INTRODUCTION. 37 



faaiily some genera belong. Thus Merismopsedia, placed by 

 Meneghini and Kiitzing in the Desmidieee, I believe, with 

 Meyen, to belong properly to the Palrnelleae. On the other 

 hand, Cylindrocystis, placed in the latter by Meneghini and 

 Brebisson, has been rightly removed to the Desmidiese by 

 Kiitzing and Hassall. Brebisson's discovery of conjugated 

 specimens of Coccochloris protuberans and Cocc. rubescens 

 brings to our notice another link connecting these families, 

 whilst some species of Scenedesmus may be allowed to have 

 an almost equal claim to rank with either. 



Respecting the uses of the Desmidiese little is known. 

 Doubtless, in common with other aquatic vegetables, they 

 tend to preserve the purity of the water in which they live, 

 and Mr. Williamson has ascertained that to a great extent 

 they furnish food for the bivalved moUusks which inhabit 

 fresh waters. 



As the Desmidiese are unattached and very minute, they 

 are rarely gathered in streams : nevertheless interesting 

 species may occasionally be obtained where the current is 

 so sluggish as to permit the thin retaining mucus to elude 

 its force. In small shallow pools that do not dry up in 

 summer they are most abundant ; hence pools in boggy places 

 are generally productive. 



The Desmidiese prefer an open country. They abound on 

 moors and in exposed places, but are rarely found in shady 

 woods or in deep ditches. To search for them in turbid 

 waters is useless : such situations are the haunts of animals, 

 not the habitats of the Desmidiese, and the waters in which 

 the latter are present are always clear to the very bottom. 



M. de Brebisson informs me that in France, calcareous 

 districts, which are so favourable to the Diatomaceae, are 

 very unproductive of Desmidieee. 1 have myself had no 

 opportunity of ascertaining whether the same fact obtains in 

 England. Mr. Thwaites and Mr. Jenner have indeed fur- 

 nished me with fewer habitats from calcareous soils, but the 

 less frequent occurrence of small pools and bogs in such di- 

 stricts may perhaps partly explain the deficiency. 



