Rhabdonia tetter a , J . Ag. 407 
central filament c, c f c, &c., and the lateral branches. It is 
not always possible to decide whether there is one or several 
central filaments. A careful study, however, of the tips of 
ordinary branches, and especially of the proliferations on the 
tetrasporic plants, strongly favours the conclusion that only 
one such filament is present. Each lateral branch ramifies so 
as to produce a corymbose cluster of branchlets ; these clusters 
are firmly held together by a common gelatinous intercellular 
substance, so as to constitute a firm parenchyma-like tissue. 
Fig. 10 shows a section passing through two such clusters. 
Cell-division is almost exclusively confined to the terminal 
cells of the branches and their branchlets ; the frond therefore 
grows only at the surface. The terminal cells divide longitu- 
dinally, transversely, or obliquely, or they cut off from the 
distal end segments on all sides by means of oblique walls. 
The method of branching is in consequence very irregular. 
The vegetative growth of the thallus conforms to the rule 
laid down by Schmitz (’83 and ’92), according to which the 
Florideae possess thalli which are composed of branching 
filaments, and only the terminal cells of these filaments ever 
divide transversely or in such a way that the dividing-wall 
lies in the organic long axis of the cell. Exceptions to this 
rule are found in the Corallinaceae (Schmitz, ’92), in the 
Delesseriaceae (Schmitz, ’92 ; Naegeli und Schwendener, ’77 ; 
Kny, ’79; and Reinke, '80), and in the cells which form the 
pericarp in Rhabdonia . In all these cases the same individual 
shows a transition, in some or all of its parts, from a fila- 
mentous to a non-filamentous mode of growth. This transi- 
tion forms an interesting parallel to the similar development 
which must have taken place in the Chlorophyceae, if they 
represent the ancestral stock from which the Liverworts 
are descended. 
A median longitudinal section of the frond passing through 
the apex, shows that the cells of the central filament, as well 
as those which surround it, become more and more elongated 
the further they are removed from the apex. At the same 
time they tend to assume a direction parallel to the long 
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