140 



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



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 0-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 Wi'egand 

 is a nearly allied distinct species. 



P. Nuttallii and P. cayugensis certainly occupy a 

 separate position within the subsection, but they never- 

 theless stand in the nearest affinity to its other species. 

 This especially becomes clear from the style, consisting 

 of submersed leaves, showing the form and Quly in a Swelling ou the ovary, the fruit-form, and 



nervation, slightly enlarged; C, Pistil, a, ^ ^ 



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

 flattened with a conspicuous pit on the sides and a 

 prominent dorsal keel, and the embryo has the spiral 

 form characteristic to the other species of the group. 

 But the linear, submersed leaves also with their ex- 

 panded areolation, and the floating leaves with petioles equal to the laminae 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 larger floating leaves and broader sub- 

 mersed ones. — P. cayugensis (Wieg.) differs from P. Nutt. by 9 — 11-nerved broader 

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





:o 





Fig. 62. P. NuttaUii Cham. & Schl. 

 A, Floating leaves, nat. size; B^ a, b. Tops 



side-view, h, back-view, '-^^; D, Longitudinal 

 section of the fruit, showing the embryo 

 («), putaiflen {b) and the flesh and dorsal 

 keel (c), f ; E, Transverse section of the 

 stem, \** showing nine bundles. In the 

 central stele eight bundles are visible, tr, 

 the compound trio bundle. 



