6 
BRITISH GALLS 
easily discernible, and I noted in many places two Oaks 
(Quercus pedunculata) growing side by side under similar 
conditions with their branches intermingled, one bearing 
a profuse crop of “ oyster ” galls, and the other almost 
or quite without them. From two such trees, near the 
Haslemere Museum, I gathered, on August 23, two twigs 
at random, one from each tree. One twig had ten leaves ; 
on these I counted 228 galls, of which no less than 188 
were “oyster” galls. All the leaves were brown at the 
margin, and presented the conditions seen in Plate I., 
where Fig. 1 shows the upper surface of one of those 
leaves, and Fig. 2 the under surface. The other twig 
had fifteen leaves: none showed acroteric death; I found 
two spangle galls (Plate I., Fig. 3) on one and a single 
spangle on another. Thirteen of the fifteen leaves were 
entirely without galls of any kind. How is the compara¬ 
tive immunity of the latter tree to be explained ? Is there 
some special substance in its plasma which has a dele¬ 
terious effect upon the egg of the insect ? I determined, 
by microscopic examination, that many of the leaves had 
been punctured, but no gall growth had followed. 
Certain species of trees rarely produce galls upon their 
leaves. I may instance the Horse Chestnut. Only four galls, 
all obscure, are known, and none have been observed in 
Britain. Mr. Alfred Sich, F.E.S., the acknowledged 
authority on leaf-mining insects, informs me that he knows 
of “no leaf-miners of the Horse Chestnut in England nor 
Europe,” a fact of great interest to cecidologists. 
Peyritsch and other investigators have shown that many 
plants can be induced to produce double flowers (stamens 
changed into petals) under the stimulus of mites. Kerner 
gives particulars of his personal observations in the case of 
Veronica officinalis , which bore double flowers when infested 
with mites. He noted that ripe seeds were produced only 
from flowers which had remained single amongst the double 
ones, and that the plants from these seeds bore single 
flowers. But V. officinalis has only two stamens in each 
