KtTNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 55. N:0 5. 



145 



P. alpinus Bale, x graiiiiiieus L. (P. nericiiis Hagstr.). — Fig. 65. 



The few specimens of this hybrid hitherto found can 

 be described as follows: 



Caulis teres subcrassus (ut in P. alpmo) simplex vel (ad 

 medium) ramulo debilissimo (interm.) instructus. Folia sub- 

 mersa lanceolata, superiora + petiolata laminis decurrentibus 

 apice ± obtusa, ramulina apice siibcuspidata ± obtusa, omnia 

 integerrima; natantia lanceolata obtuse cuspidata, petiolata 

 et laminis decurrentibus ut in P. alpino. Ligulae intermedise, 

 debiliores quam in P. gramineo, subbicarinatse, fuscovirides. 

 Pedunculus vix incrassatus. Spica sterilis. 



The colour of this plant reminds of P. gramineus more 

 than of alpinus, but in habit it resembles most the latter, 

 although the stem, at the middle, usually has a short and 

 very faint branch (influence from P. gramineus). Very signi- 

 ficant are the dark-green submersed leaves the structure, 

 nervation, and smooth margins of which are most like those 

 of P. alpinus. Their tops taper gradually or end with a slight 

 cusp and are always a little more obtuse than in P. grami- 

 neus (influence from P. alpinus). The floating leaves resemble 

 as to stalk, and the size and form of the blade, nearest P. 

 alpinus, but the apex is intermediate with a very faint and 

 obtuse cusp. All the ligules are paler than in P. alpinus, 

 which they otherwise resemble as to structure, the basal nerves 

 being by far fainter and the keels lower than in P. gramineus. 

 The styles are not in such a state that they can be examined. 



The anatomy of the stem: Cortical strands as in P. 

 gramineus, sometimes, however, a little weakened. The arrange- 

 ment of the vascular bundles of the central cjdinder varies, 

 in the common way of the hybrids, from more alpinus-\i\ie 

 to almost quite gramineus-s\\a.\}%A. Endodermis of one-sidedlj' 

 thickened it-cells as has P. gramineus, but influenced by P. The medmn bundles with common 



. . . . xylem-cavity ;is in P. qraiiiiiieiis, 



alpinus the suberisation is somewhat weaker, ihe epidermis the lateral ones free (in d) as in 

 is destitute of a hypoderma and its cells approach, as to the graJine'i"'. " ' ^ '"'" 

 length, the measure usual in P. gramineus. 



Distribution. Sweden, Nericia, Gotlunda in Hasta-rivulet, 82, Tiselius (hb. 

 Stockh.) growing together with the parents: P. gramineus f. fluctuans Tis. and an 

 alpinus-iorm with elongated upper internodes and large floating leaves. Several 

 other Scandinavian and North European plants have falsely been taken for this 

 hybrid. P. gracilis Wolfg., P. Wolfgangii Kihlm. and specimens named P. alpinus 



K. Sv, Vet. Akad. Handl. Band 55. N;o 5. 19 



Fig. 65. I'. aljiiiiKS B.4LB. X 

 fframinetts L, (P. nericiits Hagstr.}. 

 A, Top of a submersed stem-leaif, 

 |. B, Top of a branch-leaf, a 

 little enlarged. C, Floating leaf, \. 

 D, E, Diagrams of the central cy- 

 linder from different individuals. 



