20 
Evans . — Branching in the Leafy Hepaticae. 
to the Frullania type, exhibits a number of interesting peculiarities. The 
modifications which occur at the base of a branch are shown in Fig. 27, 
which represents B. trier enata, a widely distributed species in the northern 
and mountainous regions of Europe, Asia, and North America. The spirals 
shown in the figure are both dextrorse, and it will be noted that the 
distance between a leaf on the right-hand side and the next underleaf is 
much less than the distance between the underleaf and the next leaf on 
the left-hand side. An occasional leaf in B. tricrenata is bluntly tridentate 
at the apex, but most of the leaves diverge from this typical condition. 
The underleaves are broader than long, and are either entire or irregularly 
toothed or lobed at the apex. The incomplete leaf at the base of a branch 
is always undivided at the apex and acute, and its line of attachment, as 
in similar cases, is partly on the stem and partly on the branch. The 
first underleaf of a branch is smaller 
than the other underleaves and tends 
to be distinctly bilobed ; it is dis- 
placed in such a way that its line of 
attachment almost reaches that of the 
next underleaf in the stem-spiral. 
The first leaf of the branch is small, 
but the later leaves and underleaves 
show no special modifications. 
In the genera previously con- 
sidered the branches have been partly 
on one side of the main axis and 
partly on the other. This has made 
the branching axis and the branch 
sometimes homodromous and some- 
times antidromous. In Bazzania , however, Leitgeb (’71 b) brings out the fact 
that the spirals in a branch-system are invariably homodromous, although 
in some plants they may be dextrorse and in others sinistrorse. He shows 
that this condition is due to the restriction of the branches to anodic 
segment-halves. This being the case a dextrorse branching axis would 
develop branches on the right-hand side only (when viewed from the ventral 
surface) and a sinistrorse axis on the left-hand side. On account of the 
simulated dichotomy in Bazzania the shoot-system never appears one-sided, 
as it necessarily would if an obvious monopodium were maintained. 
From the standpoint of comparative morphology Velenovsky (’ 05 , 
p. 1 1 2) concludes that terminal branching, especially of the Frullania type, 
represents a true dichotomy. In his opinion evidence derived from develop- 
mental study is of no value in deciding questions of this kind. Unfortu- 
nately he fails to give reasons for his opinion in this particular case. It 
cannot be based upon the fact that the two new axes which result from 
Fig. 27. Bazzania tricrenata. Beacon Falls, 
Connecticut (A. W. E.). x 27. 
