of Coal- Measure Plants. 



147 



breadth. The tissue of the meristematic zone is imperfectly pre- 

 served, but here and there the cells of this band immediately 

 internal to the secretory zone show a distinct tendency towards a 

 radial arrangement, as already noticed in Section 1 ; the regular 

 cross- walls are evidently the product of secondary meristematic 

 activity, and indicate the beginning of secondary thickening 

 (Text-fig. 2 m). In the drawing reproduced in fig. 2, x, x marks 

 the outer edge of the corona ; in front of the depression in the 

 corona there is a leaf-trace group of tracheids with an internal 

 protoxylem, px, and this is surrounded by a few of the cells of the 

 meristematic zone a, most of which has not been preserved ; the 

 regular band of cells m marks the position of the dividing tissue; 

 at s a portion of the secretory zone is represented. The secretory 

 zone consists of large sacs and small short cells ; in a few places 

 the large spaces are occupied by ill-defined thin-walled elements, 

 but the cells are partially disorganised and present the appearance 

 of a tissue in course of degeneration. It is not always easy to 

 distinguish between actual cells in the interior of the larger sacs 

 and cracks in the mineral matrix, which have been rendered 

 more distinct by the infiltration of a brown colouring matter. 



The portion of the secretory zone shown in Text-figure 3 

 illustrates the occurrence of a few smaller elements enclosed by 



Figube 3. Transverse Section through a portion of the Secretory Zone. 



(Slide 2, x90.) 



the thin membranes which mark the outlines of the sacs ; the 

 latter usually appear as conspicuous empty spaces, as in Section 1 

 (fig. 3, PL in., s), and in the greater part of the secretory zone 

 shown in Section 2. 



The three layers of cells at the top of the drawing (fig. 3) 

 represent part of the innermost cortex, while the few elements in 

 the lower part of the section belong to the parenchyma of the 

 meristematic zone. 



