602 Church . — The Structure of the Thallus of 
Note on the Calcification of N. dumetosa. 
A comparison of sections of the calcified and decalcified 
thallus leads to the conclusion that the deposit of calcium 
carbonate is closely connected with two phenomena : the 
presence or absence of, first, an external mucilaginous layer 
on the cellulose membrane, secondly of chlorophyll-corpuscles 
(Fig. 26) ; and, in fact, that it is abundant on segments where 
both occur, but absent if either is for some reason wanting. 
The growing apex of the main axis is covered by striated 
layers of clear hyaline mucilage, and the thick mucilaginous 
sheaths of the developing appendages of the upper half-dozen 
whorls form a series of striated layers filling the cavity of what 
may be termed the apical bud. This apical bud is formed by 
the inarching of about 16 whorls of appendages, and on 
the whole surface of these the mucilaginous layer is well 
marked ; but, as soon as the filamentous portions spread out 
in the water, the thin mucilage is dissipated and no trace of it 
remains, either on the filaments or on the outer surface of the 
cortical facets. On the other hand, it persists unaffected 
lining the internal cavities of the thallus, beneath the cortical 
system, and it is only in this situation that any incrustation 
takes place. Even here, however, the pedicels of the cortical 
segments, as also of the aplanosporangia, which possess mucila- 
ginous layers, do not calcify in the adult plant, although it has 
been shown that they were slightly incrusted in Stage IV 
(Fig. 21). 
Again, with the first formation of a cortex, as in Stage III, 
the chlorophyll-corpuscles, at first evenly distributed through- 
out the plant, will tend to leave the pedicel-portions of the 
cortical segments and accumulate in the dilated, almost 
peltate, facets. In fact, it is possible that this very differentia- 
tion in form of the basal segment was induced by the assimila- 
tory requirements of the member. In the cortex of Stages 
IV and V this centrifugal migration will be still more 
pronounced ; the main axis will contain relatively fewer ; 
the basal segments few, and these exhibiting a tendency to 
