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 zoospores, 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 zoospores. These zoospores 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 oogonia are spherical and form one and sometimes many 

 eggs. The antheridia are formed on branches near the oogonia. 

 The antheridium comes in contact with the oogonium 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 oospore is formed, which after rest grows 

 into a hypha which can penetrate and infect a host. 



A pecuhar feature in connection with some of the Saprolegnias 

 is the abihty 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 oogonium, or there 

 may be no antheridium developed, and still the egg without 



