CHAPTER III. 



On the Formation of Galls by Gall-Flies. 



In the description of the oak gall-flies given in the 

 last chapter, the importance of an accurate examination 

 of the gall is repeatedly enforced. The study of the 

 flies must begin with the galls, they give in all cases 

 the best, and often the only, means of distinguishing 

 closely related species ; and lastly they play a most 

 important part in the economy of each species, because 

 they serve as the abode of the individual, as larva 

 and imago, for the greater portion of its life. I will 

 therefore endeavour to give a general idea of the 

 formation of galls. 



In spite of great differences in form, manner of 

 formation, size, and appearance, galls can all be 

 referred to a common origin ; whether they grow 

 from the bud, leaves, bark, or roots, the parent 

 tissue from which they spring has always the same 

 physiological character. This invariably belongs to 

 that zone of formative cells which is called the 

 cambium-ring ; a layer which, beginning from the 

 finest root fibres, and extending to the most distai'rt 

 leaves, wraps the entire plant in a tight-fitting garment. 

 The whole vegetable life proceeds from the cells of the 



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