23 
Evans. — Branching in the Leafy Hepaticae. 
type. It will be noted that the segments on this side follow the same 
sequence as on the other side and correspond with the leaves and under- 
leaves described above. In addition to terminal branching Microlepidozia 
shows intercalary branching from ventral segments. 
The Acromastigum Type. 
The genus Acromastigum contains the single Hawaiian species A. in- 
tegrifolium . Since a detailed description of the branching in this plant has 
already been published by the writer (’00), only the more essential features 
of the Acromastigum type will be mentioned here. The species shows in 
addition terminal branching of the Frullania type and ventral intercalary 
branching. In the development of a ventral segment the first wall laid 
down is periclinal and gives rise to an internal and an external cell, the latter 
soon dividing by an anticlinal wall into two cells or segment-halves. If the 
development continues in the usual way one of the external cells again 
divides by an anticlinal wall, so that the 
segment shows three external cells. These 
three cells give rise to three hyaline papillae, 
which are carried forward on the apex of 
the underleaf developed from the segment. 
When a terminal branch is to be formed from 
a ventral segment the apical cell of the 
branch becomes differentiated in one of the 
segment-halves, leaving a single external cell 
for the underleaf. The result is that the 
underleaf is much narrower than usual and 
shows only one apical papilla. The branch 
is situated at one side of this narrow under- 
leaf, sometimes on the right and sometimes 
on the left. In Fig. 30 the establishment of a branch of this character 
is shown diagrammatically on the right-hand side, while a branch of 
the Frullania type is shown in the lateral segment on the left. The first 
two segments cut off in the ventral branch give rise to lateral leaves and the 
third to an underleaf. It will be noted that the spiral is sinistrorse in the 
branch figured ; if the branch had been on the left-hand side the spiral 
would have been dextrorse. The same thing would have been true if the 
spiral of the main axis had been dextrorse instead of sinistrorse. In other 
words, the branches arising in anodic segment-halves are homodromous with 
the main axis, while those arising in kathodic segment-halves are anti- 
dromous, the relation being the same as in branching of the Frullania type. 
Branches of the Acromastigum type are apparently always flagelliform, and 
their minute leaves and underleaves present no modifications of special 
interest. 
Fig. 30. Diagram representing 
the establishment of a branch of the 
Acromastigum type on the right and 
one of the Frullania type on the left. 
