320 Transactions of the Canadian Institute [vol. ix 



There are three well differentiated zones. Immediately inside the 

 small celled epidermis is a mass of large thin-walled cells irregularly ar- 

 ranged. The walls of these cells are seldom straight but usually present 

 a wavy outline. They diminish in size progressively, passing in from the 

 periphery of the gall until they merge into quite a well defined nutritive 

 zone. This tissue is illustrated in Fig. 32. In this zone the cells are 

 very much smaller and are arranged in rows radiating out from the larval 

 chamber. Vascular strands pass irregularly throughout the gall. There 

 is no indication of a protective layer separating the two inner zones. 

 Cecidomyia majalis Bass. 

 _, JQuercus rubra L. 



[Quercus coccinea Muench. 



A flat- pouch-like gall on the under side of the leaf. The opening 

 which extends the entire length of the gall is on the upper side. It is 

 produced by a folding of the blade of the leaf; this fold is parallel with 

 and very close to the midrib or a main vein. 



Dimensions: — Along the line of its attachment to the leaf, diameter 

 4-7 mm. 



The gall has been formed in this case by a folding of the blade of the 

 leaf. The resulting type recalls the pouch-like form usually associated 

 with the Eriophyidae or more rarely the Aphididae. 



The part of the blade included in the fold has not a well defined 

 palisade and spongy parenchyma, the mesophyll being practically uni- 

 form throughout. The cells of this region are much larger than those 

 of the normal leaf and are placed together without intervening air spaces. 

 At the apex of the fold the leaf blade is much thicker than at any other 

 part of it (Fig. 27). 



The epidermis that lines the interior of the fold seems to remain 

 intact throughout all the developmental stages of the larva. 



Cecidomyia ocellaris O.S, 

 Host Acer ruhrum L. 



A circular ridge on the under side of the leaf and a slight convexity 

 on the upper surface constitute the chief part of this gall. In the depres- 

 sion the larva rests covered with a viscid fluid secretion. The effect of 

 the stimulation extending out from this centre is shown in the different 

 coloured concentric rings produced in the leaf blade. These colours change 

 in the course of development of the gall through various shades of red, 

 green or yellow. 



The slight depression in the leaf blade that constitutes this gall has 

 been produced in the following way. The part of the leaf blade that 

 forms the bottom of the depression has remained practically normal, 



