperms, such as Ginkgo, Stangeria and Cycas Michelozzi. 
In the Pteridophytes this type of branching is typical in the 
stems both of Lycopods and Ferns, though with transitions in both 
cases to the monopodial type, and is of widespread occurrence in 
the branching of the fronds of the more primitive Ferns, as well as 
in the venation of the “ pinnules ” throughout the group. It is also 
apparently common in the branching of the fronds of the Pterido- 
sperms. It is found in the leaves of Sphenophyllales and Psilotales 
and of some Palaeozoic Equisetales, such as Arclueocalamites, and 
in the curious isolated form Pseudobornia, though the stems of most 
of these branch monopodially. 1 
The occurrence of dichotomy in the leaves of some of the 
older Equisetales is a striking fact, and if we accept the view, for 
1 see D. H. Scott. The Present Position of Palaeozoic Botany. 
Progressus Rei Bot. 1., 1906. 
