THALLOPHYTES FUNGI 275 



which, cause diseases of plants, of which the most common are 

 pear blight, black rot of cabbage, wilt of cucumber, wilt of to- 

 bacco and related plants, crown gall of fruit trees and many 

 other plants. However, it should be remembered that some of 

 them are very useful ; among the most important of the useful 

 species is* Pseudomonas radicicola, which lives in tubercles on 

 the roots of leguminous plants and fixes atmospheric nitrogen 

 in such a form that it can be used by growing plants (see page 

 204). The study of these forms is of such great importance that 

 it is now generally recognized as. an independent subject known 

 as bacteriology (page 250). It will be readily seen that it is 

 very closely associated with the study of medicine, plant path- 

 ology, soil fertility and with some of the industries. 



The Phycomycetes are most nearly like the algae in both struc- 

 ture and reproduction. Some of them produce free swimming 

 zoospores, but this feature is not nearly so common as in the algse. 

 They may be divided into the Oomycetes and the Zygomycetes. 



The Oomycetes are heterogamous, usually aquatic species, 

 and produce zoospores. Some of the most common and important 

 genera are 8dprolegnia } Albugo and Plasm&para,. 



The Saprolegniales (Fig. 125) are the water molds. They 

 live on dead insects and other small animals and also on live fish 

 and frogs. They are sometimes very destructive in the fish hatch- 

 eries. The filaments project from the body of the host and ap- 

 pear very much like a mass of cotton. The apical cells develop 

 into sporangia which produce great numbers of biciliate zoo- 

 spores. The oogonia are terminal and spherical and bear from 

 one to several oospheres. The antheridia are tubular and may 

 rise fiom the same or another filament ; they come in contact 

 with the oogonia and send out small tubes, which penetrate 

 the oogonial wall. The contents of the antheridium are 

 discharged into the oogonium and fertilization results in the 

 formation of thick-walled oospores which germinate by the 

 formation of new hyphse. 



