302 Transactions of the Canadian Institute [vol. ix 



The anatomical structure of this gall shows it to combine the char- 

 acteristics of a pouch-gall with a type in which the epidermis lining the 

 larval cavity is thrown into folds, as described in the case of the galls on 

 Populus. The foldings of the lining of the gall cavity often coalesce, and 

 practically divide the cavity into a number of compartments. Mor- 

 phologically this is one of the most highly differentiated of our Phy- 

 toptocecidia. The lining epidermis has produced a large number of 

 acicular, unicellular trichomes; these in some cases almost fill the cavities. 

 The hairs surrounding the gall aperture are of the same type, as are also 

 the normal hairs of the leaf. Glandular cells are much more abundant in 

 the gall tissues than in those of the normal leaf. 



Eriophyes serotincB Beut. 



Chadwick's No. lOO 



Host Prunus serotina Ehrh. 



This is a club-shaped gall, produced on the upper side of the leaf 

 of the host. The aperture of exit, which is on the lower side, is sur- 

 rounded by fine white hairs. It varies in length from 5 to 8 mm. and at 

 a distance from the apex of about two-thirds the total length, it is nar- 

 rowed into a stalk with an average diameter of i mm. In color it varies 

 from green to a distinct red. 



The stimulation has not produced very marked changes in the struc- 

 ture of the leaf near the origin of the gall. Apart from the fact that the 

 spongy parenchyma has divided more actively, the mesophyll is normal. 

 The upper epidermis of the leaf that forms the outer covering of the gall, 

 has its cell walls abnormally thickened. The lower epidermis that lines 

 the gall cavity has larger cells than the unstimulated epidermis, and from 

 these cells originate elongated, unicellular trichomes with bulbous bases. 

 The hairs on the normal leaf are acicular and unicellular. While the 

 cells in the neck of the gall are arranged in rows parallel to its length, 

 the larger cells that form the main body of the gall are not regularly 

 placed. The vascular strands pass up from the leaf at a distance of 

 about three cell layers from the gall cavity. 



Summary. 



In some forms the effect of the stimulation does not extend beyond 

 the epidermis, on which the producers are located, but in other species 

 it is transmitted throughout the mesophyll of the leaf. 



The abnormal activity of the epidermis is expressed in the curving 

 and folding of the tissue as well as in the production by it of various forms 

 of trichomes. 



In general, when the effect of the stimulation extends to the mesophyll 

 the distinction between the palisade and the spongy parenchyma is lost. 

 In place of these tissues a compact mass of uniform cells is produced. 



