194 



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



the othei' factor must have wanted them, since the hybrid has very much reduced 

 upper leaves. The very narrow and long submersed leaves point in the direction 

 of P. nutans and some narrow-leawed species. Superficially observed these leaves 

 seem to be 7-nerved, but as the figure below shows, the vascular bundles are in fact 

 only three. The same figure also shows that these leaves are very rich in scleren- 

 chymatous strands. The apexes of the submersed leaves vary from rather acute to 

 roundly obtuse, see the figure! The ligules are stronglj' nerved, dark-green, and in 



the front-field five-nerved. At their base 

 there are prominent glandulous swellings 

 (nodal vittse or oil-cells). The stem is 

 high-grown, terete, at the middle thread- 

 like. It prolongs itself by one single 

 spike-bearing branch. The spikes are 

 small, 3 — 5-whorled. All these proper- 

 ties taken together undoubtedly allude 

 to the combination written above. 



If nevertheless there would be any 

 hesitation in admitting a crossing between 

 one of the largest (P. natans) and any 

 of the most delicate species it must 

 disappear on the discovering, in the 

 hybrid, of cauline turios, evidently like 

 those characteristic of some small-leaved 

 species. A closer examination proves that 

 no other species but P. trichoides together 

 with P. natans has been able to produce 

 such turios. The figure gives a better 

 idea of it than any description can do. 

 In a surprising way the stem ana- 

 tomy also confirms the morphologic and 

 biological testimonies. The central cylin- 

 der of stem, thus, has now (in narrower 

 internodes) an arrangement of bundles 

 quite as in P. trichoides, now, again, it is 

 intermediate between the two: with 2 free median and 2 free lateral bundles. The 

 subepidermal strands are also full-numbered and strong; in the bark, moreover, 

 there are one or two circles of bundles, which can be said to be a reduction of the 

 circumstances of P. natans, in which we usually find three or four circles. The en- 

 dodermis-cells are, in all the cases examined, »-cells as in P. natans. The anatomy 

 of the peduncle corresponds most closely with that of P. natans with a reduction 

 of the number of bundles here, too. The cross-section can be said to exhibit a P. 

 natans in miniature. 



The anatomy of the submersed leaves will be rendered clear by the figure 



Fig, 97. P. variifoliiis Thoee A, a, b, c, cl. Different forms 

 of floating leaves, {. B, a, b, Tops of submersed leaves, Y- 

 C, Part of a submersed leaf at tlie middle, lacunar system and 

 superficial sclerenchyma (subepid. str.) visible, \'^. D, Transverse 

 section of a submersed leaf at the middle, showing the arrange- 

 ment of the lacunae and the subepidermal strands, ics, upper, and 

 Is, lower surface, hi, midnerve, »' lateral nerves, sir, bast bun- 

 dles, 'j*. E, Turio, a, stem, b, leaf, f/7, gland, }-. 



