326 BOTANICAL GAZETTE [NOVEMBER 
The duration of swarming has been reported by various authors 
(15, p. 768), among them STRASBURGER, who found active swarm 
spores of Ulothrix zonata after three days, of Haematococcus lacustris 
after two weeks, during which the material had been kept continuously 
in darkness. ‘These figures have little significance beyond the fact 
that the plants produced motile zoospores during this time. The 
duration of swarming of the individual spores is quite unknown. On 
this point we can give little information. We found, however, that 
in our Oedogonium the zoospores came to rest and attached themselves 
within fifteen to eighteen hours, 7. e., zoospores which had escaped 
and were active in daylight came to rest and attached themselves 
during the succeeding night. 
The spot to which zoospores formed in water attach themselves is 
largely determined by the direction and intensity of the light, if there 
be any, which falls upon the culture. If light of not too great inten- 
sity fall upon a dish horizontally or nearly so, the zoospores will collect 
and come to rest mainly upon the more strongly illuminated side. If 
~ on the other hand the light fall vertically or only somewhat obliquely 
from above, the zoospores will collect at the surface of the water. 
It is frequently observed, when the light falls obliquely upon 4 culture 
of algae, that the zoospores collect in greatest numbers at the surface 
of the water on the more brightly lighted side. From this the attract: 
ive influence of the oxygen of the air might be inferred. That such 
an inference is not always justified is proved by the fact that when 
the light falls horizontally, or is reflected obliquely from below 278 
a culture, the zoospores still tend to collect at the point best | 
regardless of the greater oxygen supply near the surface of the walet- 
As to the directive influence of oxygen upon the : 
spores, we made no experiments, but it is clear from this Ke 
that in general the directive influence of light is stronger than ie 
of oxygen. When, therefore, zoospores come to the suriace 
water, they do so mainly under the influence of light. 
on reaching the surface; which is decidedly different from yr . 
zoospores coming to rest elsewhere, we shall presently descri oe . 
Germination.—When zoospores come to rest, the cilia e : walls, | : 
by cell-wa 
the naked masses of protoplasm surround themselves PY oe 
the forward end of each zoospore develops a holdfast at 
taching it! 
locomotion of 20° 
