100 TuHompson: THE MORPHOLOGY OF TAENIOMA 
observed at the joints, even in the oldest portions. The method 
of growth is by a single dome-shaped apical cell, which cuts off 
disk-shaped cells at its base, each of which will correspond to a 
single segment in the fully formed stem. When the formation of 
a branch is to take place, the apical cell divides by an oblique wall 
into two unequal parts. The smaller of the two is the cell which 
will continue the main axis; the larger increases laterally and 
divides by an oblique wall at right angles to the first. This 
second dome-shaped apical cell forms the apical cell of a secon- 
dary axis (FIG. 4). Hence it is evident that this branching is not 
true dichotomy, since the two newly formed apical cells are not 
the product of the single division of an apical cell. The cell of 
the shoot tip is the true apical cell, for the branch apical cell is 
formed secondarily by the further growth and division in a cell 
underlying the tip cell. The apical cell of the tip next cuts off 
3-6 disk-shaped segments below, in which soon appear two longi- 
tudinal divisions, separating these segments into three cells in a 
row in one plane.* Somewhat later and usually just behind the 
first branch formed from the tip, two other longitudinal divisions, 
parallel to each other and perpendicular to the first division, occur 
in the middle cell, forming the other two pericentral cells. Con- 
sequently in the fully formed stem the branches are separated by 
three to six segments, whose structure superficially resembles that 
of a simple Po/ysiphonia (FIG. 3). 
The secondary axes develop in the same way as the main 
shoot, from the apical cell mentioned above. They may be short 
and erect with but 2-6 branches (Fic. 1), or they may be long and 
recumbent, giving rise to similar tertiary axes. On them as 
lateral outgrowths appear the flat short branches, and also the 
reproductive organs, the stichidia with tetraspores, the antheridia, 
and the cystocarps. 
The flat shoots (Fic. 11) arise in the following manner: An 
apical cell of the axis divides into a growing tip cell and a branch 
cell as before (Fic. 4). This branch apical cell cuts off below 
disk-shaped segments until a filament of 9-30 cells is formed 
(FIG. 7). It then divides as did the apical cell into two, from 
: : 
Longitudinal will be oe ‘throughout as indicating a division parallel to the axis; 
transverse as across the plan 
