May, 1902 .] Galls and Insects Producing Them. 
265 
which are elongated with the surface of the gall, i. e., the direc- 
tion of growth (Figs. 8, 9, n). Projecting into the gall cavity 
are masses of irregular shaped cells (Figs. 8-n). In young galls 
these cells show a nucleus, take the stain readily and show 
indications of maturity (Figs. 9, n). 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 Scliizonc- 
ura americana 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. 4) 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 family (Figs. 13-21). 
In Colopha ulmicola Fitch (Fig. 13 a. b. ) and Pemphigus ulmi- 
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. I11 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 L. 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. 
