90 DISEASES OF FIELD & GARDEN CROPS. [OH. 



rust fungus is carried from one leaf to another, from one 

 plant to its neighbouring plant, and from weeds to food- 

 plants, in damp, rainy, or misty weather, by the microscopic 

 zoospores sailing about over moist surfaces. There can 

 be no doubt that they are also carried about in damp air, 

 in currents of wind, and that birds, insects, and other 

 animals help to carry the living conidia and zoospores 

 from place to place. 



As a rule conidia, zoospores, germ tubes, and fungus 

 spawn are very liable to perish ; too much dryness, a 

 superabundance of moisture or frost, will quickly destroy 

 them. 



Three questions now present themselves to us How 

 does the white-rust fungus tide over the winter ? Where 

 is it hidden ? How does it suddenly reappear in the 

 spring ? In the case of many plant diseases, as in the 

 surface mildew of turnips, Oidium Balsamii, Mont., already 

 described, no one is able to answer such questions ; but 

 with the white-rust fungus and several of its allies the 

 knowledge has been obtained, and a satisfactory answer 

 can be given. It has been already stated that the spawn 

 or mycelium of Cystopus grows within the leaves and stems 

 and burrows amongst the intercellular spaces of the host 

 plant. It not only bears the chains of spores already de- 

 scribed, which, when ripe, are blown away by the wind, 

 but it carries other bodies within the substance of the leaf. 

 These latter organs roughly answer to the pistils and anthers 

 of flowering plants. The first bodies are female, and are 

 termed oogonia ; these are large globular cells in which 

 the female reproductive bodies, or oospheres, or sometimes 

 zoospores, are formed. They generally grow on terminal 

 branches of the mycelium ; sometimes they are sessile 

 or nearly so, or they may be intercalated in the my- 

 celium itself. An oogonium is illustrated, enlarged 400 

 diameters, at A, Fig. 33. The oosphere, filled with 

 granular protoplasm or vital formative material, is seen 

 within. Other organs borne on the mycelium are male, 



