The Morphology of Ruppia Maritima. 81 
in the central lysigenous opening. In sections of the stem it ap- 
peared as if tracheids were also present, especially at and near the 
nodes, possessing the same characters as to their thickenings as 
the tracheae, but showing cross walls (Text-fig. 7a, 6). 
Surrounding the central cavity are thin-walled xylem parenchyma 
(Pl. I, fig. 3). Just outside of these lies the phloem region, the 
boundary between the two being impossible to determine on account 
of the apparently identical characters of the xylem and phloem 
parenchyma. 
In the phloem zone are distributed the sieve tubes, with their 
companion cells, in an irregular ring in much the same way as in 
Zannichellia (Schenck, 1886). As a rule, the sieve tubes are larger 
than the surrounding cells, and on this account and because of 
their arrangement in a ring, each with its companion cell, they 
stand out quite clearly even in 
freehand sections. Between them seas 
ay sale tort r) 
, eemte lanht 090%." 
ONE Easy 
and the endodermis lie one or tig 
occasionally two layers of par- 
enchyma cells (Pl. I, fig. 3). 
One noteworthy feature of the 
phloem region is that in the vic- 
inity of the node, i. e. immediately 
before and after the branches are 
given off from the central cylinder, 
it is localized into four distinct 
symmetrically situated portions 
(Text-fig. 8), although this is never 
evident in the internodal region. 
Such an arrangement would be 
expected when one considers the 
true nature of this fibrovascular 
area, as described below, and 
compares it with certain Pota- 
mogetons which reveal this char- 

Figure8.— Photomicrograph of cross 
section of stem, &c., similar to PI.IIT, 
fig. 8, but cut at a higher plane and 
acter more distinctly (Schenck, 
1886. pp. 40-41 and figures 35, 
36and 38), and with Zostera marina 
(Chrysler, 1907, p.172 and fig. 29). 
near a node; showing localization of 
phloem into four regions in central 
cylinder of stem. >< 35. 
In his work already cited, Schenck (1886, pp. 27 ff.) reviews in 
a most admirable manner the various interpretations of the concen- 
tric type of bundle as it exists in water plants. 
He concludes with 
Sanio and Russow that it represents phylogenetically not a sing], 
