8 THE AMERICAN MIDLAND NATURALIST 
elliposoidal cells which gradually increase in size towards the center 
of the rhizome and merge without any line of demarcation into 
the large parenchymatous cells which make up the bulk of the 
organ. Many of these contain a few starch granules (Fig. 19, d), 
and there are a number of enlarged cells with raphides (Fig. 18, 
c, Fig. 19,c). The numerous scattered wood bundles vary in type 
from the closed collateral (Fig. 19, n) to those which are rather com- 
pletely and typically amphivasal (Fig. 20). This transition shows the 
close relation that exists between these two types. The amphi- 
vasal bundles are mostly found in the center of the rhizome 
and in the older portion of its length, showing that they are a 
subsequent modification of the closed collateral. The xylem 
(Fig. 18, f, g, Fig. 19, e) is of the spiral and pitted types. The phloem 
(Fig. 18, h, Fig. 19, f) consists of the ordinary elongated sieve cells. 
Around each bundle there are one or two irregular layers of 
elongated cells which appear as a sheath (Fig. 18, d, e; Fig. 19, g), 
but these are not always distinctive. Fig. 20 shows an amphi- 
vasal bundle from the center of an old rhizome. The xylem (c) 
completely surrounds the phloem (f) and there are two sheath 
layers (a, b) separating the bundle from the surrounding parench- 
matous tissue (e). 
THE STEM (FIGS. 16. and 17). 
The stem has an epidermis of brick-like cells with greatly 
thickened walls (Fig. 16,a; Fig. 17, a), Beneath this are found three 
or four layers of large thin-walled cortical cells (Fig. 16, b; Fig. 
17, b). Next in order is a very distinctive zone of hardened schler- 
enchyma made up of cells whose walls are so much thickened 
that only a small lumen remains (Fig. 16, ce; Fig 17, ¢). This schler- 
enchyma appears to develop from the cambium layer which earlier 
in the season formed the wood bundles. One small bundle (Fig. 
17) is shown entirely embedded in the layer and most of those 
nearest it are at least partially surrounded. The remaining part 
of the stem enclosed in this schlerenchyma cylinder is composed of 
rather loosely arranged pith tissue (Fig. 17, d) which contains the 
numerous scattered vascular bundles. ‘These are of the closed 
collateral type with the xylem (Fig. 16,e; Fig. 17,e) directed towards 
the center of the stem. The xylem consists mostly of spiral ducts, 
and the principle element of the phloem is the elongated sieve 
tube cells (Fig. 16, d; Fig. 17, f). 
