220 AGRICULTURAL EXPERIMENT STATION. [Jan. 



is wholly one of kind, the latter developing from the 

 former by the simple extension of the pathological condition 

 described, over a greater surface. In this way is produced 

 what may be described as the superficial form of the disease, 

 illustrated by the lower specimen in Fig. 1 , opposite page 

 136 of our report for 1888, and by Fig. 1, accompanying 

 the present paper. The drying and browning sometimes 

 penetrates to a considerable depth, and causes the death 

 of masses of tissue of some volume, which finally become 

 destroyed by decay, frequently with the assistance of worms 

 and other animals. Their presence in this form of the 

 disease has apparently led to the belief, held by many 

 persons, that such animals are the cause of the trouble. 

 This may be called the deep form of the scab, and shows, in 

 its completest development, extensive cavities in the tubers, 

 where tissue has died and decayed. It is illustrated by the 

 upper specimen in Fig. 1 of last year's report, and by the 

 accompanying Fig. 2. Both forms of the disease coexist 

 under various conditions to such a deo:ree that the causes 

 determining the development of the deep form are wholly 

 indefinable. 



Very careful examinations were made, to determine 

 whether the present disease is caused by any plant or 

 animal, either as a true parasite or otherwise ; but no 

 organism of any sort was found constantly or even frequently 

 present at any stage of its progress, and there can be no 

 doubt that it is 7iot the result of the activity or development 

 of any living thing other than the potato plant. Various 

 experiments, referred to in the paper in last year's report, 

 above mentioned, have pointed to this conclusion, and their 

 results would be very puzzling had the present investigations 

 resulted otherwise. The search for some organism standing 

 in causal relation to the trouble, has, however, been con- 

 ducted with much care, in deference to the claims and theory 

 of Bruuchorst, quoted above, and to be discussed later. 



Since the scientific name of an organism indicates always 

 a definite and determinable thing, one can always be sure, 

 in the study of a disease plainly caused by a plant or animal, 

 as to the validity of his comparisons of his results with those 

 of others who have studied the same disease. But the 



