354 THALLOPHYTES 
is no sharp line of distinction between parasites and saprophytes. 
As long as the host is living, the Fungus is a parasite, but upon 
the death of the host it becomes a saprophyte. Thus a Fungus 
may be a parasite at one time in its life and a saprophyte at 
another. Saprolegnia is usually obtained for study by throwing 
dead insects or pieces of beefsteak into stagnant water from a 
pond, where the objects usually become infected and soon look 
like the fly in Figure 304. 
The mycelium of Saprolegnia is composed of many branched 
hyphae which extend throughout the tissues of the host. The 
hyphae are coenocytes. This coenocytic feature suggests a closer 
kinship to Vaucheria than to the other Green Algae. After the 
mycelium is well established in the host, numerous hyphae, which 
cause the fuzzy appearance, protrude from the surface of the 
host. 
The hyphae within the tissues of the host are able to absorb 
food materials directly. They are also able by means of enzymes 
to change materials to soluble forms, and in this way the Water 
Molds bring about the decay of animal bodies in water. 
Many of the hyphae protruding from the host become modified 
for reproduction. Some produce zoéspores, while others produce 
sex organs. The swollen tips of some of the protruding hyphae 
are cut off by a cross wall and form sporangia in which are pro- 
duced numerous zoéspores. These zodspores escape, swim about, 
and when in contact with another host produce hyphae that 
penetrate and infect the new host. 
Oogonia and antheridia are also formed at the ends of hyphae. 
The odgonia are spherical and form one and sometimes many 
eggs. The antheridia are formed on branches near the odgonia. 
The antheridium comes in contact with the odgonium and pierces 
its wall with a small tube through which the sperms from the 
antheridium pass and fertilize the eggs. As a result of fertiliza- 
tion, a heavy-walled oéspore is formed, which after rest grows 
into a hypha which can penetrate and infect a host. 
A peculiar feature in connection with some of the Saprolegnias 
is the ability of their eggs to develop without fertilization. In 
most plants, unless the egg is fertilized, it will not develop, but 
will soon disintegrate and disappear. In some Saprolegnias. the 
sperms of the antheridium fail to enter the odgonium, or there 
may be no antheridium developed, and still the egg without 
