PSEUDO-GALLS. 



347 



excrescence may originate in the natural growth of a shoo'^ 

 being checked by the punctures of aphides, or of those grubs 

 which we have described (page 338). 



Many of these excrescences, however, are probably 

 altogether unconnected with insects, and are simply hyper- 

 trophic diseases, produced by too much nourishment, like 

 the wens produced on animals. Instances of this may be 

 seen at the roots of the holyhock {Althea rosea) of three or 



Pseudo-galls of the Hawthorn, drawn from specimens. 



four years' standing; on the stems of the elm and other 

 trees, immediately above the root; and on the upper 

 branches of the birch, where a crowded cluster of twigs 

 sometimes grows, bearing no distant resemblance to a 

 rook's nest in miniature, and provincially called witch- 

 knots. 



One of the prettiest of these pseudo-galls with which we 

 are acquainted, is produced on the Scotch fir (^Pinus sylves- 

 tris), by the aphis pini, which is one of the largest species 

 of our indigenous aphides. The production we allude to 

 may be found, during the summer months, on the terminal 

 shoots of this tree, in the form of a small cone, much like 



