284 The Ohio Journal of Science [Vol. XVI, No. 7, 



layer), which in all cases represents a zone of parenchyma cells, 

 which change their activity from growth to thickening of their 

 walls. 



In the case of the development of a number of species of 

 galls on a particular leaf, the physical environmental factor 

 can be thrown out, since it is the same for all. It is the biological 

 environmental factor (the larva) which is now the external factor 

 controlling the internal ones operative in developing tissues. 

 These internal ones, to state them again are: The factor or 

 factors related to the orientation of mitotic spindle axes and 

 the number of mitoses carried out; the sizes attained by cells 

 after mitotic activity has ceased; the factor or factors directing 

 the distribution of differentiation products, which in this 

 morphological study has particular reference to the thickening 

 (lignification) of walls. Any theory concerning the nature of the 

 stimulus should adeciuately explain how the particular stimulus 

 exercises its control over the above factors, which factors, 

 be it noted, are the most important factors entering into the 

 growth of tissues. 



It should be noted in the above analysis care has been taken 

 to definitely distinguish between the factors related to the 

 development of particular kinds of tissues and those related to 

 the development of particular kinds of cells. These distinctly 

 intra-cellular factors making possible mitosis, growth in size 

 of cell, lignification of cell wall and the like, it would appear 

 are practically undisturbed, for from the standpoint of the cells 

 there is little or no fundamental difference between those 

 simpler ones in host tissue and those in the galls. It should 

 be remembered, however, in this connection, that highly 

 specialized cells, such as cystoliths, etc., do not appear in galls. 



Before leaving this phase of the subject, attention should be 

 called to the fact that much evidence exists to show that these 

 fundamentally new gall tissues are carrying out fundamentally 

 new functions. This material, however, would be out of place 

 in a paper intended to be purely morphological. 



Material of some interest may be forthcoming if we view the 

 above conclusions in the light of modern genetic conceptions. 



Cosens (3) states, "this much is certain that there appears 

 to be an entire lack of evidence supporting the view that the 

 protoplasm of the host has become endowed with a property 

 that enables it to produce a fairly definitely shaped but withal 



