54 DISEASES OF CROPS. 



takes place by the transfusion of the contents of the 

 antheridium into the oogonium. After fertilization, the 

 oogonium develops into an oospore (Fig. 23 E). The 

 outer coat of the oospore thickens and may become 

 ■■' spiny." The oospore hibernates for nearly a year, and 

 then gives rise to zoospores or germinates by throwing out 

 a hypha (Fig. 23 F and G). The zoospores and hyphae 

 from the oospore reproduce the disease in the next year's 

 crop, if the external surroundings are favourable for their 

 development and growth. 



The author has shown {Chemical News, vol. 53, p. 255) 

 that the spores, etc., of Peronospora infestans are capable 

 of being dried up in the dust of the atmosphere for several 

 months without losing their vitality. On the other hand, 

 if potato tubers infested with the mycelia of this fungus 

 are exposed to a temperature of —10° C. (14° F) for a few 

 weeks, they are completely destroyed. Peronospora in- 

 festans causes (as stated before) putrefactive changes to 

 occur within the tissues of the host-plant. The author 

 found lactic acid (in small quantities) in the leaves, etc., 

 of diseased potato plants. It may be that lactic acid is 

 formed from the decomposition of glucose contained in 

 the sap of the host-plant. From this it appears that 

 parasitic fungi may cause the abnormal development of 

 compounds within the living cells of infested plants ; and 

 thereby cause death by " poisoning " {Chemical News, 

 vol. 53, p. 255). 



Prevention. — (1) Sow only those varieties which are 

 hardy and have been proved to resist the attacks of P. 

 infestans. " Besides our potato-bearing Solanum tubero- 

 sum, there are, amongst the 700 species of Solanum, 

 about five which bear tubers— (S. Commersoni, Maglia, 

 immite, verrucosum, Jamesi—oi these, Solanum Commer- 



