105 
forma darkehmensis n. f. : folia ca. 100 mm longa, ca. 
23 mm lata, basi rotundato-cordato, obtusa, superiora sub- 
coriacea folia summa iloralia petiolata, 
stipulæ intermedia, Stigmata oblonga iis 
P. alpini similia. 
The anatomy of the stem of this 
form approaches more closely to that 
of P. alpinus, by which it is distingui- 
shed from P. gramin. + perfoliatus 
which it else as to habitus and leaves 
often extremely likens. The leaf-apex is 
obtuse, in P. graminens + perf. in the 
contrary acute or, at least, obtusely cus- 
pidate. 
Only one growing -place is hi- 
therto known, viz. »in der Angerapp at 
Darkehmen, East-Prussia, where it is 
collected by H. Kuehn. 
2. ser. alpinifolia Hagstr. 9—12. Pot. alpi- 
, , _ , . uns X perfoliatus (P. 
forma pelphnensis n. t. P. alpinus prussicuS) H .). 9-10. 
Balb. var. obscurus II virescens Ca- Ser.abbreviatai.dar- 
. . kehmensis Hagstr. 9. 
spary ex Asch. et Græbner, Synopsis x op 0 f a s t e m-leaf 
der mitt. Fl. I. 1897. 312. p. p. — P. alpi- showing the course of 
. . . , . _ the longitudinal ner- 
nus Balb. var. d. virescens Asch. & Gr., ves _ ^ at s j ze jq. Base 
in Græbner, Potamogetonacæ. 1907. 74. of one of the topmost 
floral leaves. Nat. size. 
p. p.: caulis habitu P. perfoliati. Folia 11 — 12 . Ser. al- 
submersa quoad formant fere omnino pinifolia f. pelplinen- 
. , , . . . , sis Hagstr. 11. Top of 
ut in P. alpmo sed margine denticuhs a branch-leaf, showing 
unicellaribus caducis instructa; folia the course of the lon- 
_ , , ... gitudinal nerves. Nat. 
tloralia lanceolata subsessiha, summa s j ze 12 . Topmost flo- 
subcoriacea. ra l l eaf - Nat - size. 
Anatomy of the stem as that of P. perfoliatus. This 
species is seldom lacking some fetv small bastbundles in 
the cortex (1 — 2, seldom more, subepidermal and sometimes 
1—2 strands or so farther in). And indeed we meet with 
in the hybrid-plant the same fact, two small cortical strands. 
