Abnormal Growth 



285 



specific modification of the tissue pattern of the host plant is caused by 

 the presence in it of an egg and larva of the parasite. A given cynipid will 

 always produce the same kind of gall on a given plant species, and the 

 galls induced by different wasps on the same plant are quite dissimilar. 

 On Celtis occidentalis, Carter found 17 different sorts of galls formed by 

 17 species of wasps. Each type of gall is related to the character of the 

 larva that develops within it. 



Doubtless the formation of these formed galls results from a specific 

 stimulus coming from the wasp or the growing larva and a specific re- 

 sponse by the tissues of the host plant, but how such a subtle control of 



Remains of the egg membrane 



Vein of the 2nd order of branching 

 Ruptured lower epidermis 



Leaf upper epidermis 



Vein connecting leaf and gall 



Cup of platform cells 



Additional sclerenchyma ring 



Funnel-shaped sclerenchyma mass 



Nutritive tissue cells 



Plate-like sclerenchyma mass 



Gall cavity containing larva 



1 mm. 



Large air-spaces region 

 Gall vascular stand 



Stellate hairs 

 Gall parenchyma 

 Gall-stalk sclerenchyma 

 Gall epidermis 



Fig. 11-7. Diagram of a longitudinal section through a cynipid gall on the leaf of oak, 

 showing its specific form and considerable structural differentiation. (From Hough.) 



the morphogenetic potencies of the host cells is exerted by the parasite is 

 unknown. Various theories have been proposed and many experiments 

 performed to throw light on this problem. It has been suggested that the 

 gall-inducing stimulus is a mechanical one, but this seems rather unlikely, 

 and most workers now believe that the stimulus is chemical in nature, 

 perhaps an enzyme or a specific formative substance. 



Many investigators have tried to extract the gall-forming agent from 

 the insect, inject it into a plant, and thus produce a gall artificially, but 

 earlier attempts all failed. Recent ones have been more successful. Parr 

 (1940) and Plumb (1953) injected extracts from the salivary glands of 

 coccids and an aphid into young needles of Norway spruce and induced 

 the formation of galls much like those normally produced by these insects. 



