KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 55- N:0 5- 



147 



P. alpinus has considerably reduced the number of the bundles, and subepidermal 

 bundles are quite absent. Interesting and absolutely decisive is the fact, that the culti- 

 vated indviduals of the original P. Drucei above mentioned vary in the stem-anatomy 

 in such a manner, that the endodermis and central cylinder answer to P. natans, 

 whereas the hypoderma and the cortical strands are quite absent as in 

 P. alpinus. In the former case P. alpinus has forced its way to the 

 central parts of the stem, P. natans, on the contrary, has mastered 

 the periphery. In the latter the fact is the very reverse. Both cases, 

 taken together, are the most excellent prove that can ever be exhibited, 

 that the two said species are represented in P. Drucei. 



The following facts forbid the combination P. natans X polygoni- 

 jolius: the hybrid is always destitute of subepidermal bast-bundles, and, 

 at least sometimes, furnished with an O-endodermis both of which could 

 not occur, provided that P. natans is the one of the parent-species (against 

 Fryer, 1907, in Graebner, Pot.). 



Distribution. England, Berkshire, River Loddon, 04, Druce (hb. 

 Stockholm., Lund.). 



P. alpinus Balb. x nodosiis Poir. (P. rectifolius Ar. Bennett, 



Notes on Potamogeton, in The Journ. of Bot., 1902, 147, as P. ameri- 

 canus X alpinus. — P. subobtusus Hag ström). — Fig. 67. 



The materials of this hybrid here accessible consists of a barren 

 and a fruiting shoot, both simple, the latter with a stem-prolongation 

 of two spike-bearing and one undeveloped branch of lst— 3d rank. 



Stem terete; basal internodes 2—3 cm, above 4 — 6 cm long. 

 Lower leaves sessile, but with a stalk-like base, narrowly lanceolate. 

 The four leaves next below the primary spike long-petioled and with 

 narrowly sloping blades (see the fig.!), the apexes of which are even 

 and obtuse. In the whole these leaves are 14—15 cm long, the blade 

 ^~^ mm, more or less coriaceous. The involucral leaves and the leaves 

 of the stem-prolongation are nearly similar to those just mentioned, 

 coriaceous but thin and narrow as usually in P. alpinus. The barren 

 shoot has the uppermost leaves endowed with longer petioles (100 mm) 

 and slowly sloping blades (100 mm), thus more intermediate. Ligules 

 deciduous, 50 — 60 mm long, without or with very low keels at the base. 

 Peduncles stout (P. nodosus), 100 mm long; spike deflorate 35 — 45 mm 

 long (P. nodosus). Style as in P. nodosus. Pollen sterile to the greatest 

 part. The stem shows the same anatomic diagram as the two parent species, the 

 structure of which is essentially the same. 



Distribution. N. America, »Not. Ebor. , E. Tuckerman Jr (hb. Upps.). — 

 The original habitat of P. rectif. is Stony Island, Chicago, 111., where it was gathered 

 30.8.01, by Rev. E. J. Hill. A specimen kindly sent from the discoverer has, 



Fig. 67. P. sitb- 

 obttt tus Hagstb. 

 One of the fonr 

 Imh es below the 

 primary spike. \. 



