May, 1916] Morphology of the Zoocecidia of Celtis 283 



bundle tissue, observable in any of the prosoplasmas, does not 

 even find an analogy in the normal structure. The cells of the 

 galls, however, all have their homologs in the normal tissues. 

 Cosens (3) states: "The conventional view to account for 

 these phenomena is that the protoplasm has been endowed with 

 entirely new characteristics and power to produce something 

 foreign to the normal host. But this probably is true only in a 

 very limited sense for according to my experience at least the 

 prototypes of such apparently new tissues, etc., have been 

 found elsewhere in the host or its relatives." He bases this 

 statement on a comparative study of special structures, such as 

 "glands, trichomes and aeriferous tissue," which reappear 

 in certain galls in addition to the definitely new tissue "forms," 

 constituting the gall as a whole. Any comparative studies of 

 cecidia and normal parts should take into consideration the 

 whole structure and when this is done the essential "newness" 

 of the cecidium appears. 



In the form characters of the gall tissues (gained through 

 growth, i. e., proliferation and differentiation of cells) we 

 have characters, which without any doubt whatever, are 

 ascribable to the specific physiological phenomena of the 

 insect. In other words the insect larva controls the growth 

 of the embryonic tissue in its immediate vicinity, this growth 

 developing a new structure, showing specific characters as 

 definite and constant as the group of characters observable 

 in the adult insects. A glance at plates 6 and 7, showing nine 

 species of itonid galls, all but one of which have been seen 

 by the author, on Celtis leaves, will demonstrate to any one 

 the validity of the above statement. 



An analysis of form character can be made, which will 

 disclose certain factors over which the insect has undoubted 

 control. 



Form character with respect to tissues in the normal plant 

 is directly related to the orientation of the mitotic spindle 

 axes and the number of divisions during growth, and the sizes 

 attained by the cells after mitotic activity has ceased. These 

 factors are of course influenced by the ever present factor of 

 environment. To the growth factors should be added the 

 factor (the nature of which is unknown) which directs differ- 

 entiation. In thejgall problem this is particularly involved in 

 the appearance of_the lignified sclerenchyma tissue (protective 



