84 EVOLUTION OF PLANTS 
botanist Thaxter, has large multiciliate zodspores 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 
“whiterust” (Cystopus 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 zodspores 
much like those of the water-moulds (Fig: 21, H). © 
These zodspores 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, h) 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 zodspores. This ordi- 
narily takes place when the leaves are wet with rain 
or heavy dew. Odgonia and antheridia are formed in 
