208 



J. O. HAGSTROM, CRITICAL RESEARCHES ON THE POTAMOGETONS. 



flexibility and toughness, which is effected by the narrow form of the central cylinder, 

 the fusion of bundles, the incrassation of the endodermis-cells, and the reduction 

 of bark. 



The close relation between P. lucens and gramineus is already pointed out by 

 E. Fries. It is obvious in the morphologic facts as well as in the anatomy and 



some biologic circumstances (rhizomatic 

 turios etc.). They also easily hybrid- 

 ize with each other, but the latter 

 is also the case in quite the same 

 degree or still more with gramineus 

 and perfoliatits. Any reason to unite 

 them into a common species is not 

 present and the cross-breeds grow 

 sterile in most cases. But there 

 are no reasons either for taking out 

 from the species and forming new 

 species of such forms as P. varians 

 MoR. and graminifolius Fryer, or 

 Wolfgangii Kihlm., ornig^'escens Fni'ES. 

 The specific differences of those so 

 called » species » are not, in reality, so 

 prominent as the descriptions give us 

 to know. Thej' belong all to the 

 varying power of the species and do 

 not go above it. 



Distrihution. Sweden. P. gra- 

 mineus is met with over the whole of 

 Sweden ; in the northern rivers the ff . 

 Wolfgangii and sepienirionalis are pret- 

 ty common. LiEST^Dius named these 

 and the hybrids of them with P. perfo- 

 liatus (see under P. nitens!) now P. 

 salicifolius, now P. lanceolatus, or P. 

 nigrescens. Plenty of specimens from 

 all provinces are preserved in hb. Stock- 

 holm., Uppsal., Lund., and Gothob. 

 A specimen from Paiala (Westbothnia), leg. LiEST. (hb. Stockholm.), bears the note 

 taljiin. X gramin. v. graminifolius? At. i Hn. ed. 12». Anything of P. alpinus is, 

 however, not to be found in this plant, being a true gramineiis-iorm. 



Norway, likewise through the whole country up to »Mager6en and Sydvaranger» 

 (Blytt, Norges Fl. 1902, 47). Materials from East Finmarkia (Elvenees, 64, Fries), 

 and other stations are in the museums of Stockholm, Uppsala, Lund, and Christi- 

 ania. — Finland, northward to Muonio, Enai'a etc. (hb. Stockholm., Uppsal., Lund.). — 



Fig. 102. P. f/ramincus L. A — E, Anatomical diagrams of the 

 central cylinder of the stem (from a single individual, gathered in the 

 lake M3;Snaren near Sodertelje, Sweden). A, nethermost internode (the 

 fusion of the lateral bundles is here more complete than what some- 

 times is the case; diameter of the internode: 2,1 mm). B, from the 

 middle of the stem (diameter: 1,5 mm). C, the internode next below 

 the primary spike (bundles separate, bundle-trio fused into a single 

 bundle like the opposite bundle; diameter 1,9 mm). D, from a pro- 

 longation branch of the second rank (a genuine prototypic diagram, 

 tf, bundle-trio), diameter: 2,2 mm. E, from a 4th rank branch (top- 

 branch), again with only two median bundles (eight-bundled diagram 

 type), diam. : 2 mm; all bundles hatched, ^'|^. F — G. P. (iraminpus 

 var. heteyophyllns Fe. f. 7ii{/r€sc€ns (Fr.) Hagstr. F, Floating leaf, -J. 

 G, Submersed leaf, y, outlined from the original plant of Arjeplong. 

 H — L, P. gyaminens L. Tops of yonngest branch-leaves, showing the 

 form and nervation, H, ^j, K, -f, L, -y. 



