336 Transactions of the Canadian Institute [vol. ix 



spring from a cambium layer produced in the pith of the bundle near 

 the ovipositor wound. The cells arising from this tissue force the severed 

 ends of the bundle apart until the vascular elements form only a narrow 

 line of cells between the gall proper and the cortex of the petiole; it is 

 shown at this stage in Fig. 85. This cortex is not materially thickened 

 but shows signs of stimulation in that there appears a small amount of 

 aeriferous tissue located near the place of entrance of the ovipositor. This 

 tissue is shown on each side of the wound in Fig. 85. This tissue was 

 not found in the normal cortex of the petiole. The cuticle is very much 

 thickened. 



Summary. 



Great proliferation of tissue with little differentiation is a common 

 characteristic of all sawfly galls. 



All the leaf tissues of the genus Salix appear to be susceptible to 

 stimulation by sawfly producers. 



The pith of the bundle produces practically the whole mass of the 

 gall when the place of origin is the petiole or the midrib of the leaf. In 

 the case of a stem gall a layer of cells bordering the pith produces the 

 chief proliferation. 



When the gall originates from the mesophyll of the leaf the bundle 

 of the midrib produces relatively only a very small part of the total 

 gall mass. 



Of the remaining tissues of the leaf the upper epidermis responds 

 more readily to stimulation than the lower and the spongy parenchyma 

 more actively than the palisade parenchyma. 



In some cases the abnormal tissues exhibit the characteristics of 

 the normal from which they have originated. The cells produced to 

 form a solid base of attachment for the gall Pontania pisum Walsh never 

 lose their arrangement in vertical rows, and the cells that originate from 

 the same tissue in P. desmodioides Walsh are also in vertical series (Figs 

 80, 81). 



Adler^ secured specimens of Nematus vallisnierii in which the larva 

 was still within the egg. I have been equally fortunate with the species 

 Pontania pomum Walsh, Pontania hyalina Norton and Pontania desmo- 

 dioides Walsh. At this early developmental stage considerable prolifer- 

 ation of tissue had already occurred. Adler even reports that the gall 

 was nearly full grown. My experience has been that the larvae are 

 invariably found feeding unless the material is secured almost as soon 

 as the galls are visible to the unaided eye. At this time little swelling 

 of the leaf tissues is apparent, but a discoloration of the leaf enables the 

 wound of the ovipositor to be detected. Owing to cell proliferation pre- 

 ceding the emergence of the larva, Adler concluded that the immediate 



