DISEASES THE "CURL;" "DRY-ROT." 53 



very weak ones, which soon fall off and yield no seed. 

 They produce no tubers, or only a few minute ones, which 

 are hard and unfit for food. These, however, when set, do not 

 always produce plants infected with disease. " Our own high 

 authority on vegetable pathology, the Rev. M. J. Berkeley, 

 tells us that the contraction of the leaves is probably caused 

 by the parenchyma (fleshy part) of the centre of the leaf 

 being multiplied, while that towards the margin remains 

 stationary : this would at once cause that peculiar appear- 

 ance at their edges from which the name "curl'* has been 

 applied to the disease. The opinions expressed in refer- 

 ence to the cause of the disease are various, and differ 

 widely from each other, some attributing it to the use of 

 over-ripe "sets," others, again, to the use of "sets" not 

 fully matured. Some consider poverty of the soil as the 

 determining influence, while the general high condition 

 of our potato land is by others considered as conducive to 

 the disease. The disease appears clearly to be a case of 

 hypertrophy (or excess of nutriment) ; and if so, it is not 

 difficult to understand why sets fully matured, and abun- 

 dant in natural nutriment, should, when planted in a 

 richly manured soil, give rise to" such an excess in develop- 

 ment of fleshy tissues as to hinder the proper expansion 

 of the plant. It would therefore appear advisable to 

 follow the recommendation given at p. 525, vol. i., and use 

 " sets" that are not fully mature, or that have been greened 

 by exposure to the light before planting. In all oases it is 

 advisable to avoid the use of " sets" the produce of plants 

 in which the particular disease, even in its mildest form, 

 had been observed. 



The " dry rot" has been more noticed in the potato 

 crops of the Continent than in this country. It has 

 been specially studied by Martius, who describes it as 

 being " characterized by a hardening of the tissues, which 

 are completely gorged with mycelium, which was often 



