KUNGL. SV. VET. AKADEMIENS HANDLINQAR. BAND 55. N:0 5. 239 



of forms p. sierilis is difficult to distinguish from P. nodosus, when sterile, spargani- 

 jolius, subrufus and Schreberi. 



It differs from P. nodosus 1) by its stem which mostly has short branches 

 with short internodes and with more or less narrow and short, upward often 

 more or less coriaceous leaves; 2) submersed leaves usually long-stalked or with a 

 stalk-like, narrow base, entire and persistent; 3) floating leaves commonly 

 thinner in textiire and with sharper points, their petioles typically shorter 

 than the blade, usually scarcely half the length of it; 4) ligules in general very 

 long, ca. 5 — 9 cm, and more durable; 5) peduncles usually conspicuously incrassate 

 at the middle; 6) spikes always barren; 7) the anatomy of stem and petioles 

 always characterized by interlacunar bast- and vascular bundles; the epidermis of 

 stem always endowed with strengthening layer and scattered subepidermal strands; 

 interlacunarly in the stem cortex there are always several circles of bundles; the 

 endodermis consists of it-cells and the median bundles of stele are always or nearly 

 always furnished with separate xylem canals. 



Characters in distinction from P. sparganifolius : 1) stem often thicker; 2) 

 lower leaves broader, longer with shorter nerve-spaces; 3) upper subcoriaceous 

 (coriaceous) leaves with more elongated, acuter or sharply cuspidate blade, 

 shorter and broader petioles; 4) ligules longer, darker, stronger and more per- 

 sistent, usually more prominently bicarinate; 5) spikes mostly larger. By the stem- 

 anatomy, again, these hybrids cannot in general be distinguished, since both usually 

 have been considerably influenced by P. nutans. Vide also under P. sparganifolius I 

 P. nutans predominates also as to the smooth margin of the submersed leaves both 

 in P. sparg. and P. sierilis. 



From P. subrujihs it is easily distinguished hy the smooth margin of the sub- 

 mersed leaves, which besides are narrower, and by the stem anatomy (see under 

 P. subrufus!). P. sterilis also exhibits a richer development of floating leaves and 

 branches with subcoriaceous upper leaves, depending on its connection with 

 P. natans. 



The difference between P. sterilis and P. Schreberi may chiefly lie in the stem 

 anatomy and the floating leaves, which as to the latter liybrid are thus described: 

 foliis natantibus coriaceis basi rotundatis vel in petiolum attenuatis ovalibus vel 

 oblongolanceolatis rarius cordato-ovatis (Fischer). 



Most difficult may it, however, be to distinguish P. sterilis from P. crassifolius. 

 The difference must lie in the influence from P. gramineus which is to be traced in the 

 latter but not in P. sterilis. The Fryer specimens of P. crassifolius are present from 

 three stations: 1) Horseload Fen Drain, Chatteris; 2) The Engine Drain, Mepal, and 

 3) Doddington, all situated in Cambridgeshire, Co. 29, Distr. 7. In spite of eager 

 and careful investigation of the numerous examples from the two first mentioned 

 places I have not been able to discover anything distinctly referring to P. gramineus. 

 All properties point to P. lucens and nutans. The Doddington plants may neverthe- 

 less really be a hybrid gramineus X hicens X natans, since they have something in 

 their habit reminding of gramineus as well as of the two other species. 



