404 DISEASES OF PLANTS. 



the twigs, catkins, and leaves of the Oak. The artichoke gall of the 

 Oak depends on an irregular development of a bud, caused by the 

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

 resembling a young cone. A cone-like gall has also been observed in a 

 species of Pernettia. On examining the galls of commerce, the pro- 

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

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

 foration 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 suberous 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 gaUs resemble parasitic fungi. Oak-spangles are galls of this 

 nature. They are attached by a central point to the outer surface of 

 the leaf, the inner side being smooth — the outer red, hairy, and 

 fringed. Each contains a single insect, which retains its habitation 

 tiU March, long after the leaves have fallen to the ground. 



It is impossible in this 'place to enumerate aU the insects which 

 attack plants. Almost every species has certain insects peculiar to 

 it, which feed on its leaves, juices, etc., and often cause great injury. 

 Those which are common ;to hot-houses and green-houses 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 gasworks 

 to kill aphides; vapour of sulphur for the red spider; sulphur for 

 the vine-disease; vapour of turpentine for the wasp; vapour of 

 crushed laurel leaves for the white bug ; coal tar for the wire-worm. 

 Carbolic acid, and sulphur mixed with soft soap, are also used. 



