458 BOTANICAL GAZETTE [JUNE 
inward, causing a slight depression in the ochreola or lowest 
sheath of the branch. The ochreola, which is much better devel- 
oped on the side away from the parent axis, possesses five teeth, 
forming in the lower part of the sheath five inconspicuous blunt 
ribs. The two which lie most externally, right and left of the 
slight median depression, are by far the most developed. Even 
in this early stage, in most cases at least, both possess a small 
vascular bundle containing two or three small tracheids. Cases 
have previously been recorded and figured in which the ochreola 
is penetrated by one vascular bundle (8, 11). 
V. Young internode of main stem 
The base of the leaf sheath is concrescent with the axis. At 
the level at which the former becomes free in the large young axis 
the tissues of the main stem have the appearance of being very 
young. ‘The cells mostly have large nuclei and are rich in contents; 
no metaxylem has been differentiated, nor is the bundle sheath 
recognizable; most of the protoxylem still persists. The cortical 
cells below the ribs of the stem, which later develop as scleren- 
chymatous fibers, still have unthickened walls. As we pass upward 
in the internode the tissues gradually assume a more mature 
appearance. This should be expected, since it is known that the 
metaxylem of an internode undergoes lignification from above 
downward (1). In this lower end of a young internode, therefore, 
we have an anatomically incompletely differentiated portion of the 
axis of E. gigantewm, the absence of which prevented GWYNNE- 
VauGHAN from definitely establishing the direction of lignification 
of the lateral metaxylem in this species. He suspected that the 
direction of lignification in Eguisetum generally was centripetal 
in the lateral strands of metaxylem. He held that this was indi- 
cated by the constantly smaller size of the peripheral tracheids 
in E. gigantewm, the species in which the lateral metaxylem is most 
abundantly developed. 
Near the base of the young internode, at the point at which the 
leaf sheath detaches itself from the axis, no metaxylem had been 
differentiated. As we pass upward one or two laterally situated 
tracheids soon make their appearance, but in most of the bundles it 
