1912] Morphology and Biology of Insect Galls 329 



thin-walled cells, shown in Fig. 68. The bud scales surrounding this 

 group of cells resemble those of the normal bud except that the cuticle 

 of the epidermis is abnormally thickened. 



Euura S. ovum Walsh. 



"On Salix cordata. An oval or roundish, sessile, monothalamous 

 swelling, .30 to .50 inch long, placed lengthways on the side of small 

 twigs, green wherever it is smooth, but mostly covered with shallow 

 longitudinal cracks and irregular rough scales which are pale opaque 

 brown. Its internal substance fleshy in the summer like that of an apple, 

 but with transverse internal fibres. When ripe in the autumn filled with 

 reddish-brown spongy matter, with close-set transverse internal fissures 

 at right angles to the axis of the twig. On cutting down to the twig at 

 any time a longitudinal slit about .20 inch long becomes plainly visible." — 

 Walsh." 



As already noted the host of this gall in this locality is Salix humilis, 

 it remains to be determined whether there are two distinct species of 

 producers or one species with two hosts. Walsh's description of the gall 

 on Salix cordata corresponds to the form occurring here on S. humilis. 



The ovipositor of the producer has in this case made a longitudinal 

 cut in the stem. A transverse section at the place where the gall is 

 located shows that this wound extends in from the epidermis to the boun- 

 dary of the pith. The activity of the young tissues, abnormally stimu- 

 lated, soon fill this fissure with a mass of small, angular parenchyma. 

 The rapid division of these cells forces the exposed edges of the cortex 

 and central cylinder apart so as to form a wedge-shaped opening which 

 is filled up with the gall mass (Fig. 71). It should be stated that the 

 newly formed cells originate mainly from the division of a cambium 

 bordering the pith at the bottom of the fissure. But other tissues also 

 respond co the stimulation initiated by the ovipositor of the insect. 

 Thus a section of the stem at a short distance from the gall shows that 

 the outlying cambium has become abnormally active and has produced 

 a layer of bast nearly one-third thicker than that found in the normal 

 stem. Likewise the activity of a cork-cambium layer has thrown off a 

 strongly cu ticularized epidermis present in the earlier developmen tal stages. 



Undescribed Sawfly Gall {Euura N.S.) on Salix serissima Fernald. 



Tig. 1.— A BMrly suture s&ll attached to a leal of the boit plant. 



