and their “tnormal’” cross sectional diameter appears to 
be from one to two centimeters. Mach stelar unit is mod- 
erately endocentric with respect to the development of 
secondary wood, 
The primary wood, in cross section, Consists of an oval 
to oblong core, comprised of groups of approximately 
tive to fifteen tracheids, each group surrounded by paren- 
chyma, which, in our specimens, is very poorly preserved, 
The primary xvlem clusters are rather evenly distributed 
throughout) the parenchymatous tissue surrounding 
them. Protoxvlem cells of the bundles cannot be distin- 
guished with certainty, though in the better preserved 
specimens they appear to be mesarch in arrangement. 
The secondary xvlem develops as an eccentric cylinder 
nround each “stele.” Unequal cambial activity is evi- 
denced in the greater development of secondary tissue 
on the inner part of each evlinder, thus producing an en- 
docentric development as found in many Carboniferous 
species of Meduflosa, The tracheids are arranged in con- 
spicuous radial rows, usually paired, and each of the 
double rows is separated by rays of varying width (Plate 
LXV. figs. 1, 2 and 3). An interesting feature of the 
pulred radial rows is the alternating position of the indi- 
vidual tracheids with respect to each other (Plate ILNTEV, 
fig. 3). This geometric arrangement is probably indica- 
tive of extreme apical elongation, or sliding growth, dur- 
ing differentiation of the cambial derivatives in the newly 
formed secondary wood. It is probably also due in part 
to changes in spatial relationship of the cells during deg- 
radation preceding mineralization. The tracheids are ex- 
tremely long and of fairly large cross sectional diameter. 
In the older secondary xylem tracheids range from 80 to 
90 «in diameter, although the outermost cells in several 
specimens are of smaller size, possibly indicating that the 
plants had been killed during active secondary growth 
[ 104 | 
