Evolution of Branching in the Filicales. 23 
branch, of a series of stages, each morphologically less complex 
than the preceding. This process naturally finds its full illustration 
in forms with reticulate steles. The basal protostele of the 
specialized branch is therefore a ccenogenetic feature, not strictly 
primitive. 
The departure from dichotomy was entered upon at a very 
early period in the history of the Filicales. 
We may look upon the simpler organization of the specialized 
branch in its proximal region as a sudden drop to a morphologically 
lower and more stable level in consequence of a loss of physio¬ 
logical conditions necessary for the production of the morphologically 
higher condition. The alleged retrospective nature of traumatic 
structures may perhaps also be regarded in the same light. 
Viewing the group of Filicales as a whole it may be said that 
the branches do not hold any regular position with respect to the 
leaves. In those cases where the branches arise in some relation 
to leaves, this association is, in its evolutionary origin, a secondary 
phenomenon attributable to possible biological advantages, one 
of which may be the protection of the young bud. Significant in 
this connection is the fact that in some plants possessing so-called 
axillary branches (Hymenophyllaceae, Ophioglossaceae) while the 
stele of the branch is sometimes inserted on that of the main 
axis independently of the subtending leaf-trace, it is some¬ 
times confluent with the latter at the base. The view here 
adopted is that in all such cases the portion of the strand common 
to leaf and branch, conveniently termed by Dr. Scott the “undivided 
trace,” is of a dual nature ; we owe this view originally to Dr. 
Scott, but he rejected it in favour of the foliar view. Professor 
Bower’s description of Cheiroplenria strongly supports the former 
view. 
The formation of new individuals from adventitious buds on 
the leaves is the most recently evolved method of branching in 
Ferns, but it is difficult to relate it to the usual method, in which 
the branches always spring from buds laid down at the growing 
apex of the mother-axis. 
My hearty thanks are due to Mr. Tansley, not only for valuable 
suggestions regarding the literature, but also for much helpful 
advice and criticism during the preparation of the manuscript of 
this paper. 1 have pleasure also in heartily thanking Professor 
Seward for references to literature. 
The Botany School, Cambridge, 
December , 1916. 
