igi2] Morphology and Biology of Insect Galls 357 



be parasitized. In some cases a tree would not yield a single perfect 

 producer, although a couple of dozen galls were examined. 



It seems safe to conclude that if this zone has ever functioned as a 

 means of defence against parasites, it is no longer operative. Apparently 

 the only protective function that can be ascribed to this tissue is the pre- 

 vention of injury to the producer by desiccation during its later larval 

 and pupal stages of development. A thick or cuticularized epidermis 

 would also afford protection in the same manner. 



Concerning the form of the elements comprising this zone, WeideH* 

 has recently made some interesting observations. He makes the follow- 

 ing too sweeping statement in his summary, — 



"Auch das gallentragende Organ der Mutterpflanze hat einem 

 Einfluss auf die Gestaltung der Elemente in der Galle, denn die blatt- 

 burtigen Gallen fuhren in der Schutzschicht einseitig verdickte, die 

 iibrigen allseitig gleichmassig verdickte Zellen." 



That this can be accepted only as a general rule, and at least requires 

 further study, is indicated by the fact that our American galls furnish 

 undoubted exceptions. Thus the gall produced on the stem of Vaccinium 

 pennsylvanicum Lam, by Solenozopheria vaccinii Ashmead has cells that 

 have a much thicker deposit of sclerenchyma on one tangential wall than 

 on the other. In some cases practically the entire cell lumen is filled with 

 sclerenchyma and the deposit has grown entirely from one side of the 

 cell. Also a number of leaf galls have their protective zones composed 

 entirely of cells with uniformly thickened walls. The following species 

 furnish examples of this: Amphiholips inanis O.S., Rhodites lent cularis 

 Bass, and Neuroterus majalis Bassett. The statement quoted is true, 

 however, in the majority of cases and is important as indicating a possible 

 effect of environmental conditions on the elements composing the gall. 



Cytology of Galls. 



Cell division in the Cynipid galls was not found to present any un- 

 usual phenomena. In the cambial layer of Dryophanta palustris O.S. in 

 which mitosis was taking place the chromosomes were found to be eight 

 in number. They are slightly curved and show a decided tendency to 

 group in pairs when moving out from the equatorial plate. The root 

 tips of the host Quercus coccinea Muench. were found to give the same 

 chromatic count and the chromosomes present the same feature of moving 

 out to the poles of the spindle in groups of two. 



In several galls amitosis was noted and very marked examples in 

 the nutritive zones of the galls Aulacidea nabali Brodie (Text Fig. 6) 

 and Aylax glechomcB Linne. Cell division did not appear to accompany 

 the phenomenon in any of the cases examined. 



