May, 1916] Morphology of the Zoocecidia of Celt is 263 



the cork proper and the phelloderm, which is but one cell 

 thick. In the region beneath the larva, the cortical parenchyma 

 has suffered some hyperplasia, but this is not at all comparable 

 in quantity to that of the phellogen. 



In the case of the mechanical, laterally diverging plates, 

 mentioned above, it is a matter of some interest to note that 

 the sclerenchymatization of the two types of cells involved 

 is perfectly uniform or continuous. While the boundary 

 between the new cambium-like tissue and the cortical tissue 

 proper is very definite, based upon the shape of the cells, the 

 wall thickening processes have gone on with an equal degree of 

 intensity in both. 



5. Pachypsylla gemma Riley. PI. XIV, Fig. 5. 



As indicated in the specific name of the insect, this is a 

 gall of the bud. The bud incept suffers extreme modification 

 in its development, an irregular sub-spherical structure being 

 formed, containing from three to eight chambers (PI. XIV, 

 Fig. 5). When the chambers are numerous the structure takes 

 on a nodular aspect. 3-5 mm. long, 4-5 mm. wide. In many 

 specimens faint outlines are present, suggesting the normal 

 scale structure, though in no case are free scales present. The 

 color is lighter than that of the normal buds. Very common. 

 A normal bud is shown in Fig. 5 c, PI. XIV. 



This gall differs from the preceding in that it is uniformly 

 polythalamous and always projects from the stem as a definite 

 (appendicular) modification of the bud. The protective layer 

 does not occur immediately beneath a cork layer, but differ- 

 entiates beneath a thick zone of tissue, which can be interpreted 

 as the homolog of the outer bud scale. Fundamentally, 

 however, the two galls are similar and they eventually may be 

 shown to be caused by the same species of psyllid. They are 

 here separated for the reason that no transition forms between 

 them have been observed. 



In the cross section of a gall (PI. XIV, Fig. 5 a) a heavy zone 

 of lignified tissue is found enveloping the nutritive tissue 

 within. The inner walls of the chambers develop somewhat 

 irregular plate-like masses of mechanical tissue to support 

 them. In the detail drawing (Fig. 5 b), the outer zone of 

 homogeneous tissue is interpreted as the homolog of a bud scale. 

 The definite row of cells on its inner border (at x) suggests 



