KUNGL. SV. VET. AKADEMIENS HANDLINGAK. BAND 55. N:0 5. 



59 



a more or less prominent boss. Endocarp thick-walled, epicarp thin, lid narrow, 

 prominently keeled. Peduncle lacking the furrows has an oval crosscut form. 



Respecting the stem-anatomy P. cris-pus is perfectly lacking interlacunar and 

 subepidermal strands but the epidermis, in compensation, is endowed with a one- 

 celled pseudohypodermal stratum. Endodermis of 0-cells, vascular bundles of the stele 

 arranged chiefly as in the following groups. The leaves are also lacking mechanical 

 strands except in the margins, where a faint strand is present. Thus P. crispus is a 

 genuine non-schlerenchymatous species. The epidermis-cells of the upper surface of 

 the leaves are not so deep, radially seen, as those of P. Robbinsii and the same is 

 the case with the next following groups. 



The rhizome has not the same cross-cut form as the stem, but a nearly cir- 

 cular or faintly oblong one, which, besides, seems to beacharacteristic of the rhi- 

 zomes of all species. The epidermis-cells are longer, about 4 — 10 times the width; 

 in the stem, to mechanical purpose, only 2—4 times (fig. 22, C, D). The air chan- 

 nels are reduced both in size and . 1 

 number. The bundles of the stele 

 are separated, and beside this 

 there are three lateral ones on 

 each side. The endodermis-cells 

 are weaker like all mechanical 

 dispositions for the rest. But 

 these arrangements return also 

 regularly in the rhizomes in ge- 

 neral. The compression of the 

 leafy shoot in this and other 

 species is therefore an evident 

 arrangement for mechanical 

 purposes by making all parts 

 more flexible and pliant in the 

 water. The enlarged, rounded borders of the stem and branches of this and other 

 species, again, make the plant float more easily by their being pierced by larger 

 air-filled channels. The fusion or reduction of the lateral bundles of the stele of 

 the stem evidently depends on the necessity of making place in the strongly com- 

 pressed stem for the essential, lacunar system situated semicircularly round the 

 lateral edges of the stele, fig. 22 B. If the channel-system is reduced, as in the 

 rhizomes, or sufficiant space can otherwise be obtained you will find a plurality of 

 free, lateral bundles in the central axis. 



According to Fieber (Potam. Bohm. 1838 p. 32) the variations of this species 

 can be classified into two varieties characterized by a pointed or obtuse leaf apex: 



Var. acutifolius, and Var. obtusifolius. — The latter is P. crispus a genuinus 

 Reichb., Icones 1845, 18, and P. crispus f. typicus Tisblius, Pot. suec. exsicc. fasc. 

 II, 1895. It is the commonest in the Swedish collections. The former is much rarer, 

 recorded from Sweden at following stations: Scania, between Skanor and Trelle- 



3 



OZX^^ct: 



E^S=^ 



rz 



Fig. 22. P. cn'spus L. A, Transverse section of the rtiizome, ^, showing 

 the central cylinder (cc), the lacunar system (/) and the terete form, B, 

 Transv. section of the stem, -^ , showing the same. C, Longitudinal section 

 of the epidermis of the rhizome, and 1), of the stem, 'Y- 



