PRESENT DISEASE IN THE POTATOE. 31 



cases I have noticed the disease to commence in the new 

 tuber itself, where the potatoe has formed new tubers with- 

 out roots, stems, or leaves. 



(115.) When the tuber is affected, parts here and 

 there become soft, discolored, and rotten. Under certain 

 circumstances this altered material becomes dry, passing 

 into a state of dry gangrene ; in other cases it remains 

 soft, or in the state of moist gangrene. 



(116.) The disease does not always commence at the 

 external surface of the tuber, for it is frequently seen a short 

 distance inwards, forming an irregular circle of disease. 

 This fact was so well known, that I observed that pota- 

 toe salesmen in the low districts were accustomed last 

 winter to exhibit a cut potatoe, to show to the buyer its 

 general condition. 



(117.) The part decidedly most liable to disease is that 

 which lies between the bark and an irregular circle, which 

 a section of a potatoe always presents. This circle is 

 formed by a layer of spiral vessels, which contain air 

 having seven or eight per cent, more oxygen than atmo- 

 spheric air. (Plate vii., fig. 3.) 



(118.) At times the ravages of the gangrene are not 

 confined to this incipient spot, but the entire potatoe suffers 

 and becomes disorganized as the disease continues to ad- 

 vance. 



(119.) Occasionally, moreover, after the lapse of a longer 

 period, we find that the internal portion of the potatoe has 

 shrivelled into irregular masses, leaving a great cavity in 

 the centre, having somewhat the form of an X. (Plate 

 vii., fig 4.) 



(120.) Besides the general effects of the disease which 

 I have now enumerated, there are some others consequent 



