142 



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



the characteristics of the species: the coriaceous floating leaves with petioles shorter 

 than the lamina and the compressed fruits (Koch), the obtuse, submersed leaves, 

 the obovate floating leaves with sloping base (Cham.). In our days the turios and 

 stem-anatomy (Raunki^e), the stigmas (Almquist) and styles (Hagsteom) have 

 been described more or less completely. 



The stem prolongs itself by branches of as far as the fifth rank. The ana- 

 tomy of the stem is constant whether the plant is growing in running or stagnant 



water. I have not been able to observe 

 D any other change than a fusion or divi- 

 sion, within certain limits, of the cen- 

 tral cylinder's vascular bundles. The 

 epidermis lacks a hypoderma and its 

 cells are large-roomed and stretched, 

 5 — 15 times the width. Bast-bundles 

 do not occur in the bark neither sub- 

 epidermally nor interlacunarly. The en- 

 dodermis always consists of 0-cells and 

 the central stele has usually six lateral 

 and two median bundles, of which 

 the one is composed of three (a trio- 

 bundle). This trio-bundle sometimes 

 dissolves into a bundle-trio and some 

 one or other of the lateral bundles 

 often splits into two (see the fig. Dl). 

 The earlier authors (Camisso etc.) 

 were of the opinion that the species ty- 

 pically developed floating leaves and held 

 the form, endowed with such leaves as 

 main-form. This is also, no doubt, cor- 

 rect. Where the species accidentally lacks 

 floating leaves, it depends on unfavour- 

 able external facts, as rapid streaming or 

 swelling of the water during the florifica- 

 tion.yn such cases a leaf-form intermedi- 

 ate between submersed and floating leaves, is developed which I have called amphibial 

 leaves, a biologic occurrence by which the species adapts itself to the existing external 

 facts. Because of that it is more appropriate to rank the variations according to the 

 shape of the submersed leaves (Chamisso, Hartman, Fries etc.) than to the appear- 

 ance, presence or absence of the floating leaves (Fieber and others). Chamisso sepa- 

 rates from the main-form a "forma angustijolia, quae P. alpimis et anmdatus Auct.,io\iei 

 habet submersa summa 4V2 poll, longa 5 lin. lata* etc. and a "forma latifolia quae P. 

 purpurascens Seidel, folia possidet submersa 6V2 poll, longa 15 lin. lata» etc. (Linnsea, 

 1827, 211 — 212). Thus the main-form becomes the commonest one with the submersed 



Fig. 63. P. alpiniis Bax.bis. A, Floating leaf, |. B, Pistils, a, 

 side-view b, from the inner side, f-. C, Longitudinal section of the 

 frnit showing the embryo (a), the putanien (endocarx^) with its thorn 

 (6) and the epicarp, c, -^. D, Transverse section of a central stele 

 with more than six lateral bundles, t>% compound bundles belonging to 

 the second leaf upwards, c/r, opposite bundle, and fb, foliar bundle, 

 belonging to the leaf next above ^. E, Part of a transverse section 

 of the stem, \^, ejo, epidermis, end, endodermis, /, lacunar system. 

 F, Top of a young ligule, "-, b — b, chief nerves, ic, front-field, ivg, 

 wing-field. 



