12 
Colorado Experiment Station 
formed. These motile spores are called swarm spores 
or zoospores. They swim about for a short while, 
then come to rest, round up and germinate as an 
ordinary spore. Swarm spores are commonly formed 
in a sack-like enlargement of the mycelium or spo- 
rangium. Swarm spores have a very thin wall or 
covering and cannot survive any unfavorable con- 
ditions. Other kinds of spores are produced, how- 
ever, which can resist very severe conditions and 
thus enable the fungi to live over winter and through 
periods of drought. These resistant or resting spores 
are usually formed by a sexual process (the fusion 
of two sexual cells, a male and female gamete) 
and are known as oospores. Oospores usually have 
a thick wall which, after living over the winter, 
breaks open to set free a number of zoospores 
similar to those 
formed in a spo- 
rangium. 
Such serious dis- 
eases as potato wart, 
late blight of po- 
tato and the many 
downy mildews are 
The formation of sexual spores in a Phycomycete. A — CaUSCd bv fuilgi of 
Young oogonium, o, and antheridium, n. B — Antheridium 
fertilizing oogonium. C — Fertilization completed. this ffrOUD 
(After de Bary.) *' ® 
Typical sporangia 
of the Phycomy- 
cetes. a — A spo- 
rangium before 
opening to free the 
spores. b — Zoo- 
spores escaping 
from the sporan- 
gium. Highly mag- 
nified. (After de 
Bary.) 
Asexual spores (conidiospores) of an 
Ascomycete. (After Tulasne. ) 
Two spore cases of an Ascomycete 
in a cross section, a — Asexual spore 
case (pycnidium). b — Sexual spore 
case (perithecium). (After Duggar.) 
