THALLOPHYTES 
habit is established and no motile reproductive cells are produced. 
However, the two groups cannot be separated rigidly upon this basis, 
for the aquatic habit with zoospores gradually merges into the aerial 
habit without zoospores. If this order of succession is true, it is an inter- 
esting illustration of the derivation of isogamy from heterogamy, which 
would mean a line of degeneracy so far as the apparent sexual apparatus 
is concerned. Illustrations of Phycomycetes may be selected from three 
important groups. 
Chytridiales. These are regarded as the simplest of the Phycomy- 
cetes, many of them being aquatic and parasitic on algae, and others 
attacking seed plants. Two of the prominent genera are as follows : 
Chytridium. A species of this genus which attacks Oedogonium 
may be used as an illustration. The zoospore has one cilium, and settling 
upon an oogonium sends a tube through to the 
egg on which it feeds. The external region of 
the parasite grows bulbous and functions as a 
sporangium, discharging zoospores which attack 
other plants (fig. 154). When the oospore of 
Oedogonium is formed, the fungus develops within 
it thick-walled resting cells ; and upon the 
germination of the oospore, these resting cells 
put out tubes that produce terminal sporangia, 
and the infection of the oogonia of Oedogonium 
begins again. These resting cells of Chytridium 
are very commonly seen in the sexually formed p IG- 154 Chytridium: 
spores of Oedogonium, Spirogyra, Cladophora, attacking the oogonium 
etc. In some species of Chytridium these resting of Oedogonium. After 
cells are said to be formed sexually ; and in 
another genus of Chytridiales there are antheridia and oogonia, which 
fuse and form the resting cell, which in that case is an oospore. 
Synchytrium. This parasite attacks the epidermal cells of many seed plants, 
the uniciliate zoospores moving over the surface of young epidermis and entering 
the cells. In the young epidermal cell the zoospore grows as a naked protoplast, 
inciting the host cell to an unusual growth until it forms a blister-like pustule, 
distorting the adjacent tissue. Finally the protoplast develops a wall and be- 
comes a resting cell, which in the next season sends out a swarm of zoospores. 
No gametes are known. 
Saprolegniales. The water molds are the most important family 
(Saprolegniaceae) of the group, being aquatics whose body resembles a 
