86 
BRITISH GALLS 
largest 2 inches in diameter. The branch which bears 
these hypertrophies is only 1 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 Schizoneura 
ulmi ; and Capsular Gall, ( b ) caused by Tetraneura ulmi. (1/2.) 
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 
* Brzezinski asserts that Bacterium viali is the cause of these 
tumours. 
