1912] Morphology and Biology of Insect Galls 335 



at a time, but occasionally the petiole of a leaf carries two or even three 

 and the midrib in rare instances two. 



The midrib galls are fairly regularly elliptical in outline with the 

 shorter diameter across the leaf. The swellings in most cases are nearly 

 equally divided between the upper and the lower leaf surfaces. The 

 petiole galls vary from spherical to ovoid in shape. In the latter case 

 the smaller end of the gall is towards the apex of the leaf. 



Dimensions of the gall: — Longer diameter 6-12 mm.; shorter dia- 

 meter 3-7 mm. 



The very marked proliferation of tissue in this gall is not accompanied 

 by a differentiation that presents many points of interest. The cells are 

 larger than those of the normal leaf and the nuclei are correspondingly 

 larger. The bundle is cut nearly through by the ovipositor (Fig. 82). 

 The free ends of the bundle thus stimulated grow out until in some cases 

 they almost surround the gall. This elongation is produced in part by 

 the increased diameter of the vessels but also by the production of new 

 cellular elements. 



The pith, exposed by the cutting of the bundle, produces almost 

 all the abnormal tissue (Figs. 83, 84), but the cortex contributes some. 

 The cells are arranged in curved lines that pass across from one elon- 

 gated end of the bundle to the other. Between these rows are many air 

 spaces which are elongated in the direction of the lines of cells (Fig. 84). 



Undescribed Sawfly Gall on Salix humilis Marsh. 



Fig. 5. — Leaf of the host with attached gall. 



This is a monothalamous gall produced by the abnormal swelling 

 of the leaf petiole of S. humilis. It is conoidal in shape with a long taper- 

 ing apex which is towards the blade of the leaf. As it is situated at the 

 base of the petiole the uniform enlargement of that organ is often pre- 

 vented by the axillary bud. This causes the gall to project to a greater 

 extent on the outside of the petiole and produces an irregularity in the 

 outline of the gall. The surface is quite glabrous in spite of the fact that 

 the epidermis of the leaf is decidedly pubescent. 



Dimensions: — Length of gall 6-9 mm.; width 3-4 mm. 



Nearly the entire mass of this gall originates from the vascular 

 bundle of the petiole which has been stimulated to activity by the in- 

 sect's ovipositor. The small thin-walled cells of the gall substance 



