Potato Leaf-curl 



leaves, and small yield. The other, which is intensi- 

 fied by cUmatio conditions, is marked by the upward 

 inward rolling of the usually small discoloured 

 leaves, shorter, more erect haulms, small flowers and 

 berries and small yield of tubers which do not decay 

 when planted. In both cases tubers from affected 

 plants produce diseased offspring. Leaf-curl some- 

 what similar to the first but with less dwarfing is 

 constitutional in certain races of tomato, and occurs 

 in beets, though in the latter it may be of parasitic 

 origin. 



The nettle-leaf disease of black currants, in which 

 the foliage is narrower and more divided than is 

 normally the case, and the yield small or none, has 

 not so far been certainly traced to any other than 

 " constitutional " causes. Red currants and goose- 

 berries are sometimes similarly affected. 



The doubling of flowers, which is generally a 

 constitutional character, may also be looked upon 

 as a diseased condition, since it generally reduces or 

 destroys the power of the plant to set seed. It is 

 often regarded as an added beauty and preserved in 

 the garden, and is of course in no way detrimental 

 to the well-being of the individual although it is of 

 the race. For some reason not at all clear double- 

 flowered plants are at times more deUcate than the 

 corresponding single-flowered type, and the degree 

 of development of the doubling is dependent to a 

 large extent on the available food substances. 



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