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. i 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. Kemer 

 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 



