KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 55. N.O 5. 



187 



B 



I) 



it does not at all strengthen its resisting power agairist the 

 pulling energy of the current water, in the latter case, by 

 sclerenchymatous tissue in the bark, but by stronger sheaths 

 round the central vascular bundles, or by transforming the 

 pith cells into mechanic tissue with thicker walls, by suberised 

 epidermis cells, and by a strengthening layer, or by lists etc. 

 Schwendener's statement (1874) upon the development of 

 mechanical tissue in P. jluitans, in running, and its disappear- 

 ance in stagnant water, is accounted for by the fact that 

 very different constituents have of old been reckoned to this 

 »species». The cortical bundles of the stem originate chiefly in 

 the petioles and the ligules and do not appear on account of 

 accidental mechanic necessities. If the petioles are void of 

 those mechanic elements and the ligules only are faintly 

 endowed with strands, the stem, as far as hitherto known, 

 lacks the bark-bundles, whether the species is growing in 

 running or stagnant water. This is the case with P. nodosus, 

 in which, however, as in other similar species one or another 

 of the ligular strands rarely decends a little piece of way into 

 the bark-parenchyma below the node. The absence of sub- 

 epidermal strands in the petioles also causes the submersed 

 leaves easily and early to decay or to be torn away by the 

 motion of water. In herbary materials, thus, we mostly see 

 very little of those leaves. The plant evidently does not need 

 them long for its existence but, instead of them, early produces 

 subcoriaceous or coriaceous leaves, wherupon it prolongs itself 

 by branches from the top according to need, thus procuring 

 a rich evolution of floating leaves during the whole vegetative 

 season. Besides, the lowest floating leaves have an incredible 

 power of stretching their petioles to reach the surface, and 

 these leaves neither fall away nor are pulled off on account 

 of the extremely strong bast-sheaths round the vascular bundles, 

 with which the species is able to supply itself. 



The submersed leaves are characterized by their abund- 

 ance of lacunae, which appear partly along all longitudinal 

 nerves, partly and especially as a broad ribbon or areolation 

 along the midrib all up to the very apex (see the fig. 95, A!). 

 The shape is lanceolate and the apex obtuse evenly narrowed 

 or with a very faint cuspidation. On the margins they are 

 beset with small forward directed one-celled denticles without sinuses in the leaf 

 and so extremely fugacious that they seldom or never are to be found in adult leaves 

 but only in the very youngest ones. In the hybrids, again, above mentioned, except 

 P. lucens X nodosus, those teeth, even in the youngest leaves, are absent or to be 



Fig. 95. P. nodosits Poiii. A, 

 Top of a submersed leaf, f ; B, 

 Marginal part of it (enlarged) show- 

 ing the serrulation. C, Floating 

 leaf, \. D, Pistils, a, lateral view, 

 b, from the inside, *-? . E, Pistil 

 with abnormal stigma, ' T 



