KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 55. NIO 5. 207 



to specimens determined by Kihlman himself, has leaves of 7 — 10 mm in width and 

 is a genuine gramineus-i orm . Of all these river- and deep-water forms, however, it 

 may be said that they now and then produce one or a few small subcoriaceous or 

 coriaceous leaves. Peduncles sometimes very long (as far as 30—35 cm). 



The form pseudo-nitens Ar. Benn. is, according to specimens from Bennett's 

 herbarium, a true nitens-iorm (i. e. P. gramineus X perfoliatus) ; f. lanceolatifolius Tis. 

 is the hybrid P. Seemenii (see belovv!). Forma marinus Tis. is identical with v. 

 lacuslris Fr., and f. ovaiifolius Tis. seems to run into f. jemtlandicus with only a 

 little broader leaves (80x10); v. debilis Tis. coincides with f. septentrionalis (Tis.). 



Of var. lacustris Fr., with leaves usually not above 50 mm (Asch. & Graebn.), 

 therc are forms with narrower (2 — 4 mm) and broader (5 — 6 mm) leaves, besides 

 which I have seen lacustris-fovms, with very short and narrow leaves, 25 X 3 mm or 

 smaller still. 



Var. platypliyllus Reiciienb., Icones, t. XLIII, 77, 78 is evidently (see the leaf- 

 apex!) a form of X P. Zizii. 



All true g ramineus-ior ms have sessile submersed leaves with tapered, lanceolate 

 base, serrulate margins and sharp apex with a nervation as in the fig. 102 H — L. 

 The cross-veins are situated more distan tly than in P. lucens, by which the nerve- 

 spaces grow more extended in longitudinal direction also in the upper half of the 

 leaf, besides which the nerves alwaj^s appear more obscure or vague than in the 

 luce ws-leaf, a characteristic of great importance, vvhen it is about to determine the 

 many hybrid forms. 



The anatomy of stem is characterized by an endodermis of narrow-roomed 

 strong M-cells laterally compressed, by always only a single circle of cortical inter- 

 lacunar bundles (the ligular circle) and a more or less full-numbered circle of sub- 

 epidermal strands. In the stele the median bundles fuse and form a common xylem 

 cavity, and the same is the case with the lateral ones and consequently the number 

 of the xylem canals becomes three (the genuine oblong type). If other conditions 

 appear at the middle of the stem, viz. weaker ?£-cells in the endodermis, two or three 

 lateral xylem canals or two median such ones in the stele, more than one circle of 

 interlacunar vascular bundles in the bark, or a complete circle of subepidermal strands 

 etc. you will, on closer examination, find it to be about any gram ineus-hyhrid. A 

 strengthening layer along the epidermis is either wholly or partly absent. At the 

 base of the stem again such a one-celled hypoderma always is to be found. The 

 increase of the thickness down the stem is chiefly produced by the increase of the 

 bark layer, whereas the central cylinder and the arrangement of the bundles are 

 only little changed. Upwards, in the internode next below the primary spike, as in 

 the branches of the stem-prolongation the thickness is caused both through the in- 

 crease of the bark layer on account of the increase, in size and number, of cells 

 and lacunse, and through the increase of the stele in volume and the spreading 

 of the bundles, too, in conformity with the facts in the hybridus-gvowp (see p. 135!). The 

 purpose is also the same, viz. to afford sufficient stiffness to this part of the plant 

 which has to support the spikes. At the middle again the plant needs the greatest 



