DISEASES OF PLANTS. 335 



attack of insects, and consists of a number of leafy imbricated scales 

 resembling a young cone. On examining the galls of commerce, the 

 produce of the Quercus infectoria, some are of a blue colour, containing 

 the larva of the insect; others are pale, and are marked with a perfora- 

 tion by which the insect has escaped. Extensive ravages are committed 

 in Elms and other trees by the attacks of Scolyti. The presence of 

 much moisture, such as the rapid flow of sap, destroys them. Mr. 

 Robert found that the flow might be promoted by taking off" the sub- 

 erous layer of the bark, and he proposes this as a method of getting rid 

 of the insects. Some galls are formed in the substance of leaves, and burst 

 through the cuticle in the form of ovate bodies, with crenate borders 

 and opercula, which are perforated in the centre. These galls resemble 

 parasitic fungi. Oak-spangles are galls of this nature. They are at- 

 tached by a central point to the under surface of the leaf, the inner 

 side being smooth the outer red, hairy, and fringed. Each contains 

 a single insect, which retains its habitation till March, long after the 

 leaves have fallen to the ground. 



702. It is impossible in this place to enumerate all the insects which 

 attack plants. Almost every species has certain insects peculiar to it, 

 which feed on its leaves, juices, &c., and often cause great injury. 

 Those which are common to hothouses and greenhouses, have called 

 for the special attention of horticulturists, and various means have 

 been suggested for their removal or prevention. Among them may 

 be enumerated, vapour of tobacco and ammoniacal liquor of gas-works, 

 to kill aphides; vapour of sulphur, for the red spider; vapour of tur- 

 pentine, for the wasp; vapour of crushed laurel leaves, for the white 

 bug; coal-tar, for the wire-worm, &c.* 



* For further remarks on the Diseases of Plants, see Henslow's paper in the Journal of the 

 Royal Agricultural Society of England. 



