18 81. FLUVIALES. 



1129. POTAMOGETON HETEROPHYLLUS, Schreb. 



Area * 2 3 4 5 6 7 8 9 10 11 * 13 14 15 IG * 18. 



South limit iii Dorset, Kent. 



North limit in Shetland, Orkney. 



Estimate of provinces 16. Estimate of counties 60. 



Latitude 50 — 61. British tj^pe of distribution. 



Agi'arian region. Inferagrarian — Superagrarian zones. 



Descends to the coast level, in Channel or Thames. 



Ascends to 100 or 200 yards, in England. 



Range of mean annual temperature 50 — 45. 



Native. Lacustral. It is to he feared that the locali- 

 ties of this species and those of P. oblongus (of British 

 botanists) are much confused in our records ; and, indeed, 

 when dra'mi out in running water, some of the plants 

 appear so intermediate that it becomes diiBficult to assign 

 a name to them with confidence. While, on the one side, 

 tliis greatly resembles the fluitant forms of P. oblongus, 

 it approximates so closely to P. lanceolatus on the other 

 side, that some of the localities reported for the latter, 

 may be deemed almost certainly to belong to P. hetero- 

 phyllus. According to Koch and Fries, this is the true 

 Linnean P. gramineus ; and yet there appears nothing 

 grassy or gi-ass-like about it, to suggest or to justify the 

 specific name ; while the P. gramineus of British botanists 

 has somewhat grass-like leaves, and would seem more 

 appropriately to receive that name. Perhaps this species 

 may be foimd more nearly general than is above indicated, 

 and also at a higher elevation. 



