GALL NUTS. 



227 



by the ovipositor of tlie insect^ and an egg is at the same 

 time deposited ; an interruption in the ordinary functions 

 of the tissues of the plant takes place at the spot where 

 the egg is inserted ; the consequence is, an excrescence of 

 vegetable matter, principally tannin, is formed round the 

 egg, and furnishes a nidus for the grub or larva when 

 hatched. When this takes place, the grub eats its way 

 out through the side of the gall, after which the vitality 

 of the excrescence either decreases or ceases altogether. 



Several varieties of galls are distinguished in commerce, 

 the principal of which are the blue and white ; the only dif- 

 ference is that the former are gathered before the insect has 

 escaped, and the latter after it has emerged. The colour of 

 the blue galls (Plate XIII. fig. 66) is a slaty-blue, and 

 sometimes a greyish- green ; the white gaU (Plate XIII. 

 fig. 67) is of a light drab colour and much lighter in 

 weight; it is also less valuable than the blue variety. 

 Nut-galls are nearly round, with a few small excrescences 

 over their surface. They yield a fine black colour, with 

 any of the salts of iron, and are used in the preparation of 

 writing-ink; the quantity imported is annually about 700 

 tons. 



A kind of gall has lately been imported from China; Dr. 



