Birbal Sahni. 
16 
intercalation, at its base, of a series of stages each morphologically 
less complex than the preceding; that, in short, the process has 
been one of retrogressive evolution in a basipetal direction. The 
appearance of relatively simple types of vascular structure at the 
base of a highly evolved branch, such as that in Alsophila, 
Nephrolepis , etc., must be regarded as a coenogenetic feature, and 
not as strictly primitive. 
In our attempt to trace the evolution of branching in Ferns we 
must constantly keep in view one fact, which was pointed out in the 
introduction to this paper. This was, that many Ferns of a 
relatively high order of evolution, such as Polypodium and Pteridium, 
commonly branch dichotomously, while on the other hand, 
relatively primitive types such as the Ophioglossaceae may show 
a monopodial type of branching. Thus we do not find that there 
is a necessary parallelism between the evolution of the modes of 
branching on the one hand, and the evolution of the plants them¬ 
selves on the other. We are driven to the conclusion that these 
two lines must have been independent of each other. 
b. The Bvanch as a Potential Individual. 
If, as we have suggested above, the evolution of the branch 
has in the main been guided by the need for vegetative propaga¬ 
tion—and, of course, the phenomenon of branching in itself 
primarily implies an amplification of the plant-body—it is of 
interest to find that the further the branch recedes from its 
ancestral form, the more nearly does it approximate to our con¬ 
ception of it as a potential individual. For the sake of an ampler 
illustration we shall again confine ourselves to the branching of 
the dictyostelic series C—F, Fig. 1. We see here that the 
stele of the branch of Alsophila excelsa, in virtue of its origin from 
a relatively minute dormant bud, has necessarily to pass, during 
its growth, from a fine thread-like beginning, through gradually 
expanding solenostelic stages, before it finally attains the adult 
type of structure. Stenzel’s description of the branch stele 
of Alsophila corresponds in the broad features with Gwynne- 
Vaughan’s description of the ontogeny of the main stele. In sharp 
contrast stands the branch of Poly podium which, having at its 
disposal from the very beginning the ample food resources of the 
plant, has been able at the outset to develop a dictyostelic vascular 
system. A striking illustration of the same phenomenon was 
recorded in this journal, 1 where it was suggested that probably 
because of the more favourable physiological conditions in which it 
1 Sahni, B., New Phytologist, 1915, pp. 263, 271. 
