viii BRITISH GALLS 



certain infective forms of inflammation, and perhaps to 

 some forms of tuberculosis. It is impossible to exclude 

 them by any definition ; at the same time their compre- 

 hension under the name of " galls " will probably be 

 very inconvenient and productive of much confusion. 

 They should, I think, be allowed to constitute a separate 

 group, and with them should be placed the diseases 

 known in England under the name of " canker." 



A gall in its special but now well-recognized meaning 

 of the word may be defined as a growth on a plant or 

 tree caused by the deposit of an insect egg in process of 

 development, or by the presence of a fungus. The 

 special kind of gall produced will depend upon the 

 endowments of the part in which the irritation occurs, 

 and will vary not only with the kind of plant, but with 

 the minute details of vital endowment of the part. 

 Thus it will be influenced by the precise part of the 

 stem, leaf, or bud which is attacked. The conditions 

 which are favourable to galls of all kinds are active 

 vitality on the part of the plant, free supply of sap to 

 the special part affected, and seasonal conditions of 

 warmth. 



The Oak, of all trees, produces probably not only the 

 greatest variety of gall structures, but the greatest 

 abundance of individual forms. 



It must be acknowledged that there is much in refer- 

 ence to peculiarities in different galls which is as yet ill 

 understood. There is no doubt that the nature of the 

 gall produced is influenced not only, although chiefly, 

 by the special endowments of the plant attacked, but 

 also by the character of the irritation which attacks it. 



