84 EVOLUTION OF PLANTS 



botanist Thaxter, has large multiciliate zoospores much 

 like those of Vaucheria. 



Besides these aquatic alga-like fungi there are other 

 Phycomycetes which are not aquatic. These may be 

 either parasites upon the tissues of living plants, or they 

 may be saprophytes either upon animal or vegetable sub- 

 stances. Of the first a good example is the so-called 

 "white-rust" QCystopus candidus) which infests the 

 common shepherd's purse, Capsella, as well as other 

 cruciferous plants. The masses of spores form con- 

 spicuous chalky-white blisters upon the stem, leaves, and 

 flowers of the host, and the growth of the fungus also 

 causes great enlargement and distortion of the parts 

 attacked. The structure of the fungus is much like 

 that of the related water-moulds, and it betrays its 

 aquatic ancestry by the formation of ciliated zoospores 

 much like those of the water-moulds (Fig. 21, H). 

 These zoospores are formed when the spores germinate. 



The fungus lives within the body of the host plant, 

 occupying the intercellular spaces and sending into the 

 cells of the host little suckers (Fig. 21, E, K) by means of 

 which it feeds. Non-sexual spores are formed in rows 

 cut off from the free ends of branches, just below the 

 epidermis of the host. The epidermis is pushed out 

 by the growth of these chains of spores, forming the 

 blisters already referred to, and finally is ruptured and 

 the spores then are shaken off as a fine white powder. 

 Under proper conditions of temperature and moisture, 

 the contents of these spores divide into a number of 

 parts which escape as biciliate zoospores. This ordi- 

 narily takes place when the leaves are wet with rain 

 or heavy dew. Oogonia and antheridia are formed in 



