3 
Evans. — Branching in the Leafy Hepaticae. 
Is anodic and the ventral wall kathodic. In Fig. 2, which represents a sinis- 
trorse spiral, segment 4 agrees with segment 3 of Fig. 1 in having its 
dorsal wall anodic and its ventral wall kathodic, while segment 2 agrees 
with segment 4 of the same figure. Speaking generally, therefore, it may 
be stated that with a dextrorse spiral the anodic walls of the lateral 
segments will be ventral on the right-hand side (when a shoot is viewed 
from the ventral surface), while with a sinistrorse spiral they will be ventral 
on the left-hand side. In the ventral segments the narrow enclosed walls 
are both lateral, but may be distinguished as anodic and kathodic. Of 
course the descriptive terms which have just been applied to the walls may 
also be applied to portions or regions of a segment. 
The early divisions in a segment follow a definite sequence, and Leitgeb 
was therefore able to assign the branches which he studied to definite 
regions. In the lateral segments the first two walls are at right angles to 
T 
Figs. 3 and 4. Diagrams representing the first two walls formed in a lateral segment and the 
establishment of the segment-halves. 
the two broad faces. The first wall (Fig. 3, 1 ) extends from the middle 
of the free face to one of the enclosed narrow faces ; the second wall 
(Fig. 4, 2 ) connects the first wall with the other narrow face. The segment 
thus becomes divided into an internal cell and two external cells, the latter 
being designated by Leitgeb the ‘ segment-halves \ 1 These halves may be 
distinguished as dorsal and ventral and also as anodic and kathodic. In 
the ventral segments the first wall brings about a division into an inner 
and an outer cell, the latter usually becoming subsequently subdivided by 
one or more anticlinal walls. In the further development of the shoot 
the internal cells of the segments are destined to give rise to the axial 
tissues of the stem, while the external cells will form the cortical tissues 
of the stem and also the leaves or other appendages. Each lateral segment 
develops normally a single leaf, which represents the entire breadth of the 
segment, and Leitgeb has clearly shown that in bilobed leaves the dorsal 
lobe has been formed from the dorsal segment-half and the ventral lobe 
from the ventral half. Even in leaves which are undivided at maturity 
the young rudiments show evidence of being bilobed, but the shallow sinus 
between them soon becomes obliterated. In leaves with more than two 
1 Leitgeb really applied the term ‘ segment-halves * to the two unequal cells formed by the first 
division wall in a lateral segment. The slight alteration in the application of the term, indicated 
above, makes it possible to homologize better the ventral segments with the lateral segments. 
