MICROSCOPIC OBJECTS. 31 



The real cause of this disease it is proposed now to 

 describe. Just before it made its first appearance much 

 rain had fallen, which was succeeded by cold frosty nights. 

 The effect of this weather upon the tuber of the potato 

 would be in the first place to saturate with water and 

 distend to the utmost the numerous cellules of which the 

 tuber consists. Intensely cold nights succeeding, this 

 water m the cellules would become fi'ozen ; during which 

 process it would expand, and the membrane inclosing it, 

 having, as before said, previously become distended to 

 the utmost, would burst; and the death of the tuber 

 would immediately result. This dead matter would then 

 be in a fit state and situation for developing the growth 

 of any fungi whose spores (seeds) might be present. 

 Many species of fimgi, it is well known, require but a 

 few hours to pass through all their stages fi'om sporules 

 to maturity and the production of innumerable sporules 

 from a single one. Hence there is no difficulty in 

 accounting for the rapid progress of this disease. 



It must be observed that the disease now found in the 

 tuber was not the cause but the effect. The almost 

 infinite number of spores of these fungi, which were 

 during the first year produced, and which exist in every 

 locality that has had a diseased potato in it, renders it 

 difficult to plant potatoes in those situations without the 

 nidus for the development of this fungus. It foUows, 

 therefore, that the only means for preventing the spread 

 of this evil would be to sow in new situations, in which the 

 disease has never appeared, perfectly sound seed potatoes, 



