2i6 DISEASE IX PLANTS. 



between transpiratory and assimilatory functions 

 of their leaves, as indicated by the less localised 

 occurrence and by their non-appearance when 

 the plant is under favourable cultural conditions. 

 Structures not unlike these have been artificially 

 induced by exposure to particular lights, and also 

 by painting spots with dilute corrosive sublimate, 

 indicating that poisons may impel the epidermis 

 cells to grow out abnormally. 



Corky zuarts. Several forms of disease are 

 known in which the pathological condition is 

 expressed by the formation of cork m unwonted 

 places and quantities. The Scab or Scurf of 

 Potatoes is a case in point. The tissue of the 

 lenticels absorbs water and the outermost cells 

 are cut off by cork and die : the cells below them 

 burst the dead bark-like masses thus formed, and 

 again cork is formed and cuts off the outer masses, 

 and the rough cork warts Scab or Scurf are 

 the result. 



The causes predisposing to scab have been 

 variously assigned to dampness, want of lime, 

 action of bacteria and fungi e.g. Sorosporium, 

 Oospora, Spongospora,- the latter making their 

 way into the ruptured tissue of the lenticels and 

 irritating the cells to further growth. 



It seems probable that several different kinds of 

 scab exist in Potatoes, as well as in roots eg. 

 Beets, and the whole subject needs further 

 investigation. The scab-like rough scaly bark of 

 Pear trees in dry districts may also be mentioned 

 here. 



