202 MRE. P. GROOM—CONTRIBUTIONS TO THE 
thick suberized walls. Thus free transport of material to or 
from the tuber takes place only through the stelar tissue. 
Just above the constricted region there is on the upperside a 
dead, brown scale, otherwise no scales occur in these secondary 
rhizomes. In the proximal portions some of the cortical cells 
may have no mycorhizal hyphe; in the inner layers of the cortex 
of this region the mycelia have been converted into the well- 
known refractive masses. 
Histology of Axis above the Upper Scale. 
The epidermis is smooth. Just above the insertion of the 
upper scale-leaf the axis is terete, but the stele stands nearer 
the side on which the scale is attached. And two minute 
vascular bundles (really meristeles) run vertically upwards on 
that same side. Higher up two ridges appear in section; thus the 
stem is raised into two folds. Within each ridge lies one of the 
two meristeles. Ascending the stem the ridges increase in size 
and gradually resolve themselves into the margins of the foliage- 
leaf, and the subjacent meristeles are the two large marginal 
nerves. Beginning at the level of the upper scale, the central 
stele commences to show changes in structure. The endodermis 
has become quite unrecognizable ; and the groups of phloém and 
xylem as we ascend from this point gradually arrange themselves 
in a collateral fashion. In particular one bundle becomes marked 
off, on the side towards the ridges, by two conspicuous medullary 
rays, from the more closely united mass of 3—4 other bundles. 
Where the foliage-leaf is apparently attached to the stem a 
large bundle runs into the leaf ; and the axis above the foliage- 
leaf has four vascular bundles forming a ring. (In my specimens 
these 4 bundles were still in an embryonic condition, consisting 
of a mass of procambium, for the flowers were still in a bud- 
condition.) 
Histology of the Leaves. 
The minute scale-leaf (s") consists simply of an outer (lower) 
and an inner (upper) epidermis and parenchyma-cells. No 
vascular bundle enters it, nor is a leaf-trace bundle given off to 
it, though a bundle goes to its axillary bud. No hairs occur on 
the leaf; the sole function possible to this minute scale-leaf is 
to protect the bud. 
The larger scale-leaf (8) is a little more complicated. It is 
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