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 

 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 perfoliatus, A ny reason to unite 

 them into a eommon 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., or nigrescens Fri es. 

 The specific differences of those so 

 called »species» are not, in reality, so 

 prominent as the descriptions give us 

 to know. They belong all to the 

 varying power of the species and do 

 not go above it. 



Distribution. Sweden. P. gra- 

 mineus is met with över the whole of 

 Sweden; in the northern rivera the ti. 

 Wolfgangii and septentrionalis are pret- 

 ty eommon. L.i:st.i;i>h's named thesc 

 and the hybrids of them with P. perfo- 

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

 salicifolius, now P. lanceolatus, or P. 

 nigrescens. Plenty of speoimens from 

 all provinees are preserved in hb. Stock- 

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

 A specimen from Pajala (Westbothnia), leg. L.est. (hb. Stockholm.), beara the note 

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

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



Norway, likewise through the whole country up to »Mageröen and Sydvarangei 

 (BLYTT, Norges Fl. L902, 47). Materials from Kast Finmarkia (Flven;es, (>4, Fkii:s), 

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

 ; , ni.i. Finland, northward to Muonio, Enara etc. (hb. Stockholm., Uppsal., Lund.). 



Fig. 102. P. gramineus L. A — E, Anatomical diagrams of the 



central cylinder of the atem (from b aingle individnal, gathered in the 



lake M&snaren near Södertelje, Sweden). A, nethermost internode (the 



fusion of the lateral bnndleB ia here more complete t han what some- 



i- the casi ; diametei of the internode: 2,1 mm). Il, from the 



middle of il'" stem (diameter: 1,5 mm). C, the internode nexl belo* 



il., primär} Bpike (bondlea Beparate, bundle-trio fased into a singlo 



i, un. II. like the opposite bnndle; diameter 1,9 mm). D, from a pro 



longation branch of the Beoond rank (a gennine prototypic diagram, 



//■, bnndle-trio), diameter: 2,2 mm. /•-'. from a 4th rank branch (top- 



branch), again with only two median bnndlea (eight-bnndled diagram 



diam.: 2 mm; all bnndlea hatched, •'■," ' '•• P. gramineus 



heterophyllus Fb. t. nign een (Fb.) Bagstb. F, Floating leaf, \. 



<;, SmI iraed leaf, J . ontlined from the original plant of Arjeploug. 



// /,. /-. gramineus I.. Topa of yonngest branch-leaves, showing the 

 form and nervation, //. ',", K, \. I,. \. 



