122 



J. O HAQSTROM, CRITICAL RESEARCHES ON THE POTAMOGETONS. 



have more flattened stem, sharp and more prominent leaf-points, 3 — 4 rows of 

 lacunae on either side of the midrib, strong lateral nerves and stronger interlacunar 

 strands in the leaves. They belong to var. p amminatus Fieb. 1. c, and are not 

 at all a pusillus-iorm but very obviously and surely mucronatus X pusillus. 



In the peduncle, like the stem of a rounded or slightly oval cross- cutf or m, the 

 vascular bundles run separated though more or less closely approached to one another. 

 The sheaths of a couple of bundles may sometimes fuse into a common one. The lateral 

 bundles casually separate into two or three, and are in the latter case also gener- 

 ally more spread or scattered. No subepidermal bast is observed in the peduncle. 

 The leaves are always, in f. tenuissimus also, three-nerved and furnished with 

 one or two rows of lacunis on either side of the midrib. The lateral nerves join 

 the midvein at a leafwidth's distance from the very point, but their ends are extreme- 

 ly faint and almost imperceptible. Within the lacunar part a few very faint 

 strands appear subepidermally and likewise in the very border a faint strand. The 



vascular bundles have also but a few mechanical 

 cells. All this causes the leaves to become 

 soft and flexible in opposition to the leaves 

 of P. panormitanus, that has narrower lacunse, 

 the midrib more elevated over the lower leaf- 

 surface, stronger strands and vascular bundles 

 and consequently stiffer and more nciihis-like 

 leaves. Typically the leaf-apex is more or 

 less rounded with a faint subobtuse point. 

 In the cases when the leaves grow longer 

 they generally also taper more gradually and 

 when growing broader they also get a little 

 more prominent cusp (v. Berchioldi). But this 

 general rule has many exceptions and the apex varies rather considerably in form 

 as is shown in fig 55, a—h. The form is somewhat different even in the same 

 individual and it proves to be very difficult to distinguish forms or varieties by the 

 form of the leaf-apex only. The width varies also with gradual transitions from 

 0,5 mm to about 1,6 mm or a little more. The largest leaves I have observed 

 have held a measure of 65 mm by 2 mm. In general the width is 1 mm. When 

 N. J. ScHEUTZ (PI. vase. Jenis.) states higher measures for pusillus-iorms from Nik- 

 androvskii ostrov, this evidently refers to the above described new species P. sub- 

 sibiricus. The colour varies from dark-green to light-green. 



The involucral leaves deserve a special mention in being often (at least the topmost 

 ones) transformed into a sort of primitive floating leaves. This proceeds so that 

 their blade increases a little in width. At the same time the lacunar part expands 

 and grows wider but the lacunse decrease in size growing narrower and especially 

 shorter than in general along the midrib. Nevertheless no such tissues are deve- 

 loped by which a difference is established between a lower and an upper surface of the 

 leaf. Only about the middle nerve a faint beginning of it is to be seen. Neither 



Fig. 54. P. pusillus L. A, Transverse section of 

 the pednncle, "y*, eh, epidermis and its supporting layer, 

 a, median, h, lateral bundles. The lacnna; are left out. 



B, Transverse sect, of the ligule, ic, the front-field, \°. 



C, Longitudinal section of the stem-epidermis, '^-^ . 



