Uronema elongatum. 163 
In a cell about to produce a zoogonidium the chloroplast 
becomes rather indefinite, and always shows abundant scattered 
starch-grains, and (generally) two pyrenoids; the cell contents 
contracts usually towards the middle of the cell, and rounds itself 
off, and a somewhat elongated red eye-spot appears. The mass 
becomes closely applied to one side of the cell-wall, the latter at 
this spot often showing a slight outward bulge, preliminary to the 
softening and subsequent dissolution of the cellulose here to form 
the pore. The latter is generally somewhat narrower than the 
zoogonidium, which in consequence becomes slightly constricted 
in its passage through it. The zoogonidium (Fig. 7a) is similar in 
form to that of U. confervicolum, and is broadly ovate, about 13 /a 
long by 10/a wide, showing an elongated stigma in the anterior 
part, 4 cilia, one or two pyrenoids, and an irregular basin-shaped 
chloroplast. It remains quiescent for some seconds after its 
emergence, hut is not enclosed in a vesicle ; the cilia, however, 
soon become very active, and the zoogonidium swims away. 
In many cases it was observed that zoogonidia-formation in a 
filamint progressed more or less regularly from the apex of the 
latter downwards, as is often the case in Ulothrix hut, as in the 
latter genus, numerous exceptions to this rule occur. The cell-wall 
of the empty zoogonidangium sometimes—hut not commonly— 
exhibited curious thickenings, as shown in Fig. 11, the appearance 
recalling somewhat the structure of the cell-wall in Tribouema, 
although the filaments never become disarticulated as in this genus. 
Such thickenings of the cell-wall were very rarely observed in 
ordinary vegetative cells, and they are certainly not a normal 
feature. Aplanospores, such as are known to occur in U. confer - 
vicolum, have not been observed. 
On germination the zoogonidium loses its cilia, becomes fixed 
to the subtratum by its anterior end, and, after taking on a thin 
cell-wall, very soon elongates to the form shown in Fig. 8, the 
acuminate apex and attenuated base being established very early 
in the ontogeny of the filament. Fig. 9 shows a very young filament 
of two cells, while an older filament with a pronounced curvature 
of the apical cell—a not uncommon feature—is shown in Fig. 10. 
An attempt was made to grow the alga in dilute nutrient 
solution, and also in filtered water from the pond, but owing to 
the numerous associated filamentatous algae which outgrew the 
Uronema the latter did not survive long in the cultures, although it 
formed numerous zoogonidia under these conditions. 
1 Cf. Chodat, Algues vertes de la Suisse, 1902, p. 264. 
