334 Transactions of the Canadian Institute [vol. ix 



This is a monothalamous gall found on the leaves of Salix humilis. 



It is spherical in shape and in that feature resembles P. pomum, 

 but in other respects it differs very markedly. It extends from the side 

 of the midrib almost out to the margin of the leaf and is divided into 

 two hemispheres by the leaf blade. In consequence the gall protrudes 

 nearly equally from each leaf surface. Usually there are two or three 

 galls on a leaf. When two are present they come in contact with the mid- 

 rib at the same place but on opposite sides, as illustrated in Fig. 79. In 

 a few cases four and even five galls were found on one leaf. The galls 

 are pubescent but not as densely as the under surface of the leaf. 



Dimensions: — Average diameter i cm. 



The mesophyll of the leaf and the upper epidermis are mutually 

 concerned in the production of this gall. In one, sufficiently immature 

 to show the relative amount of tissue arising from each source, it was 

 found that the upper epidermis had produced two cell layers, while the 

 lower had not responded to stimulation ; and that the palisade and spongy 

 parenchyma had each produced one-half of the remaining mass. The 

 hollow in the gall, present from the earliest stages, has been formed 

 between the tissue arising from the palisade and the spongy parenchyma 

 respectively. 



When only one gall originates from the midrib at any point, the 

 vascular bundle is cut approximately half through (Fig. 78). But more 

 frequently two galls are located opposite one another, one on each side 

 of the midrib, in which case the two incisions meet and completely sever 

 the bundle, as seen in Fig. 79. 



Vascular strands pass almost completely around the gall, along a 

 line half way between the epidermis and the gall cavity. These strands 

 originate from the midrib in the neighborhood of the injury and pass in 

 opposite directions. 



Undescribed Sawfly Gall on S. lucida Muhl. 



Fig. 4. — Two galls produced on the same leaf of the host. 



This gall consists of an enlargement of either the petiole or midrib 

 of 5. lucida. Neither of these organs bears, as a rule, more than one gall 



