I40 DISEASE I\ I'EANTS. 



of Notommata, which induces a different gall on 

 each of the various species of its host plants. 



It must be concluded that the formation of the 

 Vaiicheria gall is induced by the mechanical 

 irritation which the Rotifer causes in the proto- 

 plasm. These galls are comparable to the hyper- 

 trophies in Pilobolus caused by the presence of 

 Pleotr'achelus. 



Attempts to induce the development of galls 

 artificially by injecting formic, acetic and other 

 vegetable acids, poisons and other substances into 

 the tissues have, however, failed, and even the 

 substances contained in the insect or gall itself 

 only produced negative results. Nothing further 

 was obtained than slight callus formations in some 

 cases. Nor have experimenters succeeded in 

 obtaining more than slight distortions by fixing 

 insects on the growing leaves in such positions 

 that they could scratch the epidermis. 



We must therefore conclude that very com- 

 plex interactions between the plant and insect are 

 here concerned, among which may be the infiltra- 

 tion of some liquid from larva to plant many 

 of these gall larvae are strongly scented, and Kus- 

 tenmacher says that fluids excreted by the larva 

 are absorbed by the gall-tissue apparently as 

 nutriment. This would point to the symbiotic 

 character of galls and their guests. 



Notes to Chapter XIW 



With regard to the action of poisons in small closes see 

 further Johannsen, Das Aether- Verfahren hcini Fruhtreiben. 



