330 Transactions of the Canadian Institute [vol. ix 



This gall is produced by the abnormal swelling of the petiole of the 

 leaf. The leaves infected are those borne on the branchlets from which 

 spring the pistillate catkins. In the majority of cases, the leaf thac 

 bears the gall is the one from the axil of which the peduncle of the catkin 

 arises. The swelling is so close to the branchlet that after the leaves 

 have fallen, the gall appears to have originated from it. This misleading 

 appearance is due to the petiole of the leaf breaking just above the gall. 

 The galls are cone-shaped with che apex towards the blade of the leaf. 

 They are marked deeply by three or four grooves that meet at the tip. 



Dimensions: — Height of gall 7-8 mm.; 'diameter at base 6-7 mm. 



The anatomy of this gall presents a very compact tissue, owing to 

 the cells being placed close together without intervening air spaces. 

 The very much thickened cuticle of the epidermis is the greatest departure 

 from the normal tissue. Of the three bundles of the normal petiole two 

 are lacerated by the ovipositor of the insect, as shown in Figs. 69, 70. In 

 the mature gall the halves of these two are found in four widely separated 

 regions (Fig. 69), owing to the abundant production of tissue between 

 them. Indeed practically all the abnormal tissue is produced from the 

 undifferentiated cells stimulated by the cutting of the bundles. 



Pontania pisum Walsh. 



"On Salix discolor. A subspherical, pea-like, hollow, pale yellowish- 

 green gall, always growing on the under side of the leaf and almost always 

 from one of the side veins (in one case from the midrib), and attached 

 to the leaf by only a minute portion of its surface; 0.18 to 0.28 inch in 

 diameter and a few miniature, only 0.08 inch in diameter. Almost in- 

 variably there is but one gall to the leaf, but on four leaves there were 

 two, and occasionally two are confluent. Surface in some smooth and 

 even without pubescence; in others a little shriveled, generally studded in 

 the medium-sized ones with four to twelve small, robustly conical nipples, 

 which in the larger ones have burst into a scabrous brown scar. Only in 

 three out of sixty-two was there any rosy cheek as in P. pomum. The 

 point of attachment is marked on the upper side of the leaf by a brown 

 sub-hemispherical depression . ' ' — Walsh .** 



Walsh is incorrect in supposing that this gall originates from a 

 midrib or vein. A section shows that it is clearly a product of the meso- 

 phyll and is attached to that part of the leaf. The side vein, near which 

 it is always placed, is cut by the ovipositor, however, and vascular 

 strands pass out from it into the gall body. 



The mature gall consists of a peripheral layer of thin-walled cells, 

 irregular in outline surrounding a central cavity (Fig. 81). This tissue 

 is clearly derived from the mesophyll and epidermis of the leaf, but a 

 stage was not secured young enough to show the relative amounts 



