198 J. O. HAGSTRÖM, CRITICAL RESEARCHES ON THE TOTAMOGETONS. 



dorso carinatus v. rotundatus. — Anatomin, caulis: [/-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, nearlv 

 kindred with each other, further vaginans and azoricus, b}' Ar. B en n ett referred to 

 P. lucens but evidentlv distinct species, as also P. brasiliensis and v. floridanus (jragil- 

 limus m.), farthermore P. ziziiformis and nipponicus, finally with some hesitation 

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

 not described. 



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

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

 ing also close likeness to the following (Prcelongi). 



P. illinocnsis 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 bas 

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

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

 Finally I have traced it in Canada by its connections with P. amplifolius 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 f loating leaves usually a 1 it t lo longer, than those of the submersed ones, but 

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

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

 also the ehicf lateral nerves, espeeially below, aecompanied by a more or less broad 

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

 narrow, broadwise elongated form espeeially in the amphibial leaves. 7) Ligules ot 

 the stein-leaves strong, brownish, conspicuously biearinate, with broad base, finally 

 Bpreading and breaking loose from its footing. 8) Style verv short. stigma oval or 

 oblong not covering the npper surfaoe of the style. 9) The dorsal keel of the fruii 

 extends typically to t lie base of the nutlet. 10) The lid of the endoearp always keeled. 



II) The stem-anatoiny is eliaiacteii/.ed by a onc-celled pseudo-hypoderma and at least 





