3o6 Transactions of the Canadian Institute [vol. ix 



The galls on the birch leaves are produced by the fourth genera- 

 tion of Hamamelistes spinosus Shimer. Pergande described them as 

 "pseudo-galls or corrugations". 



The witch-hazel galls produced by the stem-mother of this species 



are plentiful in this locality, but Betula nigra L. is not found here. The 



Aphids have consequently been compelled to extend their list of food 



plants to include B. lenta L. and B. alba var. papyrifera Spach. 



Pemphigus vagabundus Walsh. 



Host Populus deltoides, Marsh. 



All the leaf rudiments of the terminal bud appear to be concerned 

 in the production of this gall. Yet it is in reality a large pouch-gall with 

 its wall thrown into smaller secondary folds. The apex of the stem, from 

 which the gall originates, is usually swollen to nearly twice its normal 

 diameter. These galls often remain on the host plant until the next 

 season's galls begin to appear. 



The cells composing this gall form a compact and fairly uniform 

 tissue and a nutritive layer is not clearly differentiated. About one- 

 third of the thickness of the gall wall, on the side next the larval chamber, 

 however, is composed of cells that are somewhat larger than the remain- 

 ing cells. The epidermal cells, lining the gall cavity, are elongated into 

 short trichome-like structures. The phyllome origin of this gall is re- 

 vealed by the presence of well defined stomata on its epidermis. These 

 structures are numerous and appear to be quite normal. Glandular 

 cells are plentifully distributed. Vascular strands pass irregularly 

 throughout the wall of the gall. 



Pemphigus rhois, Fitch. 

 Host Rhus typhina L. 



A balloon-shaped gall with the regularity of its outline destroyed 

 by the elongated lobes that cover its surface. A gall is shown in Fig. 14. 

 The epidermis is slightly pubescent and coloured red, shading into yellow 

 and green. It originates from the under side of the leaf, and the point 

 of attachment on the upper side is indicated by a small papilla covered 

 with a dense pubescence. These galls vary in size from very short types 

 less than i cm. to those that are 4 to 5 cm. in length. 



In the part of the leaf blade folded to form the gall, the mesophyll 

 has been entirely changed. The effect of the stimulation has even de- 

 stroyed the normal characters in the mesophyll for some distance from 

 the point of attachment of the gall. The gall consists of a compact 

 tissue composed of cells considerably larger than the normal cells of the 

 mesophyll. The cells of this tissue are arranged in layers parallel to the 

 epidermis of the gall. The vascular strands are situated about four cell 

 layers in from the gall cavity. In all pouch-galls the tracheary tissue is 



