The Morphology of Ruppia Maritima. 75 
2. Stem Structures in the Vegetative Region. 
a. Upright Stem. 
In its internal structure the stem is remarkable in many respects, 
but chiefly because of the reduction and consolidation of the vas- 
cular system into what may be termed a single axial vascular bundle, 
if we except the two minute bundles situated in the cortex. The 
morphogenesis of this reduced structure, as well as the structure 
of the whole stem as regards its ad- 
aptability to its environment, will be 
considered more in detail later. 
The epidermis surrounds a large 
zone of cortical parenchyma cells 
with a ring of lacunae in their midst; 
these cortical cells adjoin an endo- 
dermis, which encloses the axial 
vascular area (P1.I, figs. 4—5; PI. III, 
ies. 7, 8 and 9). The four parts 
—epidermis, cortex, endodermis and 
vascular system will now be de- 
scribed in order. 
(4) Epidermis. 
The epidermal cells are small in Figure 2.—Longitudinal — section 
comparison with the cortical par- through growing point of stem, 
enchyma cells and much smaller showing origin of youngest leat 
; primordium in the periblem; ¢, 
than the epidermal cells of the root qermatogen ; #, periblem ; A/, ple- 
(PL. I, fig. 5). Their walls are thin, rome; Z1, ZH, older leaves. 
and yet thicker than the walls of oe 
the interior cells, the free wall being slightly thicker than the others. 
Occasionally these cells contain a small amount of chlorophyll. 
As might be expected from a comparison with other submerged 
plants already investigated (Schenck, 1886), no stomata occur through- 
out the plant. 
In the epidermis, rather regularly distributed over the stem, but 
especially abundant in the region of the nodes, are secretion cells, 
which are, however, more numerous in the leaf, and will be de- 
scribed more in detail there (see p. 90). These cells occur scattered 
here and there also in the cortex, and some may be seen in the 
axial vascular bundle. 
(2) Cortex. 
The typical cortical parenchyma cells are large in comparison with 
the cells of the epidermis, endodermis and vascular tissue; they are 
Trans. Conn. Acap., Vol. XIV. 6 Decemper, 1908. 

