860 J. O. HAGSTRÖM, CRITICAL RESEARCHES ON THE POTAMOGETONS. 



Linnseus, the rather so as many properties really are common, and others not are so 

 important as they seem to be (the internodes and the position of leaves). G. Rouy, 

 Fl. Fr., T. XIII, has made P. densus form a separate subgenus Groenlandia, but if 

 an alteration is to be made you may go the whole figure and reestablish the genus 

 Groenlandia Gay. 



Subsectio 26. Densi Hagstr. 



Cairfis teres ramosus. Folia omnia uniform ia submersa basi semi-vaginantia. 

 PistiUum stylo tenui in rostellum postice protracto. Fruetus carinatus compressus 

 ± rostratus. Embryo spiralis. — Anatomia caulis: o-endodermis semper ex cellulis 

 tenuibus; fasciculi corticales desunt. 



P. densus L. 



Sp. plant. 1753, 126. — P. setaceus L., 1. c, 127. P. paucijlorus Lam., Fl. 



franc. III, 1778, 209. — P. oppositifolius Lam. et DC, Fl frang., III, 1805, 186. 1 



Groenlandia densa Fourr. in Ann. Soe. Linn. Lyon. nouv. sér., 1869, 169. — Fig. 119. 



This species seems to represent a very ancient type. The leaves havc namely 

 a semi-clasping base with tvvo lateral stipular auricles, free between themselves, but 

 somewhat connate with the blade. They are present only in the involucral leaves. 

 The primordial function of stipules and ligular sheaths, therefore, seems to be to 

 protect the flowers in bud-state. An additional function is to cover the youngest 

 part of the vegetative shoot especially against destruetion from the microzoa of 

 uater. In P. densus the latter purpose is obtained by the pricking serrulation of 

 the leaf-margin. 



The seedling deviates from that of all the other species by the second inter- 

 node being very strongly abbreviated. Hereby the first and the second ordinär v leaf 

 come in close neighbourhood of each other and of the bottom, where the plant 

 grows. From sucli an arrangement the plant has the profit of being able imme- 

 diately to give forth two rhizomes, by which it cares for its continuance at the 

 habitat, a very precarious matter to such a small plant. This abbreviation of every 

 other internodc once given rej)eats itself along the whole shoot. In rather deep 

 rivulets, however, the basal internodes (except, 1 think, the very nethermost ones) 

 grow elongate, according to Schur, Phyt. Fragm., 1870, 282 (f. alternifoliua Schur), 

 of course in order to reach the surface of water with the flowering spikes. 



The leaf-apex is plane and obtuse, if also of ten narrow and stretched; but 

 because the leaves are recurved, the tip is directed downwards. I have not been 

 able to discover anything in the anatomy of the leaf which can be said lo \>r extrancous 

 to the genus in the whole. But the base of the leaf, again, is singular by serving 

 as a shcatli. Its lowest part, therefore, is påle and by a peouliar flexion disposed 



1 On /'. » tintii l . , bi the index! 



