May, 1916] Morphology of the Zoocecidia of Celtis 257 



of growth at the nodes. An excessive number (2 or 3 generally) 

 of abnormal (wood reduced, pith increased) branches are 

 produced from the same bud, followed by the development 

 of an indefinite number of buds, all closely sessile in a mass at 

 the node between the "gaU" branches (PL XIII, Fig. 1). 

 The subsequent infection of the basal buds of the new branches, 

 the buds nearest to the original node attacked, accounts in 

 great part for the characteristic irregular massing of the 

 branches. If a young "broom" be stripped of its bark, (PL 

 XIII, Fig. la), this relation of the primary and secondary 

 branches is made evident. Often, however, in later years, 

 buds located at the base of the primary "gall" branches will 

 develop a shoot. After a number of years the mass of branches 

 becomes so large as to be very conspicuous and unsightly. 

 The author has investigated new branches growing on old 

 galls without finding any mites. It seems probable that the 

 condition of things grows worse after the primary infection, 

 whether or not the mites are present. 



The gall proper is altogether confined to the nodes affected, 

 in which region two prominent facts stand out in relation to the 

 histology of the parts affected: (1) The bases of the gall 

 branches have suffered an inhibition of their differentiation; 

 (2) The cortex shows definite hyperplasia. These facts are 

 shown in PL XIII, Fig. Ic, which illustrates the longitudinal 

 section of the part indicated at c, in Fig. la, which is a longi- 

 tudinal, median section through a primary "gall" branch and 

 the normal twig, from which it has grown out. The condition 

 of the xylem is an extreme case of differentiation interference. 

 Note the medullary ray cells are not very unHke those bordering 

 it, cells which should have become wood fibres and tracheids, 

 but which remain iso-diametrical, possessing simple pits 

 scattered in the somewhat thickened walls. The co-ordination 

 of the tracheae, which do form, with the cells adjoining them 

 by means of bordered pits, is not interfered with (PL XIII, 

 Fig. Id). 



Different branch bases show a wide variation in the degree 

 with which normal differentiation has been checked. The 

 extreme cases are almost uniformly composed of iso-diametric, 

 simple-pitted cells, the thickening of the cell waUs characterizing 

 the pith region with as much intensity as the xylem. The least 

 affected cases will show numerous vessels and tracheids, but few 



