The Morphology of Ruppia Maritima. 



81 



in the central lysigenous opening. In sections of the stem it ap- 

 peared as if tracheitis 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. 7 «, b\ 



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, the}- 

 stand out quite clearl}^ even in 

 freehand sections. Between them 

 and the endodermis lie one or 

 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 fibro vascular 

 area, as described below, and 

 compares it with certain Pota- 

 mogetons which reveal this char- 

 acter more distinctly (Schenck, 

 1886. pp. 40-41 and figures 35, 

 36 and 38), and with Zostera marina 

 (Chrysler, 1907, p. 172 and fig. 29). 



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 single 



Figures. — Photoraicrograpli of cross 

 section of stem.&c, similar to Pl.III, 

 fig. 8, but cut at a higher plane and 

 near a node ; showing localization of 

 phloem into four regions in centi-al 

 cylinder of stem, x 35. 



