102 THE EEPOET OF THE No. 36 



The trichomes exemplify the principle in a very similar manner to the glands. 

 When the gall produces types different from the normal, these are almost invari- 

 ably found on the reproductive axes of the host. The unicellular acicular hairs 

 of Eriophyes querci Garman, are totally unlike the stellate hairs of the leaf, but 

 their exact counterparts are found on the reproductive axes of the host Quercus 

 macrocarpa Michx. The much convoluted type of hair present in the Acarina 

 dimple gall on the leaves of Acer negundo L. (Fig. 5), are found plentifully dis- 

 tributed over the reproductive axes, although the normal leaf hairs are straight. 

 The trichome-producing activity of the protoplasm has thus been stimulated by 

 the foreign organism to a degree reached in the normal only at the time of re- 

 production. 



The production of aeriferous tissue in certain Salieaceous galls also substan- 

 tiates the principle in a very striking manner. These galls contain examples of a 

 typical aeriferous tissue, comparable indeed, to that found in such aquatics as 

 Nymphasa, Potamogeton or Saururus; w^hile in the corresponding part of the host 

 it does not occur. Indeed, this statement may be extended to include all the species 

 of the host genus. A cross section of the gall originated on Salix cordata Muhl. 

 by Rhabdophaga t7-iUcoid)ps Walsh, shows this tissue surrounding each larval cell. 

 It is present in the abnormal stem and extends entirely across the pith, as can 

 be seen in Fig. 1. While this tissue is present in the primary cortex of the normal 

 stem of both 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 appearance of this tissue in the gall cited is 

 readily explainable on the same grounds as in the case of the 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-pro- 

 ducing stimulus gives rise to unusual conditions. 



A further illustration of this principle is shown in the production 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 multi- 

 spinosus Gillette stimulates the usually unarmed stem of Rosa hlanda Ait. to the 

 production of an execeedingly 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 families of insects except the Tenthredinids 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 ovi- 

 positor 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. This fact is 

 exemplified in Fig. 13. 



From observations on the galls of Neuroterus lawiusculus 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 Amphibolips confluens Harris, secretes an enzyme capable of changing 



♦Transactions of the Canadian Institute, Vol. IX, 1912. 



