196 J. O. HAGSTROM, CRITICAL RESEARCHES ON THE POTAMOGETONS. 



of this species I have seen is very large, sharply three-carinate, and very much re- 

 minding of the fruit of some species within the amj)lifolius-group, to which, at all 

 events, P. Richardii forms a transition. 



The specimens examined are collected in Africa, Abyssinia, and labelled 

 sSchimperi iter Abyssinicum — — — 135 Potamogeton nutans L. In rivis et aquis 

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

 specimen, also belonging to the Museum of Stockholm and only labelled »P. natans 

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

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



P. OakesiailllS Robbins in A. Gray, Manual of the 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 oi'iginal description by the stem being quite 

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



Undeniably it is very much like a delicate P. natans, so like indeed as to be 

 thought to be a hybrid of that species. Knowing it from several localities, Morono 

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

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

 ability may result from a similar cause as in P. Bobbinsii, the rather as the distri- 

 bution areas of these species nearly coincide. 



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

 formis-Wke); floating leaves long-petioled (65—80 mm), their blades 45—50 X 20 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. Morokg's statement does 

 not stand in opposition hereto: — » stipules hardly keeled ». Peduncles thicker than 

 the internode next beneath, 50 — 65 mm long, spikes small-flowered of eight whorls, 

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



Stem-anatomy much reminding of that of P. natans: [/-endoderrais, numerous 

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

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

 ference chiefly lies in the reduced cortical bundles, fewer circles of lacunte, and in 

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

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



The Morongian specimens, from South Natick, again, remind as to the stem- 

 anatomy more of P. graminetis by the lateral bundles of the stele being more re- 

 duced in number. The two median ones are also here free with separate xylem 

 canals. These individuals have still smaller floating leaves, ca. 25 X 12 mm, and 

 shorter ligules. 



P. Purshii Tuck., with ^acuminate leaves», does not seem to belong hereto. 



