44 



J. O. HAGSTROM, CRIT1CAL RESEARCHES ON THE POTAMOGETOXS. 



farther to the north, for instance at Nikandrowsky ostrov, 70° 20' n. L. (coll. Lund- 

 ström, 75, Arnell, 76, hb. Stockholm.). From this boundary it spreads southwards 

 through all parts of the world, its distribution consequently being universal and sur- 

 passing all the other species of the genus. 



It is evident that a species, appearing under so different climatic conditions, 

 arctieallv (see above!) and equatorially (for instance in the Albert Edward Lake, 



Africa, coll. R. E. Fries, 1912, 

 hb. Upps.), and occurring both 

 in salt and fresh water, at the 

 shores of the sea and in small 

 lakes, in rivers and rivulets, 

 on muddy, fat ground and bar- 

 ren, sandy, clay bottom, must, 

 of course, be met with in many 

 different varieties or forms. 

 Thus we find forms with horse- 

 hair-like stems and leaves and, 

 again, stems 2 mm in diame- 

 ter at the base and with 5 mm 

 broad stem-leaves and number- 

 less transition forms between 

 those extremities; with short 

 and long sheaths (at most 50 

 mm); with large (4,5 x 3 mm) 

 and smaller (3x2 mm) fruits 

 (megacarp and microcarp forms). 

 Of all the variations of this 

 species I have observed, none 

 seems to be more significant or 

 of a greater systematic value 

 than the different forms of the 

 leaf-apex, of which the figure 

 beside should give an idea. The 

 body of the forms should be 

 divided into the fewo varieties established here below, according as their leaf-apex is 

 endowed with a conspicuous mucro or more slowly tapered. 



The somewhat peculiar v. mongolicus A. Benn. ought to be kept, and likewise 

 the P. zosteraceus Fries and /'. sirialus R. & P. All the other innumerable varia- 

 tions should after my opinion be considered only as rather acoidental and insignifi- 

 cant forms of inferior value. 



The following arrangement of the forms of /'. /xcliualus is only an attempt 

 to put in order the great multitude of forms which I have had an opportunity 

 to examine. 



Fig. 18. /'. pectinatus L. Different forms of the leaf-apex. A, It, C, /'. 

 var. diffusas in. y. E, F, G, var. ungulatut ni. ','• Hi var - striatus (B. & V), ,'•. 

 K, Middle portion of //, showing the nervcs and lacunir, J; L, var. diffustia, 

 m. y. A, basal stem-loaf, Ii, from the middle of tlio stem, C, involuoial leaf, 

 D, brauch-leaf; K, leaf from the midde of the stom, /•', involnora] leaf, Q, hasal 

 leaf of a ahort branch; It, top of a stem-leaf, material from Jujuy. Smith Ann- 

 riea. K, Middle portion of this leaf showing the position of vasoular hiiiullcs, 

 strands and lacunar walls, of. fig. 16, /'.' //, top of a basal stcnikaf with tho 

 position of nerves and laeuna visible. 



