Evolution of Branching hi the Filicales. 15 
figure given it appears that a thin protostelic strand branches off 
from the main stele and, on entering the base of the tuber, expands 
and acquires a pith. It is not said whether it becomes converted 
into a reticulate stele. Seeing that the adult rhizome of Hymeno- 
phyllum never possesses a medullate stele, the appearance of such 
a structure in the tuber carries a peculiar theoretical interest, not 
only as supporting the hypothesis offered in my paper on the tubers 
of Nephrolepis, but also as showing that under the influence of 
physiological factors an organ may, so to say, break loose from the 
morphological check which may be said to have been imposed upon 
it by the phylogenetic position of the plant. 
To take an instance from the animal world, the governing 
influence of the conditions of nutrition on growth is well illustrated 
by the rather startling results which Gudernatsch 1 has recently 
obtained by feeding tadpoles respectively on the thymus and thyroid 
glands of various mammals. While young tadpoles fed on thyroid 
cease further growth in size, prematurely pass on to the changes of 
metamorphosis, and become dwarf frogs, those fed on thymus grow 
into giant tadpoles, and if they are kept long enough under this 
influence, the metamorphosis is altogether suppressed. 
If the line of argument traced above is sound, we have in it 
a strong support for the view that in the Ferns at least the primitive 
type of branching was the dichotomous, and that the monopodial 
type was derived from it by the gradual suppression of the growing 
point of one of the arms of the fork, which in the extreme cases 
became dormant immediately after its origin. The impetus for the 
departure from dichotomy probably originated in the need for a more 
effective nourishment of one of the branches. This need was 
supplied at the expense of the other branch, whose further develop¬ 
ment was postponed till a surplus of nutrition was available. The 
most important line along which the latter branch has evolved is, 
as we have seen, specialization for vegetative propagation. As of 
secondary importance may be mentioned the modification of the 
branch to serve as a climbing organ, so well seen in Neplirolepis, as 
well as its transformation into a tuber for the storage of food and 
water ( N . tuberosa, Polypodium Brunei). 
Further, if the sequence of events has been as we have 
conjectured, we arrive at the interesting theoretical conclusion that 
the evolution of the filicinean branch has mainly consisted in the 
1 Gudernatsch, Zentralblatt fur Physiologic, 1912, Vol. XXVI, p. 323. 
