174 ANNALS OF SCOTTISH NATURAL HISTORY 



affected cells. Such galls are almost always formed by 

 parasites (such as many Gall-mites and some Aphides) for 

 their own protection and nourishment. The alterations in 

 these forms frequently amount, as stated above, to little 

 more than an enlargement of the cells of the ground-tissue 

 or of the epidermal cells, with sometimes an increase in 

 their numbers. Hairs often grow from the surface, in small 

 numbers and scattered, or so abundantly as to form con- 

 spicuous patches. These have frequently been described as 

 fungi, usually under the supposed genera Erineum and 

 PJiylleriurn. These galls belong to a somewhat specialised 

 type in that the hairs in each kind are very seldom like 

 those usually found on the host plant, while they show forms 

 characteristic of the galls. 



Very different, at least in degree, from the influence 

 exerted by the parasite in both these cases, must be that in 

 force in the production of such galls as those formed on oaks 

 by the true gall-flies ; and the difference is increased by the 

 fact that so many of these insects produce in each year two 

 galls absolutely distinct in aspect, and often very different in 

 structure. Yet these highly developed galls only manifest in 

 an extreme degree a power that in its less specialised stages 

 is possessed by many other insects, as well as by organisms 

 of numerous types lower than insects in the scale of being. 



There does not appear to be any reason why man should 

 not discover how to exercise some such power over nutrition 

 and growth of parts so as to modify structure profoundly. 

 The importance of such a discovery in its scientific interest 

 and in its practical applications is self-evident. 



The subject of Galls is far too large to be treated in its 

 fulness here ; but an outline of it may help to induce readers 

 to look for them, and to respond kindly to my request for 

 assistance in the form of specimens. 



It is somewhat difficult to define clearly what a gall is, 

 so as to include the widely different forms with which we 

 meet. But it may practically be accepted as any structure 

 in which the action of a parasite has led to increase in 

 size of the tissues of the host. The enlarged tissues very 

 generally show marked increase in the number of the cells, 

 along with enlargement of the individual cells, and (in the 



