CLASSIFICATION OF THE TRUE FUNGI. 



113 



has only been present when there was a large amount of 

 moisture. From the end of June brown spots appear on the 

 leaves, the under surface of which shows the mould-like 

 border of conidia bearers ; this is soon followed by the death 

 of the plant. Decom- 

 position of the tubers 

 then takes place; 

 dirty brown patches 

 indicate the develop- 

 ment of the my- 

 celium. On the dead 

 tubers two forms of 

 mould fungi fre- 

 quently develop : 

 Fusisporium solani 

 and Acrostalagmus 

 cinnabarinus, which 

 however have nothing 

 to do with the disease. 

 The infective fungus 

 lives through the 

 winter in the tubers, 

 reaches the field with 

 the seed, and deve- 

 lops by preference 

 during extreme 

 moisture ; it is only 

 young parts with 

 delicate membranes 

 which permit the 

 entrance of the ger- 



Fig. 4. Peronospora infestans. 

 (After de Bary.) 



A, Young twig of the fungus. 



B, Formation of swarming spores. 



(7, Swarming spores which have bored through 

 the epidermis of a potato stalk. 



D. 



a. The conidia (c) forms a secondary one (s) . 



b. Sprouting of conidia. 



minating tubes. 

 Disinfection experi- 

 ments have as yet been without result; but the local pre- 

 disposition may be influenced by avoidance of moisture, the 

 individual predisposition by the selection of resistant, tough- 

 walled kinds of potatoes, and the seasonal predisposition by 

 preserving the seed in a dry state and sowing late, a slow deve- 

 lopment of the fungus and a rapid growth of the potato being 

 obtained in all these ways. Other kinds of peronospora occur 

 on leguminous plants clover, vines, leaves of beet-root, &c. 



4. Pyrenomycetes (order : Ascomycetes). They Jive 

 partly as saprophytes and partly as parasites, on plants or 

 insects. There are usually two modes of fructification : 

 conidia and ascospores, the latter formed in perithecia. 



Claviceps purpurea, the fungus of ergot, occurs in the fruit 



8 



