May, 1916] Morphology of the Zoocecidia of Celtis 251 



The present paper is perhaps unique in that it deals com- 

 paratively with practically all of the galls on one kind of plant, 

 and with the normal tissues of that plant. The presentation of 

 the histology of the normal plant parts will be given first. 



HISTOLOGY OF THE NORMAL PLANT PARTS. 



The discussion of the histology of the normal plant parts will 

 be followed by the descriptions of the galls arranged under the 

 proper insect order and family name. 



The elucidation of the normal histology was deemed impor- 

 tant, for it is necessary to have clearly in mind the results 

 of normal differentiation to adequately understand to what 

 extent the galls have deviated in their specific structure, from 

 the normal plant characters. 



The Leaf. (PI. XII, Fig. C). The upper epidermal cells 

 are comparatively large and bear externally a thick cutinous 

 layer. Large cystolith cells (cys.) break the continuity of the 

 typical epidermal elements. The expanded internal part of 

 the cystolith shows the presence of the calcium carbonate 

 in it by staining very lightly if at all, while the stalk and the 

 round external knob takes the aniline safranin with avidity. 



The palisade zone consists of two layers of cells, the inner 

 being much less prominent and in places merging with the 

 elements of the spongy layer so as to break its continuity. 

 The spongy layer is relatively compact. 



The fibro-vascular bundles possess a more or less definite 

 bundle sheath, composed of highly elongated cells with evenly 

 thickened walls. These are especially well developed above 

 and below the bundle (PL XII, Fig. C, a). The xylem elements 

 are the characteristic tortuous tracheides of the spiral type. 

 The phloem cells are as in leaves generally of the thin-walled, 

 more or less elongate, sub-cylindric form. The end walls often 

 slant at an appreciable angle. 



The under epidermis consists of cells somewhat smaller than 

 those of the upper protective layer. The outer walls are 

 sufficiently thin to permit the protoplasts to bulge them out- 

 ward. The stomatal cells are minute, the pair being inter- 

 calated between the larger epidermal elements at their bases. 



The Petiole. The petiole as seen in transverse section, 

 shows the typical asymmetric orientation of the fibro-vascular 

 bundles, which, taken collectively, form a crescent-shaped 



