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observers to determine. The species which are already accepted by 

 botanists bear their usual names; any forms or varieties which are 

 well known to botanists are also named as we find them in the 

 works of Scandinavian writers on the genus (e. g. " P. natans 

 i. jhtriatilis Fries," a form known to be a mere state produced by 

 even transient local conditions, but which, having been named, is 

 also usefully included in this series) ; and under the true species are 

 placed, in addition to their undoubted varieties or forms, each of 

 which bears a name, a second series distinguished by Greek letters, 

 which is composed of forms much further removed from the type, 

 many of which may ultimately be found to deserve full specific rank. 

 In the meantime botanists cannot have a safer guide through the 

 intricacies of this most difficult genus than Dr. Tiselius, who is 

 unrivalled in his knowledge of the living plants, and in his great 

 ability in the selection and preservation of specimens. 



1. Potamogeton natans L. 26. P. gramineus L., 



(planta florens). f. terrestris Tis. (major). 



2. „ (planta fructificans). 27. „ „ (minor). 



3. „ f. jiuviatilis Fr. 28. „ fi. fluctuant Tis. (major). 



4. „ f. amphibia Fr. 29. „ „ (minor). 



5. P. spa rgan if alius Laest. 30. ,, y. septcntrionulis Tis. 



6. P.fluitans Roth, f. typica Tis. (major). 



7. P. plantaejineus Du Croz. 



f. typica Tis. 32. „ „ (minimus). 



„ J. debilis Tis. 

 /'. poli/f/nni/olius, Pourr. 31. P. nitm* Web. :>, f. tgpir 



10. „ f. angumfolia Fr. 



11. „ f. amphibia Fr. 



12. P. alpinus Balb. 



f. pakistris Fr. (major). 



13. „ „ (minor). 



14. „ f. viridis Tis. 



15. „ f. lonqifolia Tis. 



16. „ f.borealis Tis. 



17. „ f. gracilior Tis. 



18. „ f. iterata Tis. 

 ID. /'. dubius Tis. 



20. P.graminvuxL., f. tupira Tis. 



21. ,, f. sinuaisis Tis 



22. „ f. normal* (Fr.) Tis. 



nyustifolia Tis. 



■emis Tis. 



y. subtil m Tis. 



intermedium Tis. 

 i. innominate Tis, 



From the above list the following numbers are selected as being 

 illustrative of the plan of the work, and in most instances as of 

 special interest to British botanists. 



1-4. P. natans, the usual British forms; 4. f. amphibia Fries 

 well exhibits the phyllodes, which are always developed in the land 



