196 J. O. HAGSTRÖM, CRIT1CAL RESEARCHES ON THK POTAMOOETONS. 



of this species I have seen is very largc, sharplv three-carinate, and verv much re- 

 minding of the fruit of some species within thc amplifolius-group, to which, at all 

 events, P. Richardii forms a transition. 



Thc specimens examiaed are collectcd in Africa, Abyssinia, and labelled 

 »Schimperi itcr Abyssinicum — 135 Potamogt zton natans L. In rivis et aquis 



Btagnantibus prope Adoam U. i. 1840. d. 11 juni 1837.» (hb. Stockholm.). Another 

 specimen, also belonging to the Museum of Stockholm and onlv labelled *P. notans 

 1207» presents a little smaller leaves, 45 X 25 mm, 19 — 21-nervcd, and has the stem 

 prolonged with branches of as far as the 6th rank. 



P. OakesiailllH Robbins in A. Grav, .Manual of (lie Botany of the north. U. S., 



5 ed. 1867, 485. 



This American species is as yet rather incompletely known. The material, 

 preserved in Sweden, deviates from the original description by the stem being quite 

 unbranched as far as the primary spike, from where it prolongs itself with a branch. 



Undeniably it is verv much like a delicate P. natans, so like indecd as to be 

 thonght to be a hybrid of that species. Knowing it from scveral localities, Morong 

 also remarks, »usually not fruiting freely», which secms to confirm such a supposition. 

 Nevertheless I have not hitherto found further support of it, and its reduced fruiting 

 ability may result from a similar eause as in P. Robbin si i, the rather as thc distri- 

 bution areas of these species nearly coincide. 



The submersed leaves are narrowly linear, thin, 3-nerved, obtuse (almost fili- 

 /orwws-like) ; floating leaves long-petioled (65—80 mm), their blades 45—50x20 mm 

 with rounded or a little cordate base, and obtuse to almost rounded apex, 17- 21- 

 nerved, their ligules brown, 30 mm long, without ridges. Moronq's Btatemenl does 

 not stånd in opposition hereto: — »stipules hardlv keeled». Pedunclea thicker than 

 thc internode next bencath, 50 — 65 mm long, spikes small-flowered of eight whorls, 

 12 — 20 mm long. Deflorate spike quite without fruit. 



Stem-anatomy much reminding of that of /'. natans: [7-endodermis, numeroua 

 Eree bundles in the central cylinder (trio-type diagram), 1 ligular cirele (or 2 circles) 

 of interlacunar bundles and, besides, well developed subepidermal strands. The dif- 

 ference chiefly lies in thc reduced cortical bundles, fewer circles of lacuna-, and in 

 the absence of a strengthening layer along the epidcrmis, which consists of elongated 

 cells, ca. 4 — 7 times as long as broad. 



Thc Morongian spcciincns. Ironi South Xatick, again, remind as to the stem- 

 anatomy more of /'. gramineus by the lateral bundles of the stele being more rc*- 

 duced in uumber, The fcwo median ones are also here free with separate xylem 

 oanals. These individuals have still smaller floating leaves, oa. 25 12 mm, and 



shorter ligules. 



/'. 1'ur.shii TUOK., with acuminate lea\cs», does not scem to bclong hereto. 



