300 Transactions of the Canadian Institute [vol. ix 



The epidermis has responded to the stimulation by producing many 

 long, acicular, unicellular hairs, which are narrowed at the base. The 

 hairs are stellate on the normal leaf of the host, but are simple, uni- 

 cellular and acicular on its reproductive axis. The same feature is true 

 of Quercus robur var. pedunculata, and in all probability of the other oaks. 

 This appears to be a clear case, not of the production of a new type of 

 trichome, but of a reversion. 



Eriophyes Sp. 



Chadwick's No. 2 



Host Acer negundo L. 



A shallow dimple on the under side of the leaf, filled with a white 

 pubescence. 



In this gall as in the preceding species, the leaf blade has been very 

 much thickened by proliferation in the mesophyll. The cells produced 

 are circular in outline and of about the same size as those of the normal 

 spongy parenchyma. The hairs produced in this case not infrequently 

 consist of from 2 to 3 cells which are very much convoluted. They are 

 well shown in Fig. 4. The hairs on the normal leaf of the host are straight 

 'or only very slightly curved, but the glandular, convoluted type of hair 

 is found on the inflorescence. Both the normal and abnormal hairs are 

 composed of the same number of cells. 



Eriophyes Sp. 



Chadwick's No. 84 



Host Populus grandidentata Michx. 



A deep dimple gall with the convex part on the upper surface of 

 the leaf. The gall is light green above, with the contents of the depres- 

 sion dark red. 



The cells produced by the palisade parenchyma can be distinguished 

 in this case from those arising from the spongy parenchyma. The 

 former are almost square in outline and placed in two very regular rows 

 immediately below the upper epidermis, and parallel with it. This 

 tissue can be seen in Fig. 2. The latter constitute a tissue made up of 

 cells which are circular to elliptical in outline. These cells are much 

 smaller than those produced by the palisade layer. The outgrowths 

 from the lower epidermis described in various ways as trichomes, granules, 

 etc., are in reality produced by a complicated folding, in which only the 

 lower epidermis and the spongy parenchyma participate, as shown in 

 Fig. 2. 



A cross section of the gall shows a number of vascular strands, but 

 in my opinion the gall-producing stimulus only enlarges the veins that 

 are already present in the normal leaf; it does not originate a special 

 vascular s^^stem for the gall. 



