1912] Morphology and Biology of Insect Galls 369 



The trichomes worked out in a manner very similar to the glands. 

 When the gall produced types different from the normal they were searched 

 for successfully on the reproductive axes of the host. The unicellular, 

 acicular hairs of Eriophyes querci Garman (Fig. 6) are totally unlike the 

 stellate hairs of the leaf, but their exact counterparts are found on the 

 reproductive axes of the host Querciis macrocarpa Michx. The much 

 convoluted type of hair present in the Acarina dimple gall on the leaves 

 of Acer negundo L. (Fig. 4) are found plentifully distributed over the 

 reproductive axes, although the normal leaf hairs are straight. 



The production of aeriferous tissue in certain Salicaceous galls sub- 

 stantiates in quite as striking a manner the view I have advanced. These 

 galls contain examples of a typical aeriferous tissue, comparable indeed 

 to that found in such aquatics as Nymphaea, Potamogeton or Saururus, 

 while in the corresponding part of the host it does not occur. Indeed, 

 this statement may be extended to include all the species of the host 

 genus. A cross section of the gall originated on S. cordata Muhl. by 

 Cecidomyia triticoides Walsh shows this tissue surrounding each larval 

 cell. It is present throughout the cortex of the stem and extends entirely 

 across the pith (Figs. 34, 35, 36). This tissue is found also in the gall 

 originated on the leaf of the same willow by Pontania pomum Walsh 

 (Fig. 77), but is not found in the normal tissues; indeed, the mesophyll 

 of the leaf of 5. cordata Muhl. is peculiarly compact in structure. It is 

 figured by Cook^^ in the cortex of the stem gall produced on 5. discolor 

 Muhl. by Cecidomyia rigidoe O. S. 



With the purpose of determining the distribution of this tissue in 

 the normal stem a number of species of Salicacese were examined by 

 Mr. T. A. Sinclair and myself with the following results, a detailed 

 description of which will be published later. It was found in the primary 

 corcex of the stems of the following species and invariably more plentiful 

 at the nodes, — Salix humilis Marsh., S. alba L., S. rostrata Richards, 

 S. lucida Muhl., S. discolor Muhl., 5. nigra Marsh., 5. longijolia Muhl., 

 5. serissima (Bailey) Fernald, 5. cordata Muhl., Populus deltoides Marsh., 

 P. balsamifera L., P. tremuloides Michx. and P. grandidentata Michx. 

 It was also differentiated to some extent in the pith of the stems of P. 

 balsamifera L. and P. deltoides Marsh., P. grandidentata Michx. and 

 P. tremuloides Michx. The only indication of this tissue found in the 

 stem pith of Salix was in sections through the bases of branches of 

 5. cordata Muhl. and S. alba L. Possibly it may be present in the corre- 

 sponding region in other species. It can be traced a greater distance from 

 the growing tip in the cortex of Populus than in Salix before it becomes 

 unrecognizable owing to compression. It is apparently nearly always 

 present in the pith and cortex of the reproductive axes of Populus and 



