1896] PROCARP AND CYSTOCARP OF PTILO1A 355 
The segments of the axial siphon which bear the pinnules are 
called nodes and the segments between them internodes. 
The larger pinnule of the pair begins to develop from the 
node almost immediately after the latter has been cut off from 
the apical cell. The nodal segment first grows out to one side, 
the side that is opposite to that of the young pinnule on the 
node just below it, and a cell is cut off. This cell is the earliest 
stage of a pinnule. It assumes the réle of an apical cell, and by 
successive transverse divisions gives rise to a row of cells which 
becomes the axial siphon of the pinnule. When the larger pin- 
nule is well under way in its development, the node gives rise to 
another cell on the opposite side of the pinna, and from this cell 
is developed the second pinnule, which rarely becomes as large 
as the first and sometimes remains quite abortive. Stages illus- 
trating the above description may be seen in fig. I. 
_ Cells are cut off laterally from both sides of the axial siphon 
of the pinnule, and these by successive transverse divisions 
develop a system of lateral branches. The young pinnule 
then has the structure of a membranous tissue the thickness 
of a single cell, but it really consists of an aggregation of fila- 
ments, all in the same plane, each of which grows in precisely 
the Same manner as the axial siphons of pinne and pinnules, 
viz., from apical cells. . 
The three systems of filaments, (1) the axial siphons of the 
pinne, (2) the axial siphons of the pinnules, and (3) the lateral 
ranches from the axial siphon of the pinnules, are the frame- 
work which determines the shape of the frond. All other cells 
are part of the cortex proper. There is no cortex on the young 
Pinnules and at the tips of the pinne, but on older portions of 
the frond its gradual development may be easily traced. Short 
branches grow up over the axial siphons in older parts, covering 
them With several layers of cells. This collection of cells con- 
stitutes the cortex. 
The entire frond then consists of an elaborate system of fila- 
ments, and the growth of all parts is strictly from the terminal 
cells (apical cells) of the branches. A segment cut off from 
