May, 191G] Morphology of the Zoocecidia of Celtis 267 



sinus, above mentioned. These chambers vary in number from 

 3 to 14. The radiating waUs are very thin near the periphery 

 where they join the hard outer shelL Fig. 7b shows the gall 

 with the side removed. The chambers are nearly filled with a 

 white, flocculent, waxen mass, a secretion of the nymphs. The 

 pupae all emerge through the thin wall of the sunken area in 

 the fall, and after the last ecdysis the insects fly to the bark, 

 where they spend the winter. These galls are not common, the 

 writer's entire collection numbering but a half dozen. They 

 have not been seen in Ohio. 



Osten Sacken, B. Ent. Zeit. in Stettin, p. 422. 1861. 



Before discussing the histology, it should be noted that this 

 gall is formed in identically the same fashion as P. mamma, 

 though there are many insects concerned in its development. 

 Once the tiny cones of tissues, which are concerned in the 

 embedding of all the psyllid larvae in the petiole, have over- 

 topped them, extensive hyperplasia takes place, this hyperplasia 

 eventually forming the central core. The hyperplasia of the 

 rest of the petiole (the peripheral portion) of course keeps pace 

 w^th that just mentioned. 



A transverse section of the gall is shown in PL XVI, Fig. 

 7c. At a, is indicated a part which is enlarged nearby. An 

 outer and inner part of this has been drawn in detail in Figs. 

 7d and 7e. The first striking feature of the outer wall of the gall 

 is that of the presence of a cork layer on it. Kiister (16 [p. 206]) 

 points out that cork formation on galls is a rarity. No cork, of 

 course is ever found on the normal petiole. The cortical 

 parenchyma cells have not thickened their walls as those of the 

 normal petiole do. An extremely heavy layer of stone cells is 

 developed, but is not continuous; numerous strands of par- 

 enchyma tissue extend through it. The nutritive layer (Fig. 7e) 

 consists of the same thin walled type of tissue seen in that of the 

 other galls, but it does not possess, in the adult condition, 

 the typical cambium-like structure. The fibro-vascular bundles 

 located in the outer part of the nutritive layer are small and 

 numerous (much more so than indicated in the diagram). No 

 bundle sheath is developed. 



The central core is composed of a homogeneous mass of 

 very large sclerenchyma cells. One of these is figured, Fig. 7f. 

 The simple pits in the wall extend as far as the middle lamella. 

 Fibro-vascular tissue is entirely absent from the core, a fact 



