380 Transactions of the Canadian Institute. [vol. ix 



Fig. 25. Rhabdophaga batatas Walsh on the stem of Salix humilis Marsh. 

 A transverse section that shows the larval chamber surrounded 

 by a nutritive zone which is bounded on the outside by a well 

 defined protective sheath. X22. 



Fig. 26. A part of the protective sheath of the preceding species en- 

 larged to show the unequal thickening of the tangential walls. 



X200. 



Fig. 27. Cecidomyia majalis O.S. on the leaf of Quercus coccinea Muench. 

 Section at right angles to the midrib. The folding of the leaf 

 is shown and the uniform character of the mesophyll of the 

 gall. The epidermis lining the gall cavity is shown intact. X15 



Plate V. 

 Fig. 28. Abies balsamea (L.) Mill. Transverse section of normal leaf. 



X35- 



Fig. 29. Cecidomyia balsamicola Lintner on the leaf of A. balsamea (L.) 

 Mill. Transverse section showing the folding of the leaf and 

 the elongation of the mesophyll cells. The irregularity of 

 the cells in the strengthening layer of the resin ducts can also 

 be seen. X35. 



Fig. 30. C. balsamicola Lintner on the leaf of A. balsamea (L.) Mill. 

 Transverse section through the midrib. The chief points 

 shown are the irregularity in the development of the endo- 

 dermis, the large amount of the transfusion tissue and the 

 relatively small am^ount of the non-pitted parenchyma. Xioo. 



Fig. 31. Abies balsamea (L.) Mill. A section, through the midrib of a 

 normal leaf, corresponding to the preceding section (Fig. 30). 



Xioo. 



Fig. 32. Cecidomyia impaiientis O.S. on Impatiens biflora Walt. Sec- 

 tion through a larval chamber, showing the general nature of 

 the cells of the gall and the smaller cells of the nutritive layer. 

 The two dark masses attached to the nutriti\'e tissue in the 

 lower part of the gall cavity consist of the mycelium of a fungus. 



X30. 



Fig- 33- Cecidomyia ocellaris O.S. on the leaf of Acer rubrum L., shov.ing 

 the almost unchanged character of the leaf immediately below 

 the larva and the great amount of proliferation in the region 

 surrounding it. The general arrangement of the cells at right 

 angles to the leaf blade is also shown. X30. 



Fig. 34. Cecidomyia triticoides Walsh on the stem of Salix cordata Muhl. 

 Transverse section in which is shown the general arrangement 

 of the larval chambers and the distribution of aeriferous tissue 

 throughout the cortex and pith of the gall. X 10. 



