1877.] Reproduction in Fresh- Water Alge. 517 
an adjoining cell, blends with it, and the contents of one cell 
pass over into the other. Two filaments can frequently be found 
lying nearly parallel, the cells of the one filament conjugating 
with those of the other throughout the whole length. In conju- 
gation the deep green spiral bands are therefore destroyed, and - 
in their place the dark brown spores are produced; so that in 
a floating mass of Spirogyra, when this process takes place, the 
beautiful green is lost and a pale, sickly, dirty material is seen in 
its stead, appearing as if dead when in reality only preparing it- 
self to live over until another spring. Germination takes place 
by the contents of the spore pushing out into a filament, as shown 
in Figure 87, d, e, and f, drawn after a plate by Pringsheim, a 
noted cryptogamic botanist. 
In the genus Zygnema, quite closely related to the Spirogyra, 
the spores often do not form in either of the cells, but remain 
in the enlarged centre of the i =~ 
uniting tube, leaving both fila- @@ Ze À 
ments empty and giving a lad- à 
der-like appearance to the whole 
affair. Again, in other closely 
related genera, conjugation takes 
place between the adjoining cells 
of the same filaments by differ- 
ent methods in different species. 
In some a small tube is thrown 
out from each of the two cells, 
which meet and forma passage 
for the transfer of the contents O 
of one cell to that of the other. os 
The Nostocs grow in water, but are more commonly found on 
damp ground as gelatinous spherical masses or colonies (Figure 
88, a, a). When a small portion of one of these slippery balls is 
placed under the microscope it is seen to consist of a multitude 
of filaments, resembling strings of beads, imbedded in an amor- 
Phous jelly (4). In these strings there are at irregular distances 
Certain cells, larger than the others, ealled heterocysts. In speak- 
ing of the method of reproduction of these plants we cannot do 
r than employ the language of Thurets, who has made a 
thorough study of them. He says: “ The jelly of the old colony 
mes softened by water, the portions of the threads lying be- 
tween the heteroeysts become detached, separate from the jelly 
- Straighten, while the heteroeysts themselves remain in the. 
