May, 1916] Morphology of the Zoocecidia of Celt is 277 



position of the mites, while in the case of the lepidopterous gall 

 the greatest hyperplasia is that of the pith, the medullary rays 

 and cambium region, a condition correlated with the internal 

 position of the larva. 



Suppression of normal differentiation characterizes both. 

 The lepidopterous gall partially developes bast, but no sclerides 

 appear. The mite gall exhibits sclerides, but no bast. No 

 lignification of the undifferentiated xylem cells occurs in the 

 lepidopterous gall, but is very definitely found in the acarinous 

 cecidium. 



Compared with the normal stem, the most significant 

 single fact concerning the kataplasmas, is the marked inhibition 

 of differentiation with no substitution of entirely new tissue forms. 



Prosoplasmas. Hemipterous galls. 



Pachypsylla vesiculum (gall 3 and Fig. 3) is the simplest 

 of the psyllid galls. Compared with the normal leaf it would 

 appear that the middle cells of the immature mesophyll are 

 most susceptible to the influence of the nymph, since these 

 cells have carried out the hyperplasia. 



The other four galls are all fundamentally identical in 

 structure and mode of development with that of P. vesiculum. 

 Gall 4 is the abnormally differentiated bud primordium. 

 Gall. 5, (P. gemma) has developed for the most part from the 

 stem phellogen, a tissue in the young stem undoubtedly more 

 susceptible to control than that of the cortex. Gall. 6 (P. 

 mamma) involves all of the leaf tissues, so that the gall can be 

 considered as a mass of "new" tissue intercalated in the leaf 

 blade, but suspended below the leaf blade plane. Gall 7 (P. 

 venusta) illustrates the same mode of development seen in 

 No. 6, carried out on the petiole by a number of larvae rather 

 than one. (See description under 7). 



It can be said that the above psyllid galls, characterized 

 by little or no " umwallungen " development with rather ill- 

 defined protective layers surrounding nutritive tissue possessing 

 a cambium-like structure, constitute a generic type of gall for 

 the hackberry, a type which contrasts strikingly with the 

 generic type of itonid galls. 



It is of course evident that the specificity of the different 

 galls is in part due to the instinctive behavior of the insects in 

 choosing particular plant parts. This is strikingly shown by 

 comparing P. mamma with the gall on the side of stem (No. 4). 



