EXCRESCENCES. 



2i; 



suck the young cellular tissue e.g. Phytoptus, 

 Aphides, etc. but in others the stimulus to hyper- 

 trophy starts by the puncture of the embryonic 

 tissue of a leaf, root, etc., by the ovipositor of the 

 female insect, which then lays an egg e.g. Cynips, 

 Cecidomyia, etc. the presence of which appears to 

 intensify the irritating action, or such only occurs 

 when the young larva escapes. 



Our knowledge of the primary cause of gall- 

 formation amounts to very little. Generally speak- 

 ing, only embryonic or very young cellular tissue 

 reacts, and galls on adult leaves and branches have 

 usually been initiated long before. The same 

 gall-insect may induce totally different galls on 

 different plants, or even on different parts of the 

 same plant, and different insects call forth different 

 galls on any one plant. These facts point clearly 

 to the co-operation of both plant and insect in 

 the gall-formation, and the best hypothesis yet to 

 hand is to the effect that a gall is a hypertroph}- 

 of cells, the normal nutrition, growth, and division 

 of which have been disturbed owing to the action 

 of some poison or other irritant derived from the 

 insect, or fungus, or other organism. Attempts 

 have been made to reproduce galls by injecting 

 the juices of similar galls into the tissue, but as 

 yet without success, and this may point to the 

 co-operation of mechanical irritation during the 

 hypertrophy in normal gall-formation. 



Galls, in the broad sense, are not always pre- 

 ceded by a wound, however. Insects on the 

 outside of young tissues may cause such irritations 



