68 



J. O. HAGSTROM, CRITJCAL RESEARCHES OX THE POTAMOGETONS. 



individual or in the same spike. This fact shows that the boss spöken of belongs 

 to the species as a disposition, although it more seldom arrives at its full develop- 

 ment. This, again, seems partlv to depend on the room in each case. Where, in 

 a flower, all the four pistils have grown into fruit, the boss is lacking; if only one 

 n ut let be met with the boss is present. Partlv it may depend on the fact mak- 



ing the fruit microcarp or caus- 

 ing the partial failure of fruit, 

 viz. the reduction of numbers 

 of the median vascular bundles 

 of the peduncle. In P. acutifo- 

 lius, moreover, the rostrum is 

 more facial than in P. zosterifo- 

 lius, where it generally is more 

 apically situated. The two spe- 

 cies can thus often, but by no 

 means always, be distinguished 

 by the ripe fruit. 



In P. zosterifolius the top- 

 most part of the style is some- 

 what protracted backwards, 

 which I have not observed in 

 the parallel species. The stigma, 

 therefore, is also more elongated 

 in the former than in the lat- 

 ter. The pollen-grains of P. 

 zosterifolius are always conside- 

 rably larger than those of P. 

 acutifolius. The form of the 

 perianths has not the constancy 

 authorizing a specific difference 

 to be founded on it. Generally 



--str 



these organs have but little sys- 

 teinatic value. The number of 

 the verticils never surmounts 

 four in P. acutifolius, generally 



Fig. 25. .1. /;. C, /'. fosterifoliua Sohuk. I> O, P. acutifolius I.ink. — 



i Tranaverse Becl I a stem-leaf a Little boaeath the middle, */>; B, Trans- 



i. of pednncle, showing three median vascular bandlea, V" ''■ Pi»tU| 



i.i.i from above, e, style from the back (X 8). — D, Trane- 



■i ol a stem-leal äomewhal beneatb the middle, S |"; /••', Trausveree 



.,i pedanole ahowing the two va dian yaaoular bandlea, \ : ~; I', Piatil, a, 



aide-view, 6, atigma from above i 8); G, Btamen with redoced perianth, a, 



from within, b, ontei light, ;. — m, mid nerve, »', n", lateral nerrea, /. la- they are three, SOIlietimeS Ollly 



i ayatem, Is. lower aarface, »tr. meohanical bandlea (marginal strand <>f . ., ,, ,, 



50 30 cells), vb, vascnlar bnndUa. tW0, 111 tllC Othcr SpeClCS they 



are 7 to 9 (12). On the abor- 

 fcive fonn, see above! Auother peculiarity as to the /'. acutifolius is that usually 

 three pist ils and the two outer stamena (sometimes also one of the inner ones) of 

 each flower lail, the blade of ihv perianths also being muoh reduoed as to size and 

 of another form (see the fig. '2~k O). This peculiarity unitee (lus group elosely to 

 the group of /'. trichoiäea. In /'. zosterifolius also such a reduction of the number 



