112 A. H. Graves, 



d. Vascular System. 



This is represented by a single vascular bundle in the center of 

 the root, with a structure similar to that of the stem (PL VI, lig-. 27 ; 

 PI. VIII, figs. 46, 47). No longer a typical radial root bundle, its 

 present concentric structure admits of the same interpretation as 

 regards adaptation as does the simplified bundle of the stem (see 

 pp. 81—82). It differs from the stem bundle in only a few minor 

 details. Barel}' as large as that of the stem, in all cases observed 

 the root bundle retains intact the axial vascular area composed of 

 a few tracheae. These tracheae are peculiar in that they possess 

 no thickenings of an}^ sort. Their walls, although extremely thin, 

 can be distinctly seen in cross and longitudinal sections (PL VIII, 

 figs. 46, 47, tra). Schenck has observed a similar peculiarity in 

 the root of Potamogeton densiis. He says (1886, p. 61), " Die Ge- 

 fafie aber erfahren keine deutliche Differenzierung der Wandung; 

 auf Langsschnitten sieht man keine Verdickungen oder nur schwache 

 Spuren, so dafi die Gefafie als Gange erscheinen. tJberhaupt er- 

 fahren' in den Wurzeln der monocotylen submersen Gewachse die 

 GefalSe, obwohl sie iiberall sofort zu erkennen sind, keine weit- 

 gehende Ausbildung. Die Resorption der Querwande tritt meist 

 ein, bevor Verdickungen angelegt werden." 



Five or six sieve tubes, with their companion cells, are situated 

 at more or less regular intervals just inside the endodermis, forming 

 an irregular ring. These can be distinguished in cross-section by 

 their generally larger, nearly empty cell-cavities, their position 

 just inside the endodermis, and their accompanying companion 

 cells. In longitudinal section the sieve plates may be seen, as also 

 the accompanying companion cells with their protoplasmic contents 

 (PL VIII, figs. 46, 47 s). 



The remaining tissue of the bundle is parenchymatous, and on 

 account of its disposition in all parts of the bundle, it is impossible 

 to distinguish phloem and xylem parenchyma. It may therefore be 

 termed "connectingtissue" in accordance with Van Tieghem's (1870—71) 

 "tissu conjonctif" or Schenck's " Verbindungsgewebe " (1886, p. 59). 



e. Brief Comparative Study of Roots of Other Potamogetonaceae. 

 In general, Potamogeton pectinatus and Zannichellia palustris agree 



with Ruppia in the vascular structure of their roots. The main 

 difference is the presence of only one central trachea, which is 

 considerably enlarged (Schenck, 1886, p. 61, figs. 80, 81). I find 

 no reference in the literature to the cortex or epidermis of these 

 plants, but see no reason why it should differ to any extent 

 from that of Ruppia. Ver}- probablj- also other slender sub- 



