E. H. NEWCOMB 4^9 



Lewis and Walton (8), in a study of the formation of the cone gall of 

 witch hazel leaves caused by an aphid, found that the stem mother injects 

 into the young leaf cells minute drops of a substance "initiating, stimu- 

 lating, and directing development and differentiation." Globules con- 

 taming a crystalloid were injected by the proboscis into the cytoplasm, 

 from which they entered the nuclei and then the nucleoli. There the 

 crystalloids broke up into smaller bodies. For gall formation to continue 

 normally, repeated injections of additional material were required. 



Boysen Jensen (3), in reporting on experiments with larvae of a midge 

 forming galls on beech leaves, interpreted his results to indicate the lack 

 of a special gall-forming substance. Larvae placed on lanolin smeared on 

 a leaf caused cell division, although when lanolin on which larvae had 

 been held was smeared on a leaf, cell elongation resulted. It was assumed 

 that gall formation is caused by the larva, which is guided by instinct 

 to secrete substances similar to growth substances in definite places and 

 in different concentrations on the leaf, thus causing the latter to produce 

 the characteristic gall structure. 



Due to the minuteness of the gall-insects and the extremely small 

 amounts of growth substances which they could possess or inject, the 

 isolation and chemical characterization of such substances would require 

 enormous numbers of insects, and appears to be a formidable problem 

 indeed. Also to be remembered is the great difficulty of rearing even a 

 few of the frail and ephemeral gall-formers responsible for the highly 

 organized prosoplasmas. 



There is another approach to the study of galls, however, which might 

 yield information on the nature of the insect stimulus. This is the charac- 

 terization of gall metabolism and its comparison with that of the normal 

 tissues on which the gall occurs. It is not inconceivable that knowledge 

 of the nature and extent of change may throw light on the nature of 

 the stimulus. Such comparative data should also aid in the interpretation 

 of the role of certain metabolic mechanisms of normal tissue. 



One such difference in metabolism results in the abnormal accumula- 

 tion in galls of polyhydroxyphenols or their derivatives or condensation 

 products. Galls may contain on a dry weight basis as much as 75 per cent 

 tannin, and rarely contain less than 25 per cent (5). Such galls as the 

 Aleppo and the Chinese nutgall served for centuries as a source of tannin 

 for ink production. Both tannins and anthocyanins, as well as lignin, 

 contain polyhydroxy aromatic nuclei. It was suggested by WisUcenus 



