120 DISEASE IN PLANTS. 



seriously affected e.g. the roots are rotten and 

 cannot absorb water with dissolved minerals and 

 pass it up to the shoot, or all the leaves are 

 infested with a parasite and cannot supply the 

 rest of the plant with organic food materials, in 

 consequence of which parts not directly affected by 

 any malady become starved, dried-up, or poisoned 

 or otherwise injured by the results or products of 

 disease elsewhere. 



In a large number of cases, however, the dis- 

 ease is purely local, and never extends into the 

 rest of the organs or tissues e.g. when an insect 

 pierces a leaf at some minute point with its pro- 

 boscis or its ovipositor, killing a few cells and 

 irritating those around so that they grow and 

 divide more rapidly than the rest of the leaf 

 tissues and produce a swollen hump of tissue, or 

 gall; or when a knife-cut wounds the cambium, 

 which forthwith begins to cover up the dead cells 

 with a similarly rapid growth of cells, the callus. 

 Numerous minute spots due to fungi on leaves, 

 cortex, etc., are further cases in point, the mycelium 

 never extending far from the centre of infection. 



Many attempts have been made to classify 

 diseases on a basis which assumes the essential 

 distinction of the above cases, and we read of 

 diseases of the various organs root-diseases, 

 stem-diseases, leaf-diseases, and so forth ; or of 

 the various tissues timber-diseases, diseases of 

 the cambium, of the bark, of the parenchyma, 

 and so on. Furthermore, attempts have been 

 made to speak of general functional disease, of 



