May, 1916] Morphology of the Zoocecidia of Celtis 265 



gall. This will be described in a succeeding paragraph. In the 

 paper by Cook, the secondary chamber mentioned above was 

 inadvertently regarded as the larval chamber. 



The specimens from which serial sections were made for 

 this study were not fully mature. The old mature galls are 

 practically impossible to cut satisfactorily. Certain features 

 such as the nature and development of the nutritive layer can 

 be studied much better in a somewhat immature gall than in 

 the old ones when that layer has been disrupted. 



The gall comprises two epidermal layers, iso-diametrical 

 parenchyma tissue, sclerenchyma (protective layer) which is 

 particularly well developed near the dome-shaped nutritive 

 layer forming the central region (PI. XV, Fig. 6b). 



Fig. 6d presents the details of the blind canal region outlined 

 at d. Fig. 6b. The epidermal cells lining the canal are slightly 

 smaller than those on the other parts of the gall. The cutin 

 layer is continuous down the canal to its blind end at the inner- 

 most sclerenchyma zone. A group of sclerenchyma elements, 

 relatively large and highly pitted, occur on the inner side of 

 this zone, directly beneath the canal. Inwardly the nutritive 

 tissue adjoining these elements is composed of exceptionally 

 large cells which have stiffened their walls by criss-cross thick- 

 enings (Fig. 6h), a type of cell not uncommon in the larger 

 elements of nutritive layers. 



The cambium-like nutritive layer is detailed in Figs. 6c and 

 6e. The protective layer is well on its way in the lignification 

 of the cell walls though it must be remembered the condition 

 here illustrated is immature. In the fully mature galls, cells 

 near the periphery of the cecidium become lignified and the 

 inner cells shown in the figures finally attain walls of such 

 thickness as to be classified as stone-cells. 



The fibro-vascular bundles traverse the gall on the under 

 side of the nutritive layer. On the side next to the nutritive 

 layer the bundles commonly possess one layer of bundle sheath 

 cells (Fig. 6e). The bundles collectively form a very coarse 

 net-work over the under side of the cambium-like central 

 tissue. 



A detailed study of the cystoliths is shown at f and g. Fig. 6b, 

 PI. XV. These are illustrated in Figs. 6f and 6g, respectively. 

 The one on the edge of the gall shows marked abortion, evidently 

 possessing little calcium carbonate in its structure for it stained 



