86 BRITISH GALLS 



largest i inches in diameter. The branch which bears 

 these hypertrophies is only i foot in length and \ inch 

 in diameter. Blomfield does not think that mechanical 

 irritation initiates the hypertrophy of the cambial cells, and 

 leans to the view that it must be caused by a ferment from 

 the salivary gland of the insect. * 



The galls arising from the presence of Tetraneura ulmi are 

 not infrequently found in association with those caused by 

 Schizoneura ulmi, described above. They are, however, quite 

 distinct. The gall of T. ulmi is a capsule which completely 

 encloses the Aphis and her progeny. It begins as a blister, 

 the edges of which rise upwards until they meet over the 



Fig. 21 — Elm Leaf with Scroll Gall (a) caused by Schizoneum 

 ulmi; AND Capsular Gall, (i) caused by Tetraneura ulmi. (i/a.) 



insect. Buckton remarks that " the leaves, viewed from the 

 under side, show oblong orifices or slits at the junction of 

 the peduncle. These ventilating slits are partially closed 

 by a dense fringe of jointed threads, which doubtless pre- 

 vents the entrance of many a prying parasite. The cells 

 were full of winged insects on the 20th of July." Within this 

 pedunculated chamber the Aphis undergoes her four moults, 

 and produces her young. The latter are blackish at first, 

 becoming greenish after the first moult, and are provided 

 with a white woolly coat on the abdominal parts. A fairly 



* Brzezlnski asserts that Bacterium mali is the cause of these 

 tumours. 



