E. A. PEYOE — BOTANICAL WOEK OF THE PAST SEASON. 71 



words on the present occasion. It seems to be confined to the 

 basin of the Ash, where, however, it occurs in profusion in several 

 localities, and is undoubtedly native. This has often been confused 

 with the large-fl.owered woodland variety of M. arvensis, Hoffm., 

 and a previous record for another part of the county is on that 

 account altogether untrustworthy. It has long been known as a 

 plant of Essex, but has not been observed in any other of the 

 adjacent counties. 



Potamogeton mucronatus, Schrad. Specimens from Marsworth 

 Reservoir, in the Thame district, have been submitted to Dr. 

 Boswell (late Syme), who has coiLfirmed the correctness of the 

 name. This is the P. compresms, L., of Smith, and British 

 botanists generally, but not of the last edition of Babington's 

 'Manual,' or of the 'Student's Flora.' It is a difficult species, 

 and has been greatly confused with P. pusilliis on the one hand, 

 and P. acutifolius, Link., or ohtusif alius, Koch., on the other. It has 

 only hitherto been known, with any degree of certainty, to have 

 occurred in six counties (Dorset, SuiTey, Norfolk, Warwick, Salop, 

 and Notts, cf. ' Top. Bot.' ii. p. 400 j, and the ripe fruit, which 

 I have not yet been able to obtain, is a great desideratum. 

 P. lucens, L., perfoUatus, L., crispus, L., oUusifoUus, Koch., 

 pusillus, L., pectinatus, L., flahellatus, Bab., and Zannichellia 

 palustris, L., are all to be found in the same neighbourhood, 

 with many other interesting aquatics, and some good Carices and 

 other marsh plants : the whole district is worthy of more careful 

 examination than it has yet received.* 



Impatiens fidva, Nutt. Grand Junction Canal (near Hunton 

 Bridge). Mr. J. E. Littleboy. I do not suppose the Impatiens at 

 Hunton Bridge has originated directly from American seed, but 

 rather that its appearance in our waters is closely connected with 

 the whole history of the first introduction of the plant into Britain, 

 and its gradual extension throughout the Thames basin. It will 

 be found to afford an excellent " example of the interdependence 

 of the parts of a river-system with reference to theii- floras."! I 

 have borrowed the following particulars principally from the ' Elora 

 of Middlesex.' A North American species, which is "now so 

 thoroughly and perfectly naturalized as to give no suspicion of its 

 exotic origin, it almost certainly originated in the gardens of 

 Albury Park, Surrey. A small stream, the Tillingboume, flows 

 through these gardens, and runs into the AVey above Guildford; 

 and this in turn flows into the Thames a little above Shepperton. 

 In this way the seeds have been carried by the water-current, 

 and by barges, etc., throughout the Thames Valley district." J 



* There is a specimen of Cardamine amara labelled from Tring, and collected 

 about forty years back, in the herbarium of Professor Babington at Cambridge. 



t Flora of Middlesex, p. xix. 



X ib. p. 71. This kind of naturalization is not confined to the vegetable king- 

 dom. Breissena polymorpha, Van Ben., a byssiferous freshwater mussel, which is 

 supposed to have been originally introduced from Russia on logs of timber, has 

 now spread itself through many of the navigable waters of England, and may 



