INTRODUCTION 5 



galls of the Spruce show that in these a ferment injected 

 by the female Ckermes 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 191 1. As this 

 gall causes, when present in large numbers, marked mar- 

 ginal discoloration of the leaf, non- infected trees were 



