ON A FORM OF SCAB IN POTATOES. 39 



the small-pox. The quality of the potato itself is little altered, 

 except that it acquires sometimes a slightly earthy flavour, if 

 indeed that is really attributable to the disease ; and, according 

 to the virulence of the disease, tliere is a slight difference in the 

 quantity of fecula contained in the tubers. In this stage of 

 growth it is not uncommon to find some species of Poduridse 

 revelling in the scurfy cavity of the pustules, and threads of 

 a yellow-brown mycelium, wliicii is prol)ably that of Botrytis 

 viilgaris or some nearly allied species, traversing it, but not 

 penetrating far beneath the surface. These, however, are 

 merely accidental and secondary phenomena in the malady. 



By the time tliat the cuticle of the tubers has become well 

 fixed, the spots, which are still more rough and excavated, often 

 present a very unsightly appearance, but in every case, however 

 complicated, a tissue very much resembling that of the cuticle is 

 presented under the microscope ; for, though the pustule has 

 been excavated perhaps a line below the surface, and the ori- 

 ginal cuticle is seen surrounding the pustule, while the continuity 

 between it and the cavity of the pustule which existed for some 

 time after the appearance of the disease is quite dissolved, the 

 whole of the diseased surface exhibits a muriform arrangement 

 of the cells, those of all the strata of which it is composed being 

 for the most part coterminous ; the individual cells are, however, 

 far larger and coarser. The manner in \\ Inch these cells ori- 

 ginate is very obscure, whether from a modification of the more 

 superficial cells beneath the cuticle or from a hypertrophy of 

 those of the cuticle itself: I am inclined, however, to the latter 

 view, though it is not without difiiculty, and the analogous case 

 of cracked potatoes, which I was enabled to study, more partially 

 indeed, at the same time, has made the matter at least doubtful. 



After the summer drought and the subsequent rains, a crop of 

 very smooth-skinned potatoes was much cracked from the in- 

 ability of the tubers to accommodate the sudden increase of 

 moisture. I was surprised to find, on examination, that these 

 exhibited phases very similar to those of the scabby crop ; the 

 tissue beneath the cracks was transparent at first when cut, and 

 nearly void of fecula, and became more rapidly rusty than the 

 other parts of the tubers, and, what is of more importance, the 

 disc of the fissures exhibited under the microscope the same muri- 

 form tissue, though here also the connexion between that and 

 the cuticle was dissolved. I had no opportunity of ascertaining 

 the condition of the tissue when the fissures first took place, 

 which would probably have shown at once of which of the two 

 sets of cells, the cuticularor sub-cuticular, it was a modification. 

 In this instance I observed something which may perhaps indi- 

 cate the mode in which the muriform tissue is formed. It will 



