KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 55. N:0 5. 191 



P. liatailS Linné, Sp. pl. 1753, 126. — P. serotinus Schrader in Koch, 



Syn., 1844, 775. 



The stem of this species usually prolongs itself with only one spike-bearing 

 branch from the top, very rarely with branches of the second or third rank. Beneath 

 the primary spike the species usually has 2 — 3 floating leaves generally with longer 

 stalks and narrower, often also shorter, laminae. The leaves of the stem-prolongation 

 are mostly more conspicuously cordate and shorter than the primary involucral leaves. 

 On limiting the varieties or forms from each other by the floating leaves we, there- 

 fore, may conveniently go out from the latter leaves, comparing them as to their 

 variations. They usually are 25 — 27-nerved (in narrower forms 17 — 21-nerved) with 

 more indistinct cross-veins than in species with otherwise very similar leaves. Sub- 

 mersed leaves canaliculate, 5-nerved and of the same anatomical structure as the 

 petiols of the floating leaves; apexes always obtuse. The ligules have broad light 

 scarious borders, low, not winged ridges, and a length of as far as 18 cm, generally 

 however about 10 cm. 



The epidermis-cells of the stem belong to the shorter type of those cells, 1,5 

 to 3 (4) ti mes as long as broad; they are strengthened by a pseudo-hypoderma, one- 

 celled or more rarely two-celled. Only the very nethermost in the mud standing 

 internodes have more stretched epidermis-cells, 5 — 20 times the width. The endo- 

 dermis consists of £7-cells. Chrysler, The structure etc. 1907, Fig. 14, t. XV, shows 

 a central cylinder with 0-endodermis, which surely by mistake is signed as P. natans; 

 it seems to me to be P. amplifolius. Both stem and peduncles are rich in scleren- 

 chyma in 3—4 circles beside the subepidermal one. The cauline more or less fused 

 trio-bundle forms in the stem-prolongation a conspicuous bundle-trio with separate 

 xylem canals. As usual the peduncle lacks an endodermis round the central vas- 

 cular bundles. 



Serial sections through a node show that the subepidermal strands are inter- 

 nodal, that the outer cortical circle of the vascular bundles derives its origin from 

 the lignle, the bundles of which descend into the bark and continue to the node 

 next belovv, finally that the inner circles of the cortical bundles generally are cau- 

 line and connected with the vascular bundles of the central axis. 



The investigations have proved that the terrestrial forms of the species have 

 the same anatomical stem-diagram as the forms of running water, and that the 

 anatomical conditions are fixed. The thickness of the lower internodes is considerably 

 greater than that of the middle ones. The diameter of the 13th internode reckoned 

 from the primary spike, for instance, has once been found to be 4 mm, whereas that 

 of the 5th internode from the same spike measured only two mm. This difference 

 is chiefly dependent on a gradual diminution of the cells from below upwards, 

 whereby the lacunse also grow considerably larger in the basal part of the stem. 

 The number of the lacunar circles is about the same (5—6) through the whole stem. 



