THALLOPHYTES 



63 



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. 



Chjrtridiales. — 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 

 spores of Oedogonium, Spirogyra, Cladophora, 

 etc. In some species of Chytridium these resting 

 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. 



Synchylrium. — 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 irpportant family 

 (Saprolegniaceae) of the group, being aquatics whose body resembles a 



Fig. 154. — Chytridium : 

 attacking the oogonium 

 of Oedogonium. — After 

 Campbell. 



