557 
fronds is caused by the new cortical layers becoming darker than the foregoing ones, 
and that the lowermost parts of the fronds are the darkest because they contain the 
greatest number of cortical layers. In my opinion the dark colour of these portions 
of the frond is due to the fact that they are best protected against the destructive 
action of the light upon the phycoérythrine. I have once, at Frederikshavn (Borre- 
bjergs Reef), met with a frond having the ordinary lurid-violaceous colour, but 
bearing a branched shoot, 6.5 cm long, having a pronounced blue-green colour 
contrasting with the colour of the rest of the frond. This shoot was at least 2 or 3 
years old. It evidently contained much phycocyanin but not 
phycoërythrine. ft NAT 
M) M 
The apical meristeme of the frond is built up of 
numerous thin, closely united filaments directed vertically U y AHUU 
towards the periphery and thus diverging. The outer cell- IN DONNA? 
rows are gradually, by the growth of the frond, directed OS) u A 
vertically to the longitudinal axis of the frond. The diameter \() () Ve 
of the outer cells is about 3.5 w, and the superficial cells are (de (V © 
scarcely thicker in the older parts of the frond. These rows NY 1 
of cells branch frequently in the meristematic portion of the 
frond, the apical cell dividing by an oblique cell-wall by which (N 
a new apical cell is cut off, while a branch is produced from / Al 
the segment cell, the upper portion of which is cut off by a B 
cross wall (fig. 546 B). À 
The central part of the frond is built up of long . PE De 
Ahnfeltia plicata. A, vertical 
cylindrical cells with very rigid refractive cell-walls, giving the section of apical meristeme. 
central cylinder the character of a mechanical tissue (comp. 2; from a similar section 
= showing ramification of the 
WILLE, 1885, p. 13, 50, Plate II, fig. 12, V, fig. 52). The cells cortical cell-rows. 625 : 1. 
are terminated by transverse walls which are about as thick 
as the longitudinal ones; they do not, therefore, seem much adapted to the function 
of conducting substances, so much the more as the central pits are very thin and 
not easy to observe (fig. 547). The length of the cells is, however, great, though 
very variable; some cells were very long, up to 780 w long, others much shorter, and 
it seems that a distinction can be made between very long-celled filaments and others 
consisting of short cells; the latter are perhaps of secondary origin. At the peri- 
phery the cells are shorter and gradually approach the inner cortical cells in size. 
WILLE (1885, p. 50) attributed Ahnfeltia plicata to the Florideæ with incompletely 
developed conducting tissue; he considered the last-named tissue, situated between the 
mechanical and the assimilatory systems, as such a conducting tissue. It, however, 
seems more adapted to a conduction between the assimilatory tissue and the central 
cylinder than for conduction in a longitudinal direction. The central cylinder has not 
only a mechanical function but also serves as storage tissue; I found the cells filled 
with starch-grains in summer and autumn (July to October) whereas in fructiferous 
plants they were without starch in spring (March to May). The intermediary 
