— 271 



the thin, broad submerged leaf. Trying to decide which of the 

 two forms is the most primary one unfortunately the consideration 

 of the other characters cannot help us as we have Potamogeton- 

 species with only thin, broad submerged leaves together with 

 bundles in the bark and free bundles in the axial-cylinder as in 

 P. praelongus; and we have here species with both floating leaves 

 and submerged leaves and also in connection with bundles in the 

 bark and free bundles in the axial cylinder viz. the P. natans- and 

 the P. amplifolius-gvowp. The thin, broad submerged leaf, how- 

 ever, I feel inclined to consider the most primary: I think, though it 

 is not settled, that the Potamogeton are not originally aquatic plants, 

 but are descended from land- or marsh plants. To my view, how- 

 ever, there is no sign of the P. natans- and P. amplifolius- form 

 being transition-forms to aquatic life but rather transition-forms 

 from submerged plants to water-surface plants ; hence the P. prae- 

 longus-fovm proves to be the most primary one: a land- or marshy 

 plant that has become immerse and its leaves perfected in accor- 

 dance with the new conditions of life. Prom such a P. praelongus- 

 like form the other may have issued. P. densus that has the form 

 of leaf here regarded as the primary, has even short leaf-sheaths, 

 at least in certain leaves, what confirms my theory. 



II. 



Potamogeton fluitans. 



What is Potamogeton fluitans Roth? For more than a 

 century this question has been unsolved and often the apple of 

 discord between the botanists who studied the genus Potamogeton. 

 Here I am not going to give the history of this question or men- 

 tioning all the essays about it and remarks made on it, only 

 before entering the definitive solution of the question I must give 

 a summary of the matter at present and report the view of recent 

 Potamogeton-judges. 



To the description of P. natans in „Flora der nordwest- 

 deutschen Tiefebene" page 48 Buchenau 1 ) adds: „Die nicht seltene 

 var. fluitans Roth (als Art) .... kann ich nach Beobachtungen in 

 der freien Natur und nach Vergleichung eines (sterilen) Roth'schen 

 Originalexemplares nicht als eine eigene Art, sondern nur als eine 



') Buchenau, Fr., Flora der nord westdeutschen Tiefebene, Leipzig 1894. 



IS* 



