176 J. O. HAGSTEOM, CRITICAL RESEAECHES ON THE POTAMOGETONS. 



tago Bastard, Essai sur la flore dii dep. Maine & Loire, 1809, 64. — P. parnassi- 

 folius ScHEADER apud Mertens & Koch, Deutschlands Flora, 1823, 839, — P. uli- 

 ginosus et affinis Boemiinghausen ex Cham. & Schl., Linnaea, 1827, 216. — P. 

 paludosus BoRY ex Cham. & Schl., 1. c., 216. — P. LeschenauUii Cham. & Schlecht., 

 1. c, 223? — (P. elegans Wallich, Catal. 5178, 1828, ex Bennett, Journ. of Bot., 

 1893, 133 — 134 and P. digynus Wallich, 1. c., 5177, ex Bennett in hb. Haun., but 

 see above, p. 156). ■ — P. Hornemanni Meyer, Chloris Hanoverana, 1836, 521. — 

 P. cyprifolius Lowe, ex Graebner, Potamog., 1907, 65. — P. natans c. media, et d. minor 

 Koch et Ziz, Catal. pi. Palat., 1814, 18. — P. natans (3 acaulis Wahbeeg, Flora 

 Gotlioburgensis, 1820, 23. — P. natans S. iniermeditis et e. minor Mert. & Koch, 

 1. c, 839, Cham. & Schl., 1. c, 221. — Fig. 92. 



This species shows great affinity with P. coloratus. The styles and stigmas are 

 quite the same in the two species, and the stem-anatomy is so near alike that it 

 is practically useless for the distinction of the species. 



The styles are much swollen upward and the stigmas small and oval. The 

 stem is lacking strengthening layer along the epidermis but instead of it there are 

 strong lists of several cell-rows in the angles between the lacunar walls and the epi- 

 dermis (see the fig.!). Subepidermal bast-bundles typically present both in stem and 

 petioles, though sometimes reduced in number. Especially in P. coloratus they are 

 frequently brought down to a minimum. Interlacunar vascular bundles are absent 

 in these species both in stagnant and running water. 



The subepidermal bundles behave as is shown in the figure 88. They are only 

 internodal and disappear at the level, where the vascular bundles of the leaf and 

 the ligule enter the cortex to descend into the central axis. 



In a single case* I have observed three to four vascular bundles situated inter- 

 lacunarly in the cortex, beside a pseudo-hypoderma, but this specimen had grown 

 in shallow-water on muddy ground. Shallow-water plants of more or less high-grown 

 species show a certain disposition to procure (or maintain) properties belonging to 

 the basal part of stretched stems. They accordingly deviate accidentally (for in- 

 stance on account of the state of the bottom, see above!) in this direction from the 

 typical form growing in deeper water. As regards the anatomy those basal charac- 

 teristics are principally 1) a thickened pseudo-hypoderma, two-celled or at least 

 locally so, if one-celled at the middle of the stem, 2) increasing width of the wall- 

 cells of the lacunse (f. i. P. natans), 3) sometimes an augmented number of the 

 lacunar circles (f. inst. in P. pectinatus), 4) increase of the width of the central cy- 

 linder together with an increased number of the lateral (cauline) vascular bundles. 

 P. Cheesemanii f. frondosus furnishes example of this basal characteristic in a shallow- 

 water plant; 4) increase of the number of cortical bundles when such bundles occur 

 in a species; 5) in species interlacunarly destitute of cortical bundles (theligular circle) 

 some one or other of the bundles of the ligules and leaves may descend more or 



^ In a specimen from France, »Petits ruisseaux dans la foret d'Andaine, Domfront (Orne), 73, Coebieke» 

 (hb. Stockholm.). 



