LEGISLATION AND DISEASE 223 



of living mycelium present may suffice for continuing the 

 disease in the way indicated above. It is more than 

 probable that in many instances the rapid spread of disease 

 in a potato field, usually attributed to the conveyance of 

 spores from diseased to healthy plants by wind, rain, 

 movements of animals, etc., is in reality due to the fact 

 that slightly diseased tubers have been planted ; and that 

 under those atmospheric conditions so well known as 

 being favourable to the appearance of potato blight, the 

 disease already lurking in the haulm, and having originated 

 from the tuber, quickly manifests itself; whereas if such 

 favourable climatic conditions had not been forthcoming, 

 the fungus would have remained undeveloped in the 

 haulm, and the crop matured free from disease. When 

 slightly diseased tubers are grown for experimental pur- 

 poses, and exposed to those conditions known to favour 

 the appearance of the disease dull light, excess of moisture 

 in the air, and a fairly high temperature special care 

 being exercised to prevent external infection, the disease 

 almost invariably appears ; whereas if a second portion 

 of the same tubers used for the above experiment are 

 planted in the open air, and there is an absence of the 

 exceptional climatic conditions indicated above, no out- 

 ward trace of the disease may appear. However, if a few 

 of the leaves from the apparently healthy plants grown in 

 the open air are placed in damp air under a bell-jar in a 

 dull light, within two or three days the leaves will show a 

 copious development of potato blight, proving that the 

 fungus was present in the tissues, and only awaiting those 

 conditions favourable for its complete development. 



Phytophthora has only one form of reproduction by 

 means of spores, and these are produced only on the living 



