129 
The interior surface of a mite gall is rarely 
as smooth as in other galls, but is rough¬ 
ened by irregular folds and processes, and 
is sometimes clothed with hairs. The lat- 
Besides the purse-like 
a third abnormal formation on some plants 
\di is produced by Phytopti. It consists of a simple fold of the 
f not due apparently to any unusually rapid local growth, but 
h as could be straightened out again dici the leaf admit of a 
iciently vigorous pull. On our long-leaved willow, such a cecidium 
1 uis. It extencis the entire length of the leaf, and ordinarily there 
•ne on each side of the midrib. Dr. E. A. W. Thomas describes 
milar cecidium which occurs on the European Lonicera nigra, and 
ends around the entire margin of the leaf. The fourth kind of 
idium is formed by the crowded masses of minute leaves and 
?s whose development has been arested by the depredations of 
mites. 
’ormerly galls were thought to be due to a deposit of a liquid 
: son by insects. At present it is pretty generally believed that 
(y are caused by purely mechanical irritation. Briosi explains 
formation of the mite-gall of the vine very simply and satisfac- 
ly. He thinks that the puncturing of the cells of the leaves causes 
increased flow of sap in the direction of the injured part, the 
lit of which is that this gets an unusual supply of growth material, 
[ the cells multiply more rapidly than those of the surrounding 
ue. To make room for this additional tissue, the leaf swells 
yard, leaving a hollow below which is closed in by the subse¬ 
nt growth of the leaf. A series of ceeidii may be selected from 
uts which will illustrate the different stages in the development 
one of the purse-shaped galls. There is a cecidium on the leaves 
the box elder which consists of a dense cluster of hairs in a 
cavity on the under side of the leaf. The position of these clus- 
; of hairs is indicated above by a slight convexity differing in no 
e in texture or color from the surrounding portions of the leaf. 
! the oaks is another cecidium which is slightly more convex above 
n the preceding, and finally becomes brown. This represents a 
pnd stage. The third stage may be illustrated by galls on the 
rt-leaved willow. In these galls the opening below is pretty 
l closed and the outer surface is pitted and wrinkled. The fourth 
' last stage may be represented by the galls on the maple, as in 
m the openings are almost obliterated. 
’he galls appear with the unfolding leaves in the spring. At the 
best stage at which the leaves of the soft maple can be examined, 
mte swellings are found on them indicating the site of the future 
i s. They grow with the leaf, and by the time it has fully 
anded, have completed, their growth. No galls appear on fully 
1 eloped leaves, so that if a leaf once gets its growth, it is safe 
n attack; but when the Phytopti which cause the first galls increase 
uch an extent as to make a migration necessary, they pass along 
| branches to the terminal twigs, and may there produce new galls 
