40 



ON A FORM OF SCAB IN POTATOES. 



be seen in fig. 4 that between two contiguous radiating series of 

 cells a little wedge of cells has been interposed, of which the 

 broad end consists of double, the narrower of single cells. This 

 process repeated would clearly produce a tissue like that in 

 question. 



Fi2. 1. 



Fig. 1. Vertical section of a young pustule, showing that the structure of the pustule is the 

 same as that of the skin of the tuber. 



Fig. 2- Vertical section of an old pustule, a. Skin of tuber; 6. Cells immediately be- 

 neath the skin, containing colouring matter; c. Cells at the base of the pustule ; d Starch 

 cells. 



Fig. 3. Vertical section of a cracked tuber through the centre of a fissure. 



Fig. 4. Portion from the tissue at base of a crack more highly magnified. 



In the case of cracking, the evil is evidently mechanical ; and 

 as the phases, as far as they have been observed, are so ex- 

 tremely like tliose of the disease we have been considering, I am 

 inclined to think that tliat is in great measure mechanical also. 

 The first decay of the tissue, which is confined to a mere speck, 

 is probably caused by contact with some irritating particle in 

 the soil, and when this is established, the corrosion of minute 

 insects and the exposure of the tissue to external agents, accom- 

 panied by a constant effort to repair the damage, may be suffi- 

 cient to account for it, for the disease is absolutely confined to 

 the surface, and never penetrates to such a depth as to cause any 

 general disarrangement of the tissues. 



If such be a correct view of the case, which is 'submitted to 

 further investigation, the remedy will consist in avoiding all 



