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- 
ter are in some galls confined to the folds 
Fig. 27. Vertical section of aPhy- and processes. Besides the - purse-like 
toptus gall from a leaf of the galls, and the growths of hairs, there is 
green ash, (Fravinus viridis). 
a third abnormal formation on some plants 
which is produced by Phytopti. It consists of a simple fold of the 
leaf not due apparently to any unusually rapid local growth, but 
such as could be straightened out again dia the leaf admit of a 
sufficiently vigorous pull. On our long-leaved willow, such a cecidium 
occurs. It extends the entire length of the leaf, and ordinarily there 
is one on each side of the midrib. Dr. F. A. W. Thomas describes 
a similar cecidium which occurs on the European Lonicera nigra, and 
extends around the entire margin of the leaf. The fourth kind of 
cecidium is formed by the crowded masses of minute leaves and 
twigs whose development has been arested by the depredations of 
the mites. 
Formerly galls were thought to be due to a deposit of a liquid 
poison by insects. At present it is pretty generally believed that 
they are caused by purely mechanical irritation. Briosi explains 
the formation of the mite-gall of the vine very simply and satisfac- 
torily. He thinks that the puncturing of the cells of the leaves causes 
an increased flow of sap in the direction of the injured part, the 
result of which is that this gets an unusual supply of growth material, 
and the cells multiply more rapidly than those of the surrounding 
tissue. Tio make room for this additional tissue, the leaf swells 
upward, leaving a hollow below which is closed in by the subse- 
quent growth of the leaf. A series of ceeidil may be selected from 
plants which will illustrate the different stages in the development 
of one of the purse-shaped galls. There is a cecidium on the leaves 
of the box elder which consists of a dense cluster of hairs in a 
concavity on the under side of the leaf. The position of these clus- 
ters of hairs is indicated above by a slight convexity differing in no 
wise in texture or color from the surrounding portions of the leaf. 
On the oaks is another cecidium which is slightly more convex above 
than the preceding, and finally becomes brown. This represents a 
second stage. The third stage may be illustrated by galls on the 
heart-leaved willow. In these galls the opening below is pretty 
well closed and the outer surface is pitted and wrinkled. The fourth 
and last stage may be represented by the galls on the maple, as in 
them the openings are almost obliterated. 
The galls appear with the unfolding leaves in the spring. At the 
earliest stage at which the leaves of the soft maple can be examined, 
minute swellings are found on them indicating the site of the future 
galls. They grow with the leaf, and by the time it has fully 
expanded, have completed their erowth. No galls appear on fully 
developed leaves, so that if a leaf once gets its growth, it is safe 
from attack ; but when the Phytopti which cause the first galls increase 
to such an extent as to make a migration necessary, they pass along 
the branches to the terminal twigs, and may there produce new galls 
