1 16 THE JOURNAL OF BOTAXT 



be so founded, as they stand apart from all others, sueh as P. Boh- 

 bi/isil Oakes, a North American species, of Avhich a single leaf is 

 sufficient for identification. 



Having said this, 1 have nothing but cordial recognition to give 

 of Dr. Hagstrom's Avork. He explains difficulties that have puzzled 

 students of the genus for years, and though I differ from him in some 

 of his conclusions, I feel that more work is needed before one can 

 ventvn-e to deny them. 



With regard to the endemic species, there seems something in the 

 climatic influences that induces conditions not seen elsewhere — changes 

 in the floating leaves, etc., that are difficult to explain. In one species, 

 F. aasfraliensis, these remarkable changes range from wide mem- 

 branous leaves to narrow submerged floating leaves with very strong 

 lattice-like structure. At least three species have been made out of 

 this ; it was only when numerous examples were seen that the plant 

 was understood, hence my remark above as to single specimens. 



The author disposes of my P. disshnilis, pointing out that it is 

 really only a state of P. stria fits lluiz & Pa von, and in this I entirely 

 concur. There is, however, perhaps some slight excuse for this, as he 

 places P. striatus under three names. 



Here and there one finds conclusions arrived at in one place and 

 doubted in another. Thus the author assumes that P. nodosi/s Lam., 

 which is not now known in Denmark, might formerly have occurred, 

 and that climatic changes caused it to die out. Here of course is a 

 liypothesis that cannot be proved or disproved ; but elsewhere in the 

 instance of a hybrid he remarks, " P. nodosus cannot enter into this 

 as it is not known to occur there." If once this su])posed occurrence 

 of a species is accepted 1 do not see where the making of hybrids is 

 to cease. 



It must be remembered in dealing with the conclusions arrived 

 at that Dr. Hagstrom's material consisted of all the Scandinavian 

 lierbaria, with a few from Petrograd and Berlin, so that so far as this 

 goes liis book may be accepted as a considered exposition of these 

 lierbaria. Xo doubt there may be difference of opinion even here, 

 but the work demonstrates the value of his material. He makes 

 several North American hybrids, assuming or suspecting the occur- 

 rence of one of the parents, but still he evidently knows his plants, 

 and strong evidence must be adduced before his conclusions can be 

 controverted. 



Dr. Hagstrom shows that some species have been suggested on 

 insufficient gi-ounds, but by characters which are fully brought out 

 he places them on a level with generally accepted ones. Thus he 

 shows good grounds for accepting P. pauormiiamis Biv.-Bern., 

 indicating how it can be se])arated from P. pusillus L. ; some 

 named forms of the latter will need examination to see whether they 

 come under P. j^anormitanus. It is evident that he has not been 

 able to examine some of the old types in the Berlin herbarium which 

 he places under one name : doubtless many of these are merely 

 cliniatal or geographical forms, but they will have to be considered. 

 The Paris herbarium, which has not been consulted, will have to be 



