KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 55- N:0 5. 151 



Wilson (hb. Uppsal.), River Lligwy, 75, Bailey, and others (hb. Stockholm., Uppsal., 

 Lund.); Cambridgeshire, Burwell Fen, 80, Bennett (hb. Stockholm., Uppsal., Lund.). 

 — France, Niévre, 84, Gillot (hb. Lund.). - No other stations are known. 



P. stylatllS Hagstr., New Potamog., in Bot, Not., 1908, 98. 



I refer this species to t lie Alpini on account of the close relationship it seems 

 to have to P. alpinus, both in morphological and anatomical respect. And I catch 

 the opportunity to correct and complete the description of the stem-anatomy of the 

 plant after låter investigations of the materials (hb. Haun.). 



The anatomical difference between P. alpinus and stylatus seems to lie in the 

 facts (I) that the former has a large-roomecl epidermis without a strengthening layer, 

 whereas the latter possesses a one-layered pseudo-hypoderma and a narrow-roomed 

 epidermis, and (2) that the central cylinder of P. stylatus has an oval form instead of 

 the rounded shape outlined in the fig. 63, D, E. The bundles are arranged according 

 to the trio-ty pe (fig. 1, C) much as in P. alpinus. 



Subsectio 17. Amplifolii Hagstr. 



Caulis teres ssepe simplex. Folia natantia longepetiolata, breviter — brevissime 

 obtuseque mucronata; submersa ± petiolata, integerrima, saepe cuspidata, ut plurimum 

 cito putrescentia. Ligulai saepissime caducse. Stylus brevis, superne crassus v el incras- 

 satus. Fruetus magnus crassus lsevis carinatus vel interdum ecarinatus. Anatomia 

 eulis: O-endodeimis; fasciculi libriformes interdum adsunt, ssepius desunt. Fasciculi 

 centrales liberi, mediani duo, sa3pe quattuor. 



This group comprises a great deal of species endowed with larger or smaller 

 f loating leaves (except P. membranaceus m.,) and large fruits without bosses of t en 

 prominently keeled and more or less beaked. Styles, as far as known, usually short 

 and a little enlarged upwards. Stigmas oval or oblong often with a free part, which 

 in P. amplijolius is rather considerable and characteristic. This species has also more 

 persistent ligules with prominent ridges and richly developed sclerenchyma in the stem. 



Within this group we often meet with an arrangement of the vascular bundles 

 of the stele that must be considered to be prototypic. The median bundles belonging 

 to the second leaf, reckoned from beneath, and being in most species united into a 

 compound bundle with three phloem parts, are here often free (Ir in the figs. 76 

 and 80), whereby the median bundles grow four: the bundle trio and the opposite 

 bundle. The species with this prototypic disposition are the following: P. pulcher, 

 Fryeri, badioviridis, linguatus and amplijolius. This interesting fact, however, does 

 not prove anything of the age of these species, since the prototypic condition, 

 after extinction of many interjacent forms, in some respect or other, can return in 

 late arising species. At all events it must not be taken as an evidence that the 



