May, 1916] Morphology of the Zoocecidia of Celtis 253 



The bast fibre cylinder is differentiated directly next to the 

 layer just described. Within the bast fibre zone a layer of small 

 celled parenchyma forms a transition tissue to the phloem which 

 latter is typical and will not be discussed in detail, other than to 

 state that the outer phloem parenchyma as well as that of the 

 medullary rays in the phloem region, contain numerous sphaer- 

 raphides (calcium oxalate). 



The cambium consists of the typical, tangentionally flattened 

 brick-shaped cells, massed three to six cells deep before exhibit- 

 ing differentiation. 



In the xylem region wood fibres and tracheids of small 

 diameter predominate. The tracheae of both primary and 

 secondary xylem are of the ordinary types. It might be noted 

 in passing that the innermost tracheal element of the secondary 

 xylem co-ordinates perfectly with the adjoining vessel of the 

 primary xylem in the development of the transversely elongated 

 bordered pits, which relate the two. The medullary ray cells in 

 the region of the wood, shows the typical sclerenchymatized 

 condition, with the walls containing numerous simple pits 

 extending to the middle lamella. 



The tracheids, whose distribution in the stem is variable are 

 of particular interest in connection with this study, because of 

 the fact that it is only this kind of xylem element which is 

 found in the appendicular hemipterous and dipterous galls. 

 In the stem they are of extremely fine structure, particularly 

 those formed near the end of the season's growth, possessing 

 spiral and split-spiral thickenings of very minute size. 



Inwardly the primary xylem is bordered by some cylindrical 

 elements with slightly thickened walls forming a transition 

 tissue to the storage or "differentiated" pith, which forms the 

 periphery of the medullary cylinder. The cells of this storage 

 tissue possess as usual large numbers of simple pits. 



The large celled pith is of interest, since an exactly similar 

 type of parenchymatous tissue is found in many of the galls. 



Older stems were examined showing the products of sec- 

 ondary growth, but nothing new or of a type which possessed 

 additional significance relative to the problem in hand, was 

 found. All of the galls on the hackberry are developed from 

 the meristem functional in primary growth, the insects in all 

 cases being unable to gain access to meristematic elements 

 after the first cork layer has formed. 



