98 On the Formation of 



cambium-ring. These cells are the theatre of actual 

 metabolism ; they are not yet differentiated into a stable 

 tissue, but await a period of developmental activity. 

 A tissue that possesses these properties possesses 

 the conditions essential for gall formation. When 

 a gall-fly has inserted its ^^;g into the neighbourhood 

 of this tissue, what follows ? In the first place the 

 act of ovipositing of itself has no effect. The wound 

 to the plant-tissue inflicted by a simple prick does 

 not necessarily give the impetus to gall formation. 

 Hitherto it has constantly been stated that the prick 

 of the gall-fly and the simultaneous introduction of 

 a glandular secretion excited a specific cell-growth 

 which led to the formation of the gall. This suppo- 

 sition derived a certain amount of probability from 

 the frequent occurrences in plants of increased cell 

 formation around wounds, as seen in those swellings 

 of the bark that follow a saw cut. Judging by 

 analogous phenomena occurring in animal tissues, it 

 was thought that the cells reacted to the stimulus of 

 traumatic irritation, so that increased metabolism and 

 the production of new cells took place. This was 

 made the foundation of a most misleading hypothesis ; 

 it was assumed that gall-flies, by means of a poisonous 

 glandular secretion which they poured into the wound 

 at the time of ovipositing, instituted a specific form of 

 cell activity, and in this way each species produced 

 its own peculiarly formed and equipped gall ; hence 

 it was only required to ascribe to each individual 

 species the possession of a specific secretion. This 

 view of gall formation has been supported in recent 



