220 DISEASE IN PLANTS. 



the roots are of benefit to the plant. Urocystis 

 Leimbachii forms similar nodules at the collar of 

 young plants of Adonis. 



Heterodora javanica passes into the cortex of 

 sugar-cane roots through fissures, and makes its 

 way to the place where a young rootlet is about to 

 emerge ; here it sticks its beak into the growing- 

 point and remains fixed. 



Molliard has shown that in the roots of Melons, 

 Coleus, etc., Heterodora causes the cells in immediate 

 contact with its head, and which would normally 

 become vessels of the xylem, to swell up into huge 

 giant-cells, with their walls curiously folded, and 

 containing large supplies of proteids and numerous 

 nuclei, reminding us of the food-layer of insect 

 galls and of the tapetal layer of pollen-sacs. 

 While the stimulus exerted by the Nematode 

 thus induces hypertrophy and storage with food- 

 substances of these cells, those of the next 

 layers undergo reticulate thickenings of their walls. 

 Again instances of the evolution of new tissue 

 elements by the action of the foreign organism. 



So far" as galls on leav^es are concerned the 

 amount and kind of damage done are in pro- 

 portion to the area of chlorophyll action put out 

 of play for the benefit of the plant, and the 

 remarks already made on p. 193 apply here also. 

 Where buds are destroyed the effects may of 

 course extend further, but it rarely happens that 

 leaf-galls are so abundant as to maim a tree per- 

 manently. Nevertheless we must remember that 

 cases like Phvlloxera are notorious. 



