468 
The border of an older frond shows numerous triangular initial cells (fig. 431) 
which are the end-cells of cell-rows of the third or fourth order, which really exist in 
and the cortication proceeds at 
both sides of it. A transverse 
section of the mid-rib shows 
a row of large cells with rather 
thick walls traversing the mid- 
rib (fig. 429). These cells are 
lengthened in the longitud- 
inal direction of the frond and 
Fig. 428. show multiple pits in the trans- 
PR Arve AED versal walls. The cells situat- 
frond. 260:1. ed on both sides of the median 
row have a similar character 
to these, but they have a smaller diameter and 
are shorter (comp. WiLLE 1887 fig. 28, KyLiN 
1923 p. 70 fig. 45); they are also provided with 
(secondary) pits in the transversal walls. The 
spite of NIENBURG's assertion (comp. 1908 pp. 195, 206). These ini- 
tial cells do not grow out except by producing adventitious shoots. 
The structure and development of the mid-rib has been de- 
scribed by WILLE and Kyrın. The development begins with longi- 
tudinal divisions of the primary cell-row in the middle of the frond, 
Fig. 429. 
Phycodrys rubens. Transverse section of mid- 
rib. c, central cell with a central primary pit 
and four secondary pits. 150: 1. 
middlemost cell of the transversal cell-row must be designated as the central cell; 
its transverse walls contain the primary pit of the primary segment walls or of the 
Fig. 430. 
Phycodrys rubens. Longitudinal sections of inner cells in the mid-rib. 
A in the living state, B and C fixed and stained, showing the pits and 
the nuclei. A 200:1. B, C 450: 1. 
intercalary walls; it is shown 
in the middle of the middle- 
most cell-wall (c) in fig. 429, 
but the same wall shows four 
secondary pits in the periphery 
of the wall by which the cell 
is connected with the next 
following central cell. The 
transverse walls of the other 
cells in the transverse cell-row 
are only provided with second- 
ary pits, usually four, all situ- 
ated near the periphery of the 
wall. Fig. 430 shows longitud- 
inal sections of the inner cells 
in a mid-rib; C is a central 
cell showing at the upper end 
