78 J. O. HAGSTEOM, CRITICAL RESEARCHES ON THE POTAMOGBTONS. 



might be one of the parents beside P. zosterif., I later on, after further minute exa- 

 mination, have got still more convinced of the accuracy of the above presumption. 



Stem more or less flattened but not winged, crosscut-form oval, the longer 

 axis about twice the shorter one. Central cylinder with one cavity, round which 

 the phloem parts are grouped as in P. trichoides. Leaves narrowly linear, stem- 

 leaves 1,5—1,75 mm, with somewhat broad base, sometimes long with trichoides-]ike 

 apex, sometimes shorter with an apex more like P. zosterif olius. Stem-leaves 5-nerv- 

 ed and with strong bast-bundles in the lacunar part. Air-channels arranged as in 

 P. trichoides. Turios as to shape and other conditions intermediate, inner leaves 

 sometimes confined within the sheaths, sometimes a little protruding. The lower 

 leaves resemble most those of P. trichoides. Peduncles elongate, 3 — 5 cm long, their 

 anatomical structure intermediate: the two middle bundles in proximity to each other 

 as in P. trichoides. Spike of four verticils, always barren. A single nearly ripe 

 fruit, once found, looked very much like a trichoides-huit. About 50 p. C. of the pol- 

 len is sterile, the rest consisting of bigger {F. zosterif.) or smaller (P. trich.) grains. 



Two forms may be distinguished: 



f. apicatus n. f. — Folia elongata, (10) — 12—15 cm, in apicem acutissimum 

 sensim attenuata. 



f. breviusculus n. f. — Folia 6—8 — (10) cm longa apice citius attenuata. 



By the long leaves the former reminds of P. zosterif olius, the leaves never- 

 theless being narrower and the top unlike, the latter, again, of P. trichoides but 

 has much broader leaves. 



Distribution. Denmark, Varmingso, 97, Baagoe (hb. Stockholm.), /. apicatus; 

 Nibsa, 00, Baagoe (hb. Stockholm.), /. breviuscidus. Both forms probably originate 

 from the same crossing. More habitats are not yet known. 



Subsectio 9. Ochreati Graebner. 



Graebner, Potamog. in A. Engler, Das Pflanzenr. 1907, 100. 



Caulis compressus internodiis ± elongatis, nodis annulatis. Folia multinervia 

 obtusa vel mucronulata, basi rotundata, canalibus in medio perpaucis. Ligulse fisste, 

 multinerviae. Pistillum stylo conspicuo sequali, stigmate parvo. Fructus tricarinatus, 

 ssepe rugosus. 



This and the two following groups nearly associate with Compressi by the 

 broad, rounded leaf-base and the richly developed sclerenchyma of the leaves and 

 ligules, but differ by the style and stigma, that one more elongated, this one smaller 

 and more rounded. The group is evidently nearest related to the Polygoni, also with 

 elongated styles, though a little thickened upwards, and with a very rich lacunar 

 system in the leaves. The present group also separates from all the following through 

 the 1 — 3-celled hypodermatous layer in the stem anatomy. In this respect it seems 

 to take the first place of all the narrow-leaved groups. — Hibernating buds are not 

 known either in this group or in the two following ones. 



