122 



J. O HAGSTRÖM, CRITICAL RESEARCHES ON THE POTAMOGETONS. 



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

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

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

 at all a pusillus-form but very obviously and surely mucronatusx pasilhis. 



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

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

 The sheaths of a couple of bundles maj 7 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 lacunoe 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 börder 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. panormitaniis, that has narrower lacunar, 

 the midrib more elevated över the lower leaf- 

 surfaee, stronger strands and vascular bundles 

 and consequently stiffer and more rit(ilu$-\ike 

 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. Tlie width varies also with gradual transitions from 

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

 have held a measure of 05 mm by 2 mm. Tn general the width is 1 mm. When 

 N. J. SoHBUTZ (Pl. vase. Jenis.) states higher mcasures for pusill //.s-forms from Nik- 

 androvskii ostrov, tliis evidently refers to the above described ncw species P. svb- 

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



Tlio involucral leaves deserve a special mentioxi in being often (at least the topmost 

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

 fcheir blade increasea a little in width. At the same time the lacunar part expands 

 and growa wider but the lacunee decrease in si/.e growing narrower and espeeially 

 shorter than in general along the midrib. Nevertheleee no sucb tissues are deve- 

 loped by which •> difference is established between a lower and an upper surfaee of the 

 leaf. Onlv about t lie middle nerve a faint besinning <>t il is to be seen. Neither 



Fig. 54. /'. pugillus L. A, Transverse section of 

 the peduncle, ',". eh, epidermia and ita snpporting layer, 

 «, median, /<, lateral bnndles. The lacnnaj are lett ont. 

 /(, Transverse spet. o( the ligtile, ie, the front-fleld, '/'• 

 C, Longitndina] seetion "l the stem-epidermis, ','' 



