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



93 



little developed in the usual leaves, but in the involucral leaves it is much more 

 diffuse, fig. 36, B. We can here trace the first beginning of a differentiation between 

 true submersed leaves and coriaceous leaves: a contracted, stalk-like basal part and 

 an expanded blade-like upper part without stomata but with a well developed float- 

 ing apparatus in the lacunar system. At the same time the form of the apex is 

 altered from acute to roundish and consequently this form of the apex is an adap- 

 tion for swimming purpose. The next stage we see in the amphibial leaves with 

 both lacunar system (for swimming) and stomata (for 

 aerial life). 



The submersed leaves are mostly three-nerved, but 

 sometimes there are two marginal nerves in the lower 

 half of the leaf ending blindly in the parenchyma. In 

 such cases the leaves thus are 5-nerved. 



The stem has the usual compression (2,6 — 3 : 1) and 

 is endowed with very strong subepidermal bast-bundles. 

 The epidermis-cells are shorter than in all the other 

 species of the group. When, nevertheless, by the increase 

 of the stem the basal half of the internodes become 

 stretched, the epidermis-cells also increase in length in 

 that part, while they remain unaltered in the upper 

 half of the internode. Thus the condition is different 

 to that of P. vaginatus, the main-f., in which elongated 

 epidermis-cells never are to be seen. The lacunar sy- 

 stem is deprived of strands as usual. For the central 

 axis I refer to the diagram, fig. 1, K. 



In the peduncle the median vascular bundles run 

 near to one another or fuse together obtaining a common 

 endodermis like that of P. foliosus, confervoides and 

 triclioides. The subepidermal strands are absent, but 

 a one-layered pseudo-hypoderma fills their office. 



The fruit is very like that of P. mucronatus and 

 panormitanus, a little less than the former and a little longer than the latter (2,25 mm). 

 A little beak crowns the top. 



Spikes are evoluted according to the 5- type, Fig. 2., but with some reduction 

 of the number of branches. 



The colour of this plant is lively green but in withering state it passes into a 

 yellow tone as in P. mucronatus and panormitanus, with which this species exhibits 

 much closer relationship than with P. pusillus. 



Sometimes it occurs endowed with very few or no branches: f. simplicissimus 

 TiSEL., Pot. Suec. exs. fasc III, 1897, no. 105—106. 



Distribution. Sweden, Scania, Kristianstad, 18, lib. E. Fries (hb. Stockholm.). 

 — Ostrogothia, O. Eneby in Gusjon, 82, Starback (hb. Stockholm., Uppsal., Lund.), Vard- 

 nas in Strep, 12, Sundelin. — Uppland, FasternainMetsjon, 52, LoNROTH(hb. Stockholm., 



Fig. 36. P. rntilus Wolfg. A, Involu- 

 cral leaf developed as a semi-coriaceous 

 leaf with obtnse rounded apex ; petiole and 

 blade moie conspicnously differentiated than 

 in P. pusillus, -{-. B, The apex of a leaf 

 more magnified. C, Top of a submersed 

 leaf, ca. y. D, Transverse section af a 3- 

 nerved submersed leaf. The lacunar part 

 is endowed with a strong bast-bundle on 

 either side of the midvein (ttt), n\ lateral 

 nerve, Z, lacuna. Marginal strands very 

 strong, »|». E, Pistil, lateral view, y. 



