Corky warts. - Several forms of disease are known in which the pathological condition is expressed by the formation of cork in 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 - e.g. 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.
Cork-wings are well known on the young branches of Elms, Maples, etc., some varieties of which have received specific names on this account.
Lenticels are also formed on some leaf-galls, and are remarkable as being structures not normal on leaves.