— 2G0 — 



As all the species are not yet examined anatomically, the 

 groups here established can only be looked upon as provisional ones; 

 most likely they will be altered later on and some of them per- 

 haps expunged while new ones must be established. Until we have 

 got a thorough study of all species, it is difticult to form a sure 

 construction of the reciprocal position of the species groups, o: the 

 phylogenesis of the genus. Intending to get a clear understanding 

 of the true connection of the species-groups, it is necessary first to 

 have recognized the evokition-direction of the single characters, 

 that is: to have proved which variation of a single character is to 

 be looked upon as the most primary one. In order to reach that 

 point of view we must closer examine the principal characters 

 used in the preceding synopsis. 



The bark-lMiiulIes. It seems to me most probable that the exist- 

 ence of bundles in the bark is the primary stage from which the 

 evolution has spread into two directions of which one has led to 

 a reduction of the bark-bundles to but one circle of subepidermal 

 bast bundles, while the other has resulted into an absolute- lack of 

 bark-bundles. In some cases, however, the first case has most 

 probably been the thoroughfare to the latter. 



The axial cylinder. The occurrence of isolated bundles in the 

 axial cylinder is no doubt primary. It must be owned, however, 

 that free bundles may develop from bundles once joined, but this 

 could hardly be the case in the genus Potamogeton, all its species 

 being aquatic plants that are always disposed to have the vascular 

 bundles more or less closely united in the middle of the stem. 

 Thus the outer conditions of the life of those species speak against 

 an evolution from united bundles into free ones. 



The sheaths. When comparing the Potamogeton with other Mono- 

 cotyledones it seems most natural to suppose that the leaf end- 

 owed with sheath (and ligula) is more primary than the sheathless 

 but with a free axillar ligula endowed leaf). This difference has 

 even been regarded as so important, that the species endowed with 

 sheaths, at least the European, have been grouped as a separate 

 subgenus, Coleogeloii. It may be doubted, however, whether all 

 the species with sheaths take their origin from one and the same 

 primary typo; it is not excluded that the evolution having passed 



') G. Raunkiar, I. c, page TA and 10(;— 108. H. Gliick, Die Stipulargehilde 

 der Monokotyledonen. Heidollierg 1001. 



Botanisk TicUskrift. 25. Uiiiil. IS 



