140 



J. O. HAGSTROM, CRITICAL RESEARCHES ON THE POTAMOGETOKS. 



with shorter lamina. Submersed leaves usually 5 — 7-nerved with tapering base and 

 top, generally 3—5 mm broad, with a lacunar portion filling, in the lower half of 

 the leaf, the whole space between the midrib and the chief lateral nerves, by which 



the species shows affinity to the Euhybridi and Java- 

 nici. Pollen spheric and small-grained. Stem com- 

 pressed in the proportion of 2:1, and sometimes 

 slightly furrowed. The epidermis is destitute of a hy- 

 poderma, but instead of this its cells are very short, 

 often cubic or 1.5—2 times as long as the width. This 

 is a significant and excellent characteristic of this spe- 

 cies, especially of great importance for determining its 

 hybrids. The cortical bast- and vascular bundles 

 appear only laterally in the utmost interlacunar wall 

 (see the fig!) as in P. Robbinsii, and sometimes with 

 one or another bundle so near to the epidermis that 

 it might almost be called subepidermal. Proper subepi- 

 dermal bast absent. Endodermis of O-cells. The cen- 

 tral cylinder has two median bundles, one compound 

 and one simple, and three lateral on either side, all 

 separated from one another by mechanical tissue. This 

 anatomical diagram of the trio-type is very constant 

 in this species. The submersed leaves are destitute of 

 sclerenchymatous tissue in the lacunar part but possess 

 a strong bast-bundle in either margin. 



The species does not vary much, chiefly to the 

 size only. Var. portoricensis Graebner does not belong 

 to this species (see below!). Var. cayugensis Wiegand 

 is a nearly allied distinct species. 



P. N utfallit and P. cayuycnsis certainly occupy a 

 separate position within the subsection, but they never- 

 theless stånd in the nearest affinity to its other species. 

 This especially becomes clear from the style, consisting 

 only in a swelling on the ovary, the fruit-form, and 

 the curvature of the embryo. The fruit, namely, is 

 flattcned with a conspicuous pit on the sides and a 

 prominent dorsal keel, and the embryo has the spiral 



central stele eight bundles are viaible, tr, fonil eharaeteristic to the other species of the gioup. 

 tho eomponnd trio-bnndle. * . 



But the linear, submersed leaves also with their ex- 

 panded areolation, and the floating leaves with petioles equal to the laminsB accom- 

 plish the likeness to the preceding series while at the same time the species can be 

 said to form the transition to species with I arge r floating leaves and broader sub- 

 mersed ones. - /'. cayugensis (Wibg.) differs from /'. Nult. by 9 — 11 -ner ved broader 

 ;iikI obtuser submersed leaves, by the ligules being densely 10 — 15-nerved in the front- 



Fig. 62. /' tfuttallii dun a Bchii. 

 .1, Floating leaves, nal size; Ii, a, b, Topa 

 1. 1 Bnbmersed leaves, showing the form and 

 nervation, Blightly enlarged; C, Pistil, a, 

 tide-view, i>. baek-view, ',"; /', Longitndinal 

 iection "f the fi nit, Bhowing the embryo 

 (a), potamen ('<) and the flesb and dorsal 

 keel i* i. ;; /•.', Transverse sectioo of tho 



