XVJ 



LEPIDODENUHON 



147 



Some of the sections in the Binney Collection (Sedgwick 

 Museum, Cambridge) show early stages in the production of 

 secondary xylem : in the section represented in fig. 167 the 

 secretory zone is succeeded on its inner face by a zone of radially 

 elongated cells, vi, which are clearly in a meristematic condition. 

 The same section shows also the more radially extended form 

 of the xylem of a leaf-trace with its internal protoxylem, px, in 



Fio. 167. Lepidod^ndron fuligi'nosum. 

 (Binney Collection, Cambridge.) 



contrast to the tangentially elongated form which is assumed 

 during its passage through the cortex (cf figs. 165, 166). 



Some sections of Lepidodendron fuliginosum in the Man- 

 chester University Collection are of special interest from the 

 point of view of the method of secondary thickening. In 

 the section reproduced in fig. 168, B, the meristematic zone 

 is seen to consist in part of radially elongated elements, vi, 

 with parallel cross-walls evidently of recent origin. The same 

 tissue is shown also in fig. 168, C, a, D, a, and in fig. 169, A, a 



