INTRODUCTION 
5 
galls of the Spruce show that in these a ferment injected 
by the female Chernies is the primary cause of the abnormal 
growth, and that its action is not strictly localized, but 
spreads from cell to cell. 
A particular part of a plant—for example, the leaf—may 
produce very dissimilar galls, even when attacked by closely 
related insects, as may be seen by referring to the frontis¬ 
piece which depicts galls on Rose leaves, all of which are 
induced by species of Rhodites. 
Galls are always remarkably constant in form, etc., even 
to minute peculiarities in the epidermic covering, and are 
thus easily recognizable, though their causers are often— 
especially amongst the Cecidomyidae —so nearly alike that 
it is difficult to distinguish them. In America the Willow 
Salix humilis is attacked by ten gall-gnats which cause 
distinctive galls, but it is almost impossible to diagnose 
the insects, the galls affording the best clue to specific 
identity. 
Galls are not transmissible to the descendants of the 
plant producing them. A tree grown from a seed of a 
Birch much infested with mites will not produce “ witches’ 
brooms,” unless it is visited by mites of the particular 
species which causes them. The interesting questions of 
the undoubted susceptibility of some trees to the attacks of 
parasites, and the comparative immunity of others of the 
same species, deserve careful investigation. A Birch tree 
laden with “witches’ brooms,” and having its branches in 
actual contact with those of another Birch quite free from 
them, is no uncommon spectacle. It may be suspected that 
the descendants of the former would show the same pre¬ 
disposition to attack, and the descendants of the healthy 
tree the same immunity. 
The so-called “oyster” gall of the Oak (caused by 
the presence of the larvae of Andricus ostreus) was extra¬ 
ordinarily abundant during the summer of 1911. As this 
gall causes, when present in large numbers, marked mar¬ 
ginal discoloration of the leaf, non - infected trees were 
