GALLS 



111 



(hypoplasy). Usually the plant stores up quantities of food in the 

 tissues of the gall and these are later used by the parasitic symbiont. 

 Partly because of these large stores of reserve materials most galls 

 are very resistant and if a plant bearing galls is cut down, the galls 

 often remain fresh and green long after the other parts of the plant 



are dead. 



Galls are found on all kinds of plant parts. They are most com- 

 mon on stems and leaves but occur on roots, flowers, and fruits also. 



Fig. 53.— Insect galls on white oak. 



They exhibit a great variety of form and color, some of them being 

 very fantastic in form and appearance, and some of them are very 

 beautiful. They are caused by many kinds of plants and animals 

 but chiefly by fungi and insects and more especially the latter. The 

 kinds of insect galls are almost innumerable (Fig. 53). The great 

 majority are produced by gall-wasps {Cynipidae) or gall-gnats 

 {Cecidomyidae). Each species of insect produces galls always on the 

 same species of plant or on a few closely-related species and, while 

 insect galls are widely distributed in the plant kingdom, there are 

 some groups of plants that are peculiarly susceptible. Of about 450 

 known gall-wasps, for example, more than 350 are found on oaks 

 and nearly 30 on roses. The gall-gnats are somewhat less restricted 



