392 Hasten. — The Structure of Mesoxy ton Sutcliffii ( Scott). 
section shown on PI. XXXIII, Fig. 2, a wide space occurs between the two 
regions of the pith on the left, while on the right they are perfectly con- 
tinuous. 
The central pith> as shown on PI. XXXIII, Fig. 2,c./., consists of some- 
what irregular, but on the whole transversely arranged, diaphragms with 
intervening spaces. Between the horizontal plates of tissue there are 
numerous fine connecting strands, and the discoid structure is clearly the 
result of longitudinal tension and contraction during the growth of the plant. 
In the transverse sections (PL XXXIII, Fig. i) the discoid pith presents 
quite an irregular appearance, owing to the fact that the transverse dia- 
phragms are rarely strictly horizontal (see Fig. 2) in the somewhat con- 
tracted condition in which they are preserved. In transverse sections, the 
cells of which the diaphragms are composed appear nearly round in form, 
with a diameter in the central region of 0*15 mm. to 0*20 mm. In the longi- 
tudinal sections the cells appear somewhat flattened, and are seen to be 
arranged in more or less horizontal rows. All the cells have a somewhat 
disorganized appearance. The cells in the central parts of the diaphragms 
are larger than any of those in the outer persistent pith. 
The outer continuous zone of the pith consists entirely of more or less 
isodiametric parenchymatous cells, many of which possess dark brown 
contents. This zone is well shown in transverse section on PL XXXIII, 
Figs. 3 and 4, and in longitudinal section on PL XXXIII, Pig. 6. As is 
shown in the latter figure, the cells of the outer pith are arranged in vertical 
rows when seen in longitudinal sections. The cells frequently present the 
appearance of not being in contact, an appearance which is probably due to 
contraction of the contents prior to fossilization. The sections (PL XXXIII, 
Figs. 3, 4, 5) usually show some differentiation of the cells of the outer pith 
in concentric regions or zones, which pass, however, gradually into one 
another. Immediately adjacent to the outer boundary of the discoid pith, 
and passing gradually into it, occurs a zone of empty-looking cells with thin 
walls (PL XXXIII, Fig. 3,/L, Fig. 4 ,/'., Fig. 6,f.) ; next follows a zone in 
which the cells are usually filled with brown contents (PL XXXIII, Fig. 3,/"., 
Fig. 6,f '.) ; outside this zone of cells with dark contents a number of smaller 
empty cells are sometimes seen next to the xylem (PL XXXIII, Fig. 4 ,p'"' f 
and these in longitudinal sections are found to be vertically elongated cells 
with rather thick walls, and quite different in shape from the somewhat 
tabular (short vertically) dark cells within (see PL XXXIII, Fig. 6,p".) 
The zone of contents-filled cells itself usually consists of elements of 
two kinds, some of the cells containing pale-brown contents, while the others 
are filled with very dark-brown, sometimes almost black, material. The very 
dark cells frequently occur as a kind of subzone outside the paler-coloured 
ones (PL XXXIII, Figs. 4 and 5,/"'.). The subzone of cells with very dark 
contents is not always clearly differentiated in the transverse sections, and 
