14'6 



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



grammeus var. graminifolius Fk. by S. Almqtjist in Bot. Not., 1889, 66, are not this 

 hybrid, see under P. gramineus L. — Norivay, Senjen, Bryggehaug, 10, Noto. 

 P. nericius must until further be looked upon as a most rare plant. 



P. alpimiS Bale. X natailS L. (P. Driicei Fryer, Potamog. of the British Isles, 



1898, 31, t. 21). — Fig. 66. 



There can scarcely be any doubt of the hybrid 

 nature of the Berkshire plant named P. Drucei (Fryer 

 1899, as a full species). The large floating leaves di- 

 stinctly indicate that P. nutans is to be considered as 

 one of the originators. A closer investigation of all 

 the organs of the plant does not contradict the sup- 

 position either. But which the other parent species 

 may be is not quite a matter of course. The first 

 proposition of Druce on P. alpinus, however, seems 

 to be favoured with all probabihties. The long petioles 

 of the floating leaves certainly seem to be adverse to 

 it, but they are likely to originate from local condi- 

 tions (River Loddon). The leaf-base is sloping as in P. 

 alpinus and in the river-form of P. naians which also 

 has long petioles. The floating leaves besides appear 

 very early, already below the primary spike, which 

 confirms the supposition that either of the two parent- 

 species is furnished with coriaceous leaves. The sub- 

 mersed leaves are also long-stalked as the other natans- 

 hybrids. The ligule can be said to be intermediate, 

 75 — 85 mm long (P. nat.), brown (P. alp.) with faint 

 ridges. The styles and stigmas remind of P. polygoni- 

 folius almost more than of nutans, but the smallness of 

 the stigma, especially, may depend on deformity, a 

 usual occurrence in the hybrids. We are so much the 

 more inclined to think so, having seen specimens culti- 

 vated in the Botanical Garden of Copenhagen with al- 

 pmMs-stigmas. 



A cross-section of a spike-bearing stem shows a 

 structure very obviously confirming the presumption 

 Fig. 66, p. Drucei fkyer. A, Made about P. ulpinus and nataus as the true origin of this 

 ot the youngest leaf of a spike-bearmg hybrid. Eudodermis and central stele nearly typical 



slioot, sloping at the base, rather thin, \. ^ J J i. 



B, The lowest leaf but one, on a barren P. alpinus, cpidcrmis witli a pseudo-liypoderma and 



shoot from the Bot. Garden of Copenha- inii ti • ^^^ ii-r. 



gen, |. scattered bark-bundles inliuenced by P. nutans. Yet 



