32 Evans. — Branching in the Leafy Hepaticae . 
of branching can have been directly derived from another. Ordinarily, when 
two organs are genetically related, they are alike at the beginning and 
acquire the differences which they show at maturity during the course of 
their development. In other words they represent diverging lines. This is 
clearly seen in such organs as the scale leaves and the foliage leaves of the 
higher plants, the earliest rudiments of which are quite indistinguishable. In 
dealing with structures of this kind, Goebel (’98-’01, p. 577 ) has shown by 
experiment that it is possible to alter the course of development of a 
particular rudiment. He has shown, for example, that a rudiment, which 
from its position on a growing point would normally develop into a scale 
leaf, can be acted upon in such a way that it will develop into a foliage leaf 
instead. Nothing of the sort is conceivable with regard to the branches of 
the Jungermanniales. These show their distinctive differences at the 
beginning and tend to become more and more alike as development proceeds. 
Instead of representing diverging lines of development they represent con- 
verging lines (except in cases of marked specialization), and it would be 
quite impossible to convert a young branch of the Frullania type into 
a branch of the Radula type. To do so would necessitate a change in the 
position of the first branch-segment or else in the direction of the branch- 
spiral, either of which would of course be out of the question. On the 
whole it seems most logical to consider the differences in the branches as 
really due to mechanical causes, which in some way determine the position 
of the first branch-segment and the direction of the branch-spiral. This 
would obviate the necessity of looking for a genetic connexion between 
the various types. 
The development of a branch is undoubtedly to be looked upon as 
a response of the shoot to some kind of a stimulus. The different kinds of 
branches might then be due to the same stimulus acting upon segments of 
different ages or to distinct stimuli. To a certain extent at least the latter 
hypothesis seems to correspond better with the facts, and there are even 
reasons for believing that intercalary branches in different positions may 
sometimes be induced by unlike stimuli. These ideas are best supported 
by the study of species in which branching is a more or less unusual 
phenomenon. To illustrate the conditions under which terminal branches 
are developed Cephalozia bicuspidata may be selected, while Plagiochila 
Sullivantii and P. asplenioides may be chosen to illustrate intercalary 
branching. 
In Cephalozia bicuspidata it will be found that terminal branching is 
not only associated with exceedingly young segments and with young 
individual plants, but that the conditions necessary for a vigorous vegetative 
development must be present before the appearance of such branching can 
be induced. Under ordinary circumstances, therefore, a plant will go 
through life without branching at all until the characteristic intercalary 
