May, 1916] Morphology of the Zoocecidia of Celtis 285 



abnormal structure. Such a pronounced change would surely 

 be expressed in the heredity characteristics, yet there is not a 

 vestige of proof tending to show that insect galls ever produce 

 the slightest variation in the descendants of the host." The 

 "protoplasm" referred to above is the germ plasm and, used in 

 this sense the statement made, is correct. While nothing is 

 known concerning the difference in the meristematic tissues 

 of gall bearing plants as opposed to non-gall bearing forms, there 

 is no reason for hypothesizing a special constitution for the 

 germ plasm of the gall bearing flora. Nearly all of the orders 

 of the Anthophyta possess gall bearing plants. 



On the contrary, morphogenetical studies constantly and 

 definitely point to the germ plasm of the insect as the place of 

 origin of gall forms. These gall forms (tissue forms taken 

 collectively) are almost without exception found to be specif- 

 ically related to the insects associated with them, this being 

 exhibited in the most striking manner in all of the higher 

 prosoplasmas. In the prosoplasmas it can, with certainty, be 

 said that we have the remarkable and unique case of the over- 

 lapping, as it were, of an animal hereditary constitution on 

 that of a plant ; a situation in which the plant's tissue "forming" 

 factors (not tissue growing factors) are suppressed and new ones 

 substituted. In this connection it should be remembered that 

 in the early stages of all prosoplasma ontogenies, the larval 

 insect is in contact with the undifferentiated plant tissue; a 

 contact as intimate as that between one part of a growing 

 plant and an adjoining part. Fockeu (6) correctly states that 

 the early phenomena observed in the reaction of the plant part 

 is "en rapport" with the "phenomenes vitaux" of the gall 

 inducing form. 



Since science knows little or nothing concerning the mechan- 

 ism by which hereditary factors are enabled to come to expres- 

 sion in form and otherwise, it is suggested that in the 

 zoocecidological field, we have a unique place to attack this 

 problem. Hybridization of gall insects to see if the Fi and 

 succeeding generations of galls would follow known hereditary 

 laws, undoubtedly would prove an extremely suggestive line 

 of investigation. But the great discovery which will undoubt- 

 edly go far toward helping us understand the mechanism of 

 heredity will be that of the exact nature of the stimulus involved 

 in producing these problematic plant tissue forms, comprising 

 the prosoplasmatic zoocecidia. 



