OR, PLAIN TEACHING. 



200 



Various parts of the oak are 

 subject to attacks of insects. 

 Among these, the most remark- 

 able are gall flies, 210, so named 

 from the excrescences which they 

 produce upon the leaves and 

 stems, &c, and which are de- 

 signed to protect the delicate 

 ova and larvce of certain insects. 



Entomologists say that the 

 tumours on the leaf-stalks, and 

 those on the fruit-stalks, are pro- 

 duced by different species of 

 cynipidce; that the galls on the 

 branch are produced by a dis- 

 tinct fly from that which pro- 

 duces the galls on the leaf. The 

 largest species of British galls 

 is generally called the oak apple, 

 or oak sponge, 19. These oak 

 apples are much sought after 

 19 annually on 



the 29th of 

 May, usu- 

 ally called, 

 "Oak-day," 

 the anni- 

 versary of 

 the resto- 

 ration of 

 Charles II. 

 579. It is a 



remarkable circumstance, that 

 although some of these galls are 

 very large, only a single insect 

 is enclosed therein. The grub 

 lives for a time upon the sub- 

 stance of the gall, and then makes 

 its escape through a passage 

 which it eats to the surface, 20. 



Besides the oak apple, and 

 that species styled the " gall 

 nut," there are several other 

 excrescences observable upon the 

 o^k. The small round currant 

 gall 9 1, is produced by a/^/of a 



greyish colour, the wings being 

 marked with an elongated cross. 



580. 



581. 



582. 



The leaf galls, 2, are also pro- 

 duced by a similar insect. 



There is a description of gall 

 called the arti- 

 choke gall, 3, 

 produced also 

 by another 

 insect,* and 

 which arises 

 from the in- 

 sect's punctu- 

 ring the leaf- 

 buds, so that 

 the leaves, 

 instead of being developed, are 

 transformed into the small scales 

 composing the artichoke gall. 



The substance from -which pure tannin is 

 most frequently obtained for chemical pur- 

 poses, is vutgalls. It may be procured also from 

 several other sources, such as the bark of the 

 oak, chestnut, sumach, &c. 



The basis of the skins of animals is composed 

 of a substance to which the name of ge'atine 

 is given. One of the properties of this substance 

 Ls, that when combined with tannin, it forms 

 the compound of tannate of gelatine, or leather. 

 It is found that, for purposes of tanning, there 

 is nothing superior to good oak bark. 



The manner in which galls are formed by the 

 insects may be thus described :— The parent 

 insect punctures the vegetable tissue, and 

 thereby causes the shoot or bud to swell, and 

 become an excrescence in which the lai va, or 

 grub, is developed. When the insect has eaten 

 its way out, the gall has lost much of its value. 

 The best galls, known in commerce as black or 

 blue galls, from their bluish tint, are gathered 



* Loudun'u Encyclopaedia of Trees and Shruha 



