262 J. O. HAQSTEOM, CRITICAL RESEARCHES ON THE POTAMOGBTONS. 



in the groups 1—25 (also in the Hyhridi), the latter in P. densus only. By this 1) 

 the seed gets another shape than in all the other species, and 2) the cavity of the 

 pericarp the above-said shape, 3) the lamella the said short form, as it extends onty 

 to the point where the sack-like expansion begins. 



The embryo has a more or less curved form, inasmuch as the cotyledon has 

 a tendency to coil itself like a spiral. The most usual case is that the embryo is 

 curved like an angling-hook, f of a turn (Fig. 63, C), more seldom in a complete 

 circle (f. i. P. Rohbinsii), or coiled li turn (P. spirilliformis), H turn (P. Nuttallii 

 Fig. 62, D), 11—11 \{P. dimorphus). In P. densus, again the embryo is coiled 

 3—4 turns, but the spiral is enclosed by the testal sack, the whole reminding of 

 the coiled embryo of the Zannichellice. 



As regards the stem-anatomy P. densus differs from all the other species by 

 its endodermis-cells, which do not show any tendency to get thickened walls. Be- 

 sides, the stem is characterized by the perfect absence of mechanical tissue. The 

 two median bundles of the stele unite into a single one (oblong type), but in the 

 peduncle the primordial condition again appears, for which reason we there find 

 four bundles, two median and two lateral ones all approached to each other, but 

 without a distinct common endodermis. This arrangement reminds of P. perfoliatus 

 and of the Pusilli and nearly related groups. The stem is destitute of, but the 

 peduncle is endowed with a pseudo-hypoderma. 



The different forms are most conveniently distributed on the three varieties 

 of old: a. latifolius Wallr., P lancifolhis Meet. & Koch and v angustijolius Wallr. 



Distribution. The most northern locality of ^. densus is, in Siveden,ll3l\sin&: 

 Sondrum in MoUegardsbacken, 74, Neuman and others (hb. Stockholm., Lund.), 

 where the plant is to be considered a remainder from a milder climate. — Norway, 

 Fredriksstad, 83 (hb. Christian.). — To the south of the Scandinavian peninsula it 

 spreads through Denmark, Germany etc. to Italy (hb. Christiania) and North Africa, 

 Algeria, El-Kantara, 96, Murbeck & Olin (hb. Stockholm., Lund.). Westward it 

 occurs in England at many stations (hb. Stockholm., Uppsal., Lund.), the most 

 northern known to me is Howden, 45, Storey (hb. Lund.). How far it ranges in 

 Asia is not fully known. I have seen it from Armenia turcica, coll. Sintenis (hb. 

 Lund.). But it is also recorded from countries east of it: Afghanistan and North India. 



On p. 127 I have mentioned a plant from Lake Quinsigamond, Mass., U. S. A., 

 which I consider a hybrid of P. pusillus with P. Vaseyi. Daring the printing of 

 this treatise I have seen another most interesting plant, which has grown together 

 with P. dimorphus in the same lake. This is evidently a hybrid of P. dimorphus 

 with P. pusillus and may briefly be described as follows: 



