198 J. O. HAGSTROM, CRITICAL EESEAROHES ON THE POTAMOGETONS. 



dorso carinatus v. rotundatiis. — Anatomia ca,ulis: CZ-endodermis, fasciculi libriformes 

 et vasculares corticales ± numerosi semper obvii. — Turiones rhizomatici adsunt. 



Beside the species here below treated of, I include in this group, which I have 

 called Dentati in Bot. Not. 1908, 101, also P. dentatus and Gaudichaudii, nearly 

 kindred with each other, further vaginans and azoricus, by Ar. Bennett referred to 

 P. lucens but evidently distinct species, as also P. brasiliensis and v. floridcmris {fragil- 

 limus m.)j furthermore P. ziziiformis and nippojiicus, finally with some hesitation 

 P. distinctus, sumatrantis, and Schweinfurthii, the leaf margin of which is as yet 

 not described. 



Closely related to the Nodosi the group belongs to the Potamogetons which, 

 just as the group next before, are rich in sclerenchymatous tissue, anatomically show- 

 ing also close likeness to the following (Prmlongi). 



P. illiiioensis Morong. 



In Botanical Gazette V, 1880, 50. — Fig. 99. 



The distribution-area of this species is said to be » Illinois to Iowa and Min- 

 nesota* (Morong in Britton & Brown, 111. Fl., 1896, 70; Robinson & Fernald, 

 Gray's New Man., 1908, 73). It has, however, a much wider distribution than has 

 hitherto been thought. Beside from Mexico and Texas (see below!), I have also seen 

 it from Florida (ca. 28° 30' north, lat., O. Vestbrlund, 1889), and from Michigan. 

 Finally I have traced it in Canada by its connections with P. amqylijolius and gra- 

 mineus. It varies considerably in habit, occurring like P. gramineus both with flo- 

 ating leaves and without. No doubt it is often confounded with P. Zizii, which it 

 often resembles in habit. 



The following properties are common to all forms I have hitherto observed: — 



I) Stem typically always more or less branched, at least upwards, prolongs itself 

 willingly by short-jointed branches of first to 2d rank. 2) Upper internodes, especi- 

 ally those of the stem-prolongation, short, 2 — 3 (5) cm or less. Sometimes the 

 branches are rosette-like with extremely short internodes (see below!). 3) Submersed 

 leaves serrulate and short-petioled, or at least with stalk-like base. The petioles of 

 the floating leaves usually a little longer, than those of the submersed ones, but 

 generally shorter than the laminae. 4) The apex is cuspidate and has a character- 

 istic broad shape (see fig. 99, A, Bf). 5) The midrib of the submersed leaves, often 

 also the chief lateral nerves, especially below, accompanied by a more or less broad 

 lacunar part. 6) Cross-veins usually very dense, by which the nerve-spaces get a 

 narrow, broadwise elongated form especially in the amphibial leaves. 7) Ligules of 

 the stem-leaves strong, brownish, conspicuously bicarinate, with broad base, finally 

 spreading and breaking loose from its footing. 8) Style very short, stigma oval or 

 oblong not covering the upper surface of the style. 9) The dorsal keel of the fruit 

 extends typically to the base of the nutlet. 10) The lid of the endocarp always keeled. 



II) The stem-anatomy is characterized by a one-celled pseudo-hypoderma and at least 



