370 Transactions of the Canadian Institute [vol. ix 



Salix. The leaf petioles of the following species were found to contain it, — 

 Populus balsamifera L., P. deltoides Marsh., Salix humilis Marsh., 5. alba 

 L. and S. cordata Muhl. The tissue is developed much more plentifully 

 on the side adjacent to the stem. 



In general then this tissue is indicated in the pith of the stem of 

 Populus but is restricted in Salix to the bases of the branches. It is well 

 represented in the primary cortex of the stems of both Populus and 

 Salix, rather better so in the case of the former genus. It is abundant 

 in such primitive regions as the reproductive axes, nodes and leaf traces. 

 Thus the unexpected appearance of this tissue in the galls cited is readily 

 explainable on the same grounds as in the case of glands and trichomes, 

 namely, the power to produce this tissue is latent in the protoplasm of 

 the host and it becomes sufficiently active to reinstate the tissue only 

 when the gall-producing stimulus gives rise to unusual conditions. 



Concerning the nature of this powerful stimulating agent there is 

 at present a growing tendency to ascribe it to enzymatic action. It is 

 difficult to say just how wide the application of this method of stimulus 

 may be, but as plants present so many features in common in their re- 

 actions to produce the different types of galls, universal enzyme action 

 would seem to be at least a safe working hypothesis. It is only, however, 

 in the case of the Cynipidse that we have any experimental evidence 

 concerning enzymatic action. As described in a previous part of this 

 section, I have been able to prove, in the case of the gall Amphibolips 

 confluens Harris, that the larva secretes an enzyme capable of changing 

 starch to sugar. It is now my purpose to discuss this fact in its relation 

 to gall production. 



Kiister,^* after experimenting with Cynipid galls in culture solutions, 

 arrived at a conclusion that furnishes some experimental data on the sub- 

 ject. "Bei normaler Entwickelung wird der Inhalt der Nahrgewebe von 

 den Gallentieren verzehrt; unter abnormalen Verhaltnissen kann aber 

 das Nahrmaterial von den Pflanzenzellen selbst verbraucht werden. 

 Gallen von Pediaspis Aceris (Cynipide), die von ihren Bwohnern be- 

 freit und auf nahrstoffarmen Losungen oder auf gewohnlichem Leitungs- 

 wasser belassen werden, bleiben wochenlang am Leben; der Inhalt der 

 Nahrgewebe schwindet dabei. Werden Gallen gelicher Art ceteris paribus 

 auf Zuckerlosung verbracht, so bleibt der Inhalt der Nahrgewebe un- 

 verbraucht oder erfahrt noch eine geringe Vermehrung." 



These experiments prove that a gall is able to extract nourishment 

 from the nutritive zone to assist in its growth in general. It appears 

 axiomatic then that the greater the quantity of soluble food there is in 

 the nutritive layer, in excess of what the larva requires, the larger is the 

 supply the gall has at its command and the more marked will be the 



