May, 1916] Morphology of the Zoocecidia of Celtis 261 



sense that it has a real function in preserving the deHcate 

 nymph within from mechanical injury. At x, is observed the 

 primary cone (now flattened) which grew up and around the 

 young nymph and at y, the rounded papilla, which represents 

 the original downward evagination, which lowered the larva to 

 the center of the leaf, making possible the comparatively 

 greater hyperplasia of the central mesophyll. 



The histology of the left part of the section shown in the 

 diagram is delineated in PI. XIII, Fig. 3a. The epidermis is 

 not widely aberrant from the normal, though the cystolith cell 

 has been partially aborted, which was uniformly the case when 

 these occurred over the affected mesophyll. The upper 

 palisade layer has maintained its integrity and the lower part 

 of the spongy layer, nearly so, for stomata are present leading 

 into small air spaces. The central mesophyll has, it is evident, 

 been the tissue concerned in developing the "blister." It is of 

 some interest to note that the thickness of the blister has been 

 attained, not by a striking difference in the number of cells, 

 comparing the periphery with the tissue near the chamber, but 

 by the increase in size of the hyperplasia cells, the number of 

 cells at the periphery and near the chamber being approximately 

 the same. The protective layer appears broken, though if the 

 adjoining sections are taken into account, the layer is found 

 to be continuous in the fashion of a sieve. The sharpness with 

 which the lignified cells are delimited from the outermost layers 

 on both sides of the leaf is a prominent fact. The smaller 

 veins of the leaf which traverse the region affected show very 

 little if any modification. They pass between the lower 

 epidermis and the protective layer. They do not, however, 

 develop sheath tissue on the upper side, the side next the 

 sclerenchyma layer. 



4. Pachypsylla sp. (gemma? See next). PI. XIX, Fig. 3. 



This gall is a lateral, oval swelling of the stem, generally 

 found near or involving the nodal region. 5-7 mm. long, 

 2^-33^2 mm. wide. Color and surface texture that of the 

 normal bark. Predominately monothalamous ; confluent cases 

 occur forming a two-chambered and even a three-chambered 

 gall. Very common on the terminal twigs of the hackberry. 

 Remnants of old galls can be made out on stems 5-10 years old. 



