100 



J. O. HAGSTBOM, CRITICAL RESEABOHES ON THE POTAMOGETOKS. 



str 



branches, which in their turn shoot out branches of the second rank and so on, in 

 accordance with the length of the vegetative season and other influences (type D). 

 The stem is terete or faintly compressed (IV2 : 1) with elongated internodes, 

 the upper ones 6—10 cm, and with the branches usually crowded at the base, from 

 where spike-bearing long branches grow forth (^-type). In the axil of the scale-leaf 

 at the base of those branches the small and slender turios are usually developed, which 

 in P. pusillus on the contrary occur apically on the branches. The anatomy of the 

 stem does not essentially deviate from that of the nearly related species. In the stele 

 the bundles fuse into a compound bundle with the phloem parts in the periphery and a 

 common xylem cavity in the middle (circular stele-diagram). In the peduncle there 



appear seldom more than two separat- 

 ed bundles (the median ones), in P. 

 pusillus' four or more. The peduncle 

 is terete, slender, 15 — 30(45) cm long. 

 By the prominent midrib, elevated 

 over the lower surface of the leaf and 

 the more or less strong marginal scle- 

 renchyma the leaves are stiffer than 

 in P. pusillus having softer leaves 

 with low and broader midrib-portion. 

 P. panormitanus never has the large- 

 celled lacunse along the mid-rib, by 

 which the leaves are lacking the 1 to 

 2 light stripes, characteristic to P. 

 fusillus, and the mid-rib looks greyish 

 and not transparent. Only one row 

 of narrow lacunse, outwardly appear- 

 ing like a delicate light stripe, occurs 

 now and then. Those differences are 

 of no small consequence, since the lacunar system of the leaves constitutes an important 

 part of the swimming apparatus of the plant. With its abundance of leaves P. pusillus 

 needs a more developed apparatus for floating. Common to both species is the trans- 

 formation of the involucral leaves in this direction, though more often occurring in P. 

 panormitanus and also in (3 minor. The leaves are always 3-nerved; in the broader 

 leaves there appears occasionally a pair of nerves beyond the usual number. Apex 

 commonly sharper and longer than in the pusillus-leai in general. The ligules have 

 a light-brown colour, are more persistent than in P. pusillus, and 6— 10-fibrous in 

 the front-field, the uppermost broad and pretty long maintaining an inflated appear- 

 ance owing to the inclosed spike. This consists of 2 — 3, a little separated, 2-flo- 

 wered whorls and the perianths are endowed with a long claw. The styles are short 

 and very thin, consequently the rostrum of the fruit is typically short and thin. 

 Stigma covering the whole upper surface and even projecting a little beyond. The 

 fruit is olive-coloured and smooth, in P. pusillus more green or dark-green and more 



Fig. 38. -P. panoymlUtnus Biv. v. ntlnor Biv. A, Transverse 

 section of a stem-leaf, middle portion, ^y**, m, midvein, n, lateral 

 vein, sir, bast-bnndles. B, Transverse section of the stem, ^^, ep, 

 epidermis, end, endodermis, cc, central axis, /, lacunar system, str, 

 stibepidernial bast-bnndles. C, Tops of leaves showing the nervation 

 and different form (l, 2) of apex, 'i'. D, The frnit of the f. steno- 

 carpns FiscH., side-view, \-^. E, Fruit, side-view, \'^; the dotted line 

 shows the fruitform of the f, platycarpus FiscH. 



