68 



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



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individual or in the same spike. Tliis fact shows that the boss spoken of belongs 

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

 ment. This, again, seems partly 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 

 nutlet be met with the boss is present. Partly 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- 

 Ihis, 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 

 these organs have but little sys- 

 tematic value. The number of 

 the verticils never surmounts 

 four in P. arAitifolius, generally 

 they are three, sometimes only 

 two, in the other species they 

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

 Another peculiarity as to the P. acutifolius is that usually 

 three pistils and the two outer stamens (sometimes also one of the inner ones) of 

 each flower fail, the blade of the perianths also being much reduced as to size and 

 of another form (see the fig. 25, G). This peculiarity unites this group closely to 

 the group of P. trichoides. In P. zosterifolius also such a reduction of the number 







Fig. 25. A, B, C, P. zosterlfoUiL'i Schum. D — G, P. acutifolius Link. — 

 j1, Transverse section of a stem-leaf a little beneath the middle, ^^^; B, Trans- 

 verse sect, of peduncle, showing three median vascular bundles, \'*; C, Pistil, 

 «, side-view, b, stigma from above, c, style from the back (X 8). — D, Trans- 

 verse sect, of a stem-leaf somewhat beneath the middle, ^^; E, Transverse 

 sect, of peduncle showing the two median vascular bundles, ^■^•, F, Pistil, a, 

 side-view, b, stigma from above (X 8); G, Stamen with reduced perianth, a, 

 from within, 6, outer sight, J. — jn, mid nerve, n', n", lateral nerves, I, la- 

 canar system, h, lower surface, sfr, mechanical bundles (marginal strand of 

 50 — 80 cells), vb, vascular bundles. 



tive form, see above ! 



