May, 1902.] Galls and Insects Producing Them. 265 



which are elongated with the surface of the gall, z, e., the direc- 

 tion of growth (Figs. 8, 9, 11). Projecting into the gall cavity 

 are masses of irregular shaped cells (Figs. 8-1 1 ). In young galls 

 these cells show a nucleus, take the stain readily and show 

 indications of maturity (Figs. 9, 11). Trichomes are always 

 found extending from the walls of the cavity (Figs. 8-1 1) of 

 young galls, but disappear as the galls approach maturity. In 

 these galls we evidently have a repeated puncturing of cells by 

 the animal and an increased activity on the part of the plant in 

 its effort to recover from the wound, the wound never being suf- 

 ficient to cause the death of that part of the plant. 



My results on the Phytoptus galls agree with those of H. 

 Fockeu, except in minor points. 



3. The Aphididae Galls. In this family we find the 

 simplest form of galls discussed in this paper, of which Schizone- 

 ura america7ia Riley (Fig. 12) may be taken as a type. In fact 

 it is a mere curling of the leaf and not what is usually considered 

 a gall. According to E. Perris it would be classed as a galloide. 

 However, the structure is very similar to that of a typical gall of 

 this family of insects and I see no reason why it should not be 

 -considered a true gall. 



When compared with the normal leaf of U. americana L. 

 (Fig. i\) the palisade cells are observed to have lost their identity 

 and to have assumed mesophyll characters and the mesophyll has 

 become more compact, both distortions being characteristic of 

 true galls of this famil}^ (Figs. 13-21). 



In Colopha tdmicola Fitch (Fig. 13 a. b. ) and Pemphigus iilvii- 

 fusus (Walsh.) Oestlund (Fig. 14 a. b.) both of which are also 

 characteristic galls on the elm, we find practically the same 

 structure as in S. americana. In both the outer (upper) epi- 

 dermis is much elongated ; the same being true of the inner 

 (lower) epidermis of C. ulmicola, but not in P, ulmi-fusus. The 

 identity of the palisade cells is entirely lost, the cells now being 

 slightly elongated parallel to the surface of the gall. The 

 mesophyll cells are more compact than in S. americana and far 

 more compact than in a normal leaf (Fig. 4). 



A granular, dark brown, often black substance in the cells was 

 characteristic of the elm and other galls of this group. This was 

 probably tannin, and its presence seemed to depend on the host 

 plant rather than on an insect producing the gall. 



The Hormaphis hamamelis Fitch ( Fig. 15 a. b. ) on the 

 Hamamelis virginiana Iv. showed the same general structure as 

 the preceding galls of this order, except that the epidermal cells 

 were not so much elongated and in the inner (lower) epidermis 

 the cells were much smaller and showed thicker walls, and the 

 dark granular contents of certain cells was restricted to layers 

 near the outer (upper) surface. 



