390 



what in structure, the former having become rhizome-hke by 

 the anastomosing bundles coming closer to each other, so that 

 the cortex is thicker than in the internodes. The cortex and the 

 pith are not broken down and lignification often occurs in the 

 latter in the Danish specimens. The bundles are without stereom. 

 The internodes are somewhat different in structure in the Danish 

 specimens, according as to whether the land-or the submerged 

 form is in question. The bundles in the latter are not accom- 

 panied by stereom , while the former has an external weak, 

 and an internal stronger arch of bast and of lignified parenchyma 

 respectively (Fig. 34, Ъ and vp). The specimens from Greenland 

 I, were similar in structure to the Danish land- 

 form. The bundles number from three to 

 seven. The vessels are few in number, but 

 large. Some small-celled parenchyma occurs 

 between the vessels and the inner arch, and 

 Fig. 34. R. repfans. g^^^g wood-parenchvma between the vessels 



Transverse section of an 



thlTanï-foîm^'(Denmark ^nd the sicvc-tissue. No interfascicular ligni- 



-°/i); ft, bast; ph, leptome; ^ j , rnu л j • • 



a;, xylem,; p, parenchyma, ued раГеПсЬута OCCUrS. Ihe eudodcrmiS IS 

 vp, wood-parenchyma; I. 



lacuna. indistinct. The pith is broken down in all 



the specimens which have been investigated from all localities. 

 The epidermal cells have only slightly thickened walls, but 

 they are thicker in the land-form than in the submerged form, 

 and the former has also a more distinctly striped cuticle than 

 has the latter. The living cortex consists everywhere of but 

 2 — 4 layers. 



The structure of the leaf varies greatly according to ex- 

 ternal conditions. As there is a gradual transition from the 

 submerged, linear leaf to the linear-lanceolate leaf of the land- 

 form, so there is also from the nearly radial to the dorsiventral. 

 The dorsiventral leaf (Fig. 35, Ä) has one palisade-iayer the 

 cells of which are barrel-shaped and somewhat irregular; they 

 form about one-half of the thickness of the leaf. There is a 

 gradual transition to the spongy cells, which are slightly bran- 



