16 



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



B end 



sept 



D 



central cylinder of the peduncle exhibits the same structure as the upper portion of 

 the stem, but the lateral bundles are often still more reduced or have disappeared. 

 The anatomy of the leaf (fig. 4, D) exhibits a median vascular bundle surrounded 

 by small air-channels, on either side of which one always meets with a row of wider 

 lacunse and subepidermally a faint bundle in the very margin, the leaves accordingly 

 being three-nerved, although to the naked eye only one-nerved. The so-called cross- 

 veins are the extremely thin cross-walls of the lacuna?. 



The species varies rather much, though considerably less than P. pectinaius. 

 The upper part of the style (not the stigma) grows often in this species and in P. 

 strictus somewhat protracted backwards forming a little beak. The fruit of the usual 



Baltic form as well as of most 

 forms both from brackish and 

 fresh water is small, but there 

 also occur large-fruited forms 

 especially on calcareous mud 

 end (formce megacarpce, P. pycnosle- 

 gus KiHLivi.), in which the walls 

 of the very stone grow thicker 

 than they usually do. But the 

 shape of the lid and the ven- 

 tral part is unaltered. The lid 

 is always rounded on the back, 

 of about the same form drawn 



Fig. i. P. filiformis Peks., A, Transvorsc section of stem, upper part, show- in tlie fig. 116, H. Tlie leaf- 

 ing two barkbundles (str), central stele outlined, \°. D, Transverso section . . . . .. _ _ , 



of central cylinder, more enlarged (outlined) exhibiting two median bundles and WlCltll VarieS sllglltly irOIU 0,5 — 1 



two lateral ones, a, b-b corresponding to a, b-b in the cross-section of the leaf mrn nr\A flio cliooflic linlrl pnm 



(below) and of the sheath (fig. 3, B). C, Transverse section of the peduncle IIUI1 d,lu Ule »»eaiUS IIOICI GOIll- 



with fonr bundles (outlined). D, Transverse section of a stem-leaf middle part, monlv 5 15 111111 ill ldl^tll 



V. E, Longitudinal section of the epidermis of the stem, B , li . F, Longitudinal sec- •' > _ ® 



tion of leaf-epidermis, r i . ep, epidermis, hp, pseudo-hypoderma, in the stem The differeilt habit of tlie Speci- 

 one-layered, in tho leaf two-layered, /, lacuna', x, xylem-canals, p, pith, end, . , 



endodermis, sept, cross-walls of the lacann consisting of very thin stellnlar cells, IHenS mOStlV QepeilClS 011 tiie 



ho, longitudinal waiis ce, stele. variations in lengtli of the inter- 



nodes and peduncles. 

 Synopsis of the forms and varieties. 

 I. Narrow-leaved forms. Stem-leaves usually 0,5—0,75 mm broad. 



1. f. vulf?aris Tiselius, Potamog. Suec. exs. fasc. 111, 1897 n. 111 — 112: — The 

 common Baltic form which also occurs in many other currents of water; usually of 

 low growth (10 — 15 cm.) but with elongated peduncle (± 10 cm), small-fruited; to 

 be considered as the typical form of the species. — Fig. 3, E. 



2. f. luxuriosus Hagstr., Potamog. in Neuman, Sveriges Flora 1901, 794: — 

 Ca. 30 cm altus, foliis longissimis 0,6 mm latis, spicas valde superantibus. — On 

 ground rich in nourishment. 



This form is met with in Sweden, the vicinity of Stockholm: Vermdö, Vikings- 

 hill, 1897, Hagström. — Lapponia rossica, Varsuga?, Tsohawanga in aqua dulci, 89, 

 Kmr.MAN (hb. Stockholm.), very like /. alpimts Bl. — North America, Riv. St. Mary, 

 Midi., U. S. A., 82, Morong (hb. Stockholm). 



