NATURE OF DISEASE. 139 



by no means necessary that the tissues should be 

 wounded. Moreover, the egg as such is incapable 

 of stimulating the plant tissues, but when it 

 hatches, the resulting larva, beginning to feed 

 on the cells, irritates the tissues and rapid 

 growth and cell-division occur, as in the case of 

 other wounds or of fungus attacks. The actual 

 wound made by the ovipositor heals up at once. 

 It is evident from numerous recent researches 

 that these true galls are not due to any poisonous 

 or irritating liquid injected by the parent, but that 

 the stimulus to the tissue formation is similar to 

 that exerted by a wound. The young gall is in 

 fact a callus enclosing the living larva, and it is 

 the continued irritation of the latter which keeps 

 up the stimulation. The final shape and consti- 

 tution of the gall depend on mutual reactions 

 not as yet explained in detail between the species 

 of plant and the species of gall-insect concerned, as 

 may readily be seen from the extraordinary varia- 

 tions in size, shape, colouring, hairiness and other 

 structural peculiarities of the galls on one species 

 of, for instance, the common oak. From what we 

 have learnt about fungus parasites, however, there 

 can be little doubt that reactions between the 

 cells and the larva of the insect occur, resembling 

 those which take place between the cells and the 

 hyphae of the fungus, and this is borne out by 

 the study of other hypertrophies due to animals ; 

 (?.^. Nematode worms in roots, and the remark- 

 able galls the simplest known on Vaucheria, 

 caused by the entrance into this alga of a species 



