4 J. O. HAGSTEOII, CRITICAL EESBAECHES ON THE POTAMOGETONS. 



C. J. LiNDBERG and E. Trana. The splendid work of Dr. G. Tiselius, Potamogetones 

 suecici exsiccati, is to be found at Stockholm, Upsala, Lund and Gothenburg, and at 

 the three places first mentioned you will also find E. Fries's Herbarium Normale. 



On dividing all the species of this genus in the following twenty-six subsections 

 or groups I have considered their morphologic, as well as their biological and ana- 

 tomical characters. 



No doubt the fructification is of the greatest importance for indicating the af- 

 finity. The pistils and stigmata, however, are as yet little studied. As to the ripe 

 fruit, it is true, the keels and outgrowths are mostly described, but sometimes there 

 is no difference made between false and true keels. The curvature of the embryo 

 establishes a close relation between P. NiUtallii and Euhybridi. 



As regards the leaves, the principal interest is attached to the submersed ones, 

 especially to their margin, if smooth or rough, their apex, and the nervature. But 

 the form of the basis and the extent of the lacunar system must also be kept in 

 view. Obversely the absence or presence of floating leaves is of much less impor- 

 tance in fixing the relationship. Several species, typically provided with floating 

 leaves, are able to dispense with them under certain circumstances. Some others, 

 in which they are commonly wanting, occasionally supply themselves with a sort of 

 leaves as to structure presenting an intergradation between true floating leaves and 

 submersed leaves: more lacunous, often furnished with scattered stomata, and with 

 an obvious tendency to form an expanded blade, in the cases when the submersed 

 leaves are narrowly linear. Their biological function is twofold: to serve at the same 

 time as submersed leaves (water-leaves) and as a sort of aerial leaves (air -leaves). 

 For this reason I am inclined to term them amphibial leaves (folia amphibia). 

 Yet I do not intend to say (with Nolte) that all submersed species are able to pro- 

 duce coriaceous or subcoriaceous leaves. Submersed species in proper sense are really 

 to be found and in them you as a rule look in vain for stomata or other characte- 

 ristics of floating leaves. Such species are P. lucens, P. perfoliatus, prcelongus etc. 

 C. Satjvagbatt in his paper Sur Us feuilles de quelques monocot. aquatiques, p. 270—271, 

 certainly mentions stomata in P. lucens, but most of his »lucens» seems to have 

 been P. coloratus (»la variete non mucronee»). 



Hybrids of wholly submersed species with coriaceous-leaved species are often 

 provided with those amphibial leaves, a fact which affords a good help in studying 

 several difficult forms. Besides, they occur in the region between the true water- 

 leaves and the air-leaves in species with floating coriaceous leaves. 



The leaf-sheaths and ligules are also significant in pointing out the affinity. 

 Especially we must set weight upon if they are open or connate, but also other 

 conditions lay hold on our attention much more than has hitherto been the case. 



The affinity between groups and forms is further manifested in the form and 

 internal structure of the stem. Nearly allied forms have the same or similar ana- 

 tomical characters. Especially the cells constituting the peculiar endodermis seem 

 to be most prominent in importance. Three types can be conveniently distinguished: 

 cells without incrassate walls (= thinwalled o-cells), cells inside with round about 



