282 Bower . — Some Normal and Abnormal 
contains a single nucleus. The transition from the simple fila- 
mentous protonema to the flattened expansion is sometimes 
gradual (Fig. 31) ; but in most cases it is sudden, and occurs at 
the apex of the filament (Figs. 28, 33, 35, 36). The succession 
of cell-divisions is more regular in the early stages of formation 
of the flattened ribbon than at any other time ; thus, it will be 
seen how in Fig. 35 the filament has widened laterally, and 
two of its cells have divided longitudinally, while the terminal 
cell of the filament retains its position, the only modification 
being a slight curvature of its septum, which is a natural con- 
sequence of the lateral widening of the filament. The transition 
from this to the more advanced cases, shown in Figs. 28, 34, 36, 
is obvious enough, consisting chiefly in the farther lateral 
extension and segmentation of the cells already represented 
in Fig. 35. As the lateral distension proceeds, the curvature 
of the septum cutting off the apical cell may increase, and 
a change take place to growth with a two-sided wedge-shaped 
apical cell, as in the young prothallus of most Ferns; this is 
seen clearly indicated by the darker drawn anticlinal lines in 
Fig. 37 ; but this growth with an apical cell is of only short 
duration, and both the segments and the apical cell itself 
undergo partition by periclinal and anticlinal walls, and pass 
over to the condition of permanent tissue. The apex is 
not unfrequently elongated as an irregular terminal process, 
which may be regarded as a continuation of the original fila- 
ment from which the expansion sprang (Fig.39) 1 . These ribbon- 
like prothalli are never more than one layer of cells in thickness, 
and such appendages as rhizoids and protonemal branches are 
always produced from their margins, the surfaces remaining 
smooth. It may therefore be concluded from their form, dis- 
tribution, and the arrangement of the cells composing them, 
that the development of the ribbon-like expansions is a simple 
variant on the filamentous protonema, and that its origin is 
based on a lateral dilatation of the simple filament. 
Gemmae . — A marked phenomenon in connection with these 
1 Compare also Mettenius, l.c. Taf. v. Figs. 1 a , g. 
