260 J. O. HAGSTROM, CKITICAL RESEAKCHES ON THE POTAMOGETONS. 



Linnaeus, 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. Rotjy, 

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

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

 Oroenlandia Gay. 



Subsectio 26. Densi Hagste. 



Caulis teres ramosus. Folia omnia uniformia submersa basi semi-vaginantia. 

 Pistillum stylo tenui in rostellum postice protracto. Fruchos 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. pauciflorus Lam., Fl. 

 franQ. Ill, 1778, 209. — P. oppositifolius Lam. et DC, Fl frang.. Ill, 1805, 186.' 

 — Groenlandia densa Fourr. in Ann. Soc. Linn. Lyon. nouv. ser., 1869, 169. — Fig. 119. 



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

 a semi-clasping base with two 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 destruction from the microzoa of 

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

 the leaf-margin. 



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

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

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

 grows. From such 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 internode once given repeats itself along the whole shoot. In rather deep 

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

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

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



The leaf-apex is plane and obtuse, if also often 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 to be extraneous 

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

 as a sheath. Its lowest part, therefore, is pale and by a peculiar flexion disposed 



' On P. serratits L., see the index! 



