before cell enlargement was completed. It seems less 
likely that an actual diminution of the size of mature 
cells occurred during ontogeny of the cambium. Alter- 
nate, crowded, circular bordered pits occur on the radial 
walls of the tracheids. ‘The pits on the larger tracheids 
range from five to six rows on each radial wall (Plate 
LXNITI, fig. 3). The pit orifice, in cells with a sufficiently 
preserved secondary wall residue, tends to be elliptical 
with the long axis oriented transversely to the axis of 
the cell. 
Tyloses occur in great abundance within the lumina 
of the tracheids, and are of the sclerotic type, with thick 
secondary walls. ‘Tyloses are quite rare in the secondary 
xylem of living Gymnosperms, although occasionally 
they occur in roots of coniters. The tyloses of MZ. Olseniae 
ure one of the most conspicuous features in longitudinal 
sections of the secondary wood, as shown in Plate LNTV, 
figs. 4 and 5. Despite poor preservation of histological 
detail in the specimens at hand, it is apparent that they 
developed by protrusion of the cytoplasm from adjoin- 
ing ray cells through the bordered pits of adjacent tra- 
cheids. Inasmuch as nearly every tracheid was in direct 
contact with ray cells throughout its length, the capacity 
for tylose formation can be partially explained. How- 
ever, the tendency for such outgrowths to develop must 
have been unusually strong, as evidenced by the forma- 
tion of a tylose within a tylose as shown in the tracheid 
figured in Plate LNTLYV, fig. 6. Such a cytological anom- 
aly aus the latter is very difficult to interpret, yet this 
seems the only explanation for the structure shown. 
The rays are poorly preserved and in large part tan- 
ventially crushed to an amorphous or alveolar mass of 
cell wall residues. In better preserved areas the rays are 
seen to be of two types: uniseriates and very high multi- 
seriates. The latter are four to six cells in width, of which 
[ 195 | 
