27 
Evans . — Branching in the Leafy Hepaticae. 
In the Radula type the formation of the branches is longer delayed, the 
sequence of the branch-leaves is less definite, the spirals conform less rigidly 
to rule, and, in the vegetative branches of the Lejeuneae at least, basal 
sheaths are present, whatever the morphological nature of these sheaths 
may be. In intercalary branching the formation of the branches is still 
longer delayed, the sequences of the branch-leaves are indefinite, the spirals 
show no conformity to rule, and the basal sheaths are constant and evidently 
represent ruptured cortical tissue. The peculiar features of intercalary 
branches are illustrated below by concrete examples. 
According to Leitgeb (72, 75) intercalary branches almost invariably 
arise in ventral segments. Even in cases where they appear lateral he 
considers that they are probably ventral in origin but displaced through 
subsequent inequalities of growth. The only case in which he assigns 
intercalary branches to lateral segments is in Jubula Hutchinsiae , a rare 
species of western Europe (75, p. 36 ). In this plant the vegetative 
branches conform closely to the Frullania type and the subfloral innovations 
to the Radula type. The branches which he regards as intercalary are the 
short male branches, which are situated behind normal leaves, bearing the 
same relation to them as the subfloral innovations do to the perichaetial 
bracts. His opinion is based upon the fact that each branch has a distinct 
basal sheath, and, since he was unable to demonstrate the boundaries of 
coalesced leaves and saw in the sheaths evidence of active cell-division, he 
concluded that they represented cortical tissue. Unfortunately his ex- 
planation and figure are not convincing, and there seems to be no imperative 
reason why these sheaths should not be interpreted in the same way as the 
very similar sheaths in the Lejeuneae. 
Although this doubtful case is the only one recognized by Leitgeb, the 
writer would refer certain intercalary branches in a number of species to 
lateral segments rather than to ventral. This course seems justifiable from 
the position which these branches occupy on adult shoots, although it has 
unfortunately been impossible to trace out their development from the 
earliest beginnings or to assign them to definite portions of the original 
segments. The genera where branches of this kind occur are the following : 
Micropterygium , Diplophylleia , Scapania , Sphenolobus , Cephaloziella (sub- 
genus Prionolobus), Odontoschisma , Anomoclada , Plagiochila , and Chilo - 
scyphus. In all probability this list could be considerably increased. As 
an illustrative example of lateral intercalary branching the genus Micro- 
pterygium of tropical America may be selected. In M. P terygophyllum, 
a widely distributed species (Fig. 34 ), the large underleaves are very much 
like those of Bazzania , but the lateral leaves show a very peculiar structure. 
They are distinctly complicate, but only the dorsal lobe is attached to the 
axis, the ventral lobe being formed by a folding back of its lower margin. 
Along the keel thus formed, in well-developed leaves, a wing of variable 
