112 



NUTRITIVE CONJUNCTIVE SYMBIOSIS 



than the wasps but large numbers are found upon oaks, hickories, 

 willows, roses, legumes and composites, especially goldenrods and 

 asters. 



The exact nature of the stimuli that cause the production of insect 

 galls is not definitely known but in most cases it is probably due to 

 mechanical irritation caused by the feeding activities of the young 

 insects after hatching. 



An interesting group of galls is that of the witches' brooms that 

 are formed on various kinds of trees (Fig. 54). Those occurring on 

 hackberry (Celtis occidentalis) are caused by mites, minute animals 



Fig. 54. — Witches' broom on Acer negundo. 



closely related to spiders, while most other witches' brooms are 

 caused by fungi. The formation of a witches' broom is quite com- 

 parable to that of an ectotrophic mycorrhizal cluster. That is, the 

 attack of the parasite on a shoot inhibits the further elongation of 

 that shoot and causes an abnormal number of branches to be pro- 

 duced back of the ti]). These branches are in turn attacked by the 

 parasite and still other branches are produced the final result being 

 a thick broom-like cluster of short branches. 



Other common galls caused by fungi are the "cedar apples" which 

 occur on Jimiperus virginiana, the "black knot" of plum and cherry 



