38 ON A FOKM or SCAB IN POTATOES. 



where these form a considerable paVt of the manure in old tilths, 

 for it is by no means confined to new ground, but is to be found 

 in a greater or less proportion in most crops, in some instances 

 every individual tuber being attacked, in others the scabby tubers 

 making the exception to the smooth and healthy appearance of 

 the sample. 



It commences at a very early stage in the growth of the tubers, 

 whether in those produced immediately from the sets or those 

 wliich often make tlieir appearance with the first heavy 

 rain after a long-contiimed drought, I have seen during the 

 l^resent autumn potatoes already attacked wliich did not exceed 

 a quarter of an inch in diameter, and it is on these young tubers 

 only that the early stage of the disease can be studied. Its first 

 appearance is that of a minute brown speck, paler on the edge, 

 and staining the subjacent cells of the cuticle, which, as is well 

 known, consists of a variable number of layers of muriform 

 tissue, and which should seem rather to be considered as the 

 outer jDortion of the bark of the underground stem than as of the 

 nature of epidermis. It is thei-efore only in a 23opular sense 

 that it is here termed cuticle. The discolouration does not pro- 

 ceed beyond the cuticular cells, nor is the cuticle at all 

 thickened, but is occasionally more or less corroded in the 

 centre. Even in this early stage of growth, when the sur- 

 face of the cuticle is still but little injured, and tlie disease 

 confined to a ditaolouration of a few of the cells of which it is 

 composed, on th-:^ removal of the outer coat little pale specks are 

 visible on the smooth subjacent surface, indicating precisely the 

 situation of the incipient pustules. In the coui'se of a few days 

 the spots spread, become paler, and are abraded in the centre ; 

 and if the cuticle is now torn off, it will be found that where the 

 spots are situated it is protruded into the substance of tlie tuber, 

 the subjacent cells being at the same time absorbed, so tliat the 

 exposed surface of tlie tuber is covered with little pits of a paler 

 colour than the rest, and more transparent, and, in red varieties, 

 distinguished by the absence of colouring matter. If a section 

 be made through the pits, tlie cells will be found to contain but 

 little fecula, and their walls to change shortly to a reddish brown. 

 The cells of the base of the pustule still form a portion of the 

 cuticle, which I ';.s, however, been more or less incrassated as the 

 outer layers of its tissue were corroded. 



The pustules now become confluent and deeply excavated, 

 their inner surface being more or less scaly and corroded. As 

 long as the cuticle is easily separable they separate with it, 

 whether fresh or boiled, so tliat the tubers, when served at table, 

 appear shining and smooth if the cuticle is nicely jieeled, the 

 diseased spots being indicated by scars resembling those left by 



