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



endodermis of lucens X jjrcelongus is usually more conspicuously one-sidedly thickened 

 than in lucens X perfoliatus. Further, see P. Bahinglonii! P. decipiens sometimes 

 lacks subepidermal bast, but when some scattered strands, as usually is the case, 

 are present they always appear near the epidermis in contradistinction to the other 

 hybrid. The cortical bundles do not generally appear so near to the endodermis as 

 in P. Babinglonii. The bast is often reduced to 1 — 2 circles by influence from P. ferfo- 

 liatus, which is devoid of sclerenchyma. 



5) There is no doubt at all that the Swedish forms, preserved in our herbaria 

 and Museums under the names of P. decifiens and P. wpsaliensis, and by Tiselius, 

 in his Potamog. suec. exs., II, 1895, distributed under the numbers 68 — 74, 77 — 84, 

 belong to a single hybrid. And all the German forms I have seen, named P. deci- 

 piens, those also distributed by Nolte himself, belong to the same hybrid as the 

 Swedish forms. As regards the Fyris forms (P. upsaliensis), most reminding of P. 

 prcelongus, Tiselius remarks, 1. c, p. 20, that P. prcelongus is not met with in that 

 river. On the other hand P. lucens as well as P. perfoliatus are growing there abun- 

 dantly. Having once seen a true lucens X prcelongus and studied its peculiarities (the 

 aspect of the leaf-margin, the very apex of the leaves etc.) you will soon be able to 

 distinguish between this very rare hybrid and the much commoner P. decipiens 

 Nolte. 



6) The Noltean name of P. decipiens ought to be used of this hybrid with simi- 

 lar right as the P. nitens of Weber is used of all forms of the hybrid P. gramineus 

 X perfoliatus. The name salicifolius ought not to be used as series-name for the 

 long-leaved decipiens-iovms, since P. salicifolius according to the description as well 

 as several by Wolfgang himself distributed specimens (hb. Petersb. etc.) belongs 

 to the hybrid gramineus X p)erfoliaius. 



7) Three series of forms may properly be established: 



a latifolius n. var. — Folia caulina superiora ca. 100 mm longa, 25 — 40 

 mm lata. 



Hereto belonging forms are: 



f. typicus Tis., 1. c, no. 70: The Noltean specimens, see below! 



f. major Tis., 1. c, nos. 68 — 69: — Folia latiora, ca. 40 mm. 



f. transieiis Tis., 1. c, nos. 71 — 72 and 77 — 78 (P. upsal. f. major): — Folia 

 subelongata, ca. 120 mm. 



p brevifolius n. var.: — Folia breviora, angustiora. 

 f. pulchellus Tis., 1. c, nos. 73—74: — Foha 65—75 x 15—20 mm. 

 f. lithuanicus (GoRSKi) n. f. — P. upsaliensis Tis. (3 jdsevadensis Tis., 1. c, nos. 

 82-84: — Foha 70— 90(— 120) x 12—18 mm. 



Y longifolius n, var.: — Folia longiora subangusta, 150—200 mm longa, ± 20 

 mm lata. 



f. upsaliensis (Tis.) n. f. — P. upsaliensis Tis. f. genuinus Tis., 1. c. nos. 79 

 —80 et f. intercedens, no. 81: — Folia caulina (superiora) basi rotundata. 



