THE CANADIAN' ENTOMOLOGIST 383 



Stem and extends entirely across the pith. While this tissue is 

 present in the primary cortex of the normal stem of hotli Salix and 

 Populus, and indicated in the pith of the latter, it is entirely absent 

 from the pith in the corresponding part of the stem of Salix. It 

 is abundant in such primitive regions of Salix as the reproductive 

 axes, nodes and leaf traces. Thus the unexpected anpsarance of 

 this tissue in the gall cited is readily explainable on the same 

 grounds as in the case of the glands and trichomas — 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 con- 

 ditions. 



A further illustration of this principle is shown in the produc- 

 tion of cork in an aphid gall on the leaf of Passiflora suberosa. 

 While this tissue is entirely absent from the unstimulated leaf, the 

 stem produces it normally. Also, Rhodites muUispinosus Gillette 

 stimulates the usually unarmed stem of Rosa blanda Ait to the 

 production of an exceedingly spiny gall. The production of 

 spines, however, is a marked characteristic of the genus and a 

 dormant activity has again been aroused. 



Concerning the mode of application of the stimulus by the 

 parasite, it may be stated that in none of the orders of insects 

 except the Tenthredinidre is there any evidence that indicates the 

 beginning of gall formation before the hatching of the larva. In 

 this family the source of the stimulus is in all probability the 

 ovipositor of the insect, since it has been conclusively shown that 

 the gall structure is well advanced while the larva is still within 

 the egg membranes. 



From observations on the galls of Neuroterus IcBvinsciilus and 

 Biorhiza aptera, Adler concluded that cell division commenced 

 only after the larva emerged from the egg. Weidel lately has 

 shown that such is the case in the gall produced by Neuroterus 

 vesicator Schlecht. It may, as a consequence, be accepted as 

 proven that the source of the stimulus in the galls produced by the 

 Cynipidae is the larva of the producer. 



As already published* the writer has proven by a series of 

 experiments, that the larva of AmphiboJips cofifluens Harris 

 *Transactions of the Canadian Institute, Voluni,^ IX., 1912. 



