INTRODUCTION. lo 



in number of species, but in somewhat undulating, 

 virgin, sandy districts they occur in considerable 

 abundance. A few are met with among the sub- 

 merged plants in slow streams and rivers, and some 

 species usually occur on faces of rocks over which 

 water is constantly trickling, growing associated with 

 filamentous Alga? such as species of Stigonema and 

 tfrlii::ntli //,/'. Some occur in more rapidly running 

 water among other Alga?, as Vauch&ria geminata and 

 Sirogonium xtii-tinni/ ; or among mosses such as I>lin<li<i 

 iti'/ita, Fontinalis tuifipi/i-rfint, Amblystegium yln/n-inn, 

 and Rhacomitrium nci<-ulnre ; or among such Hepatica? 

 as tfntjHiititi undulata, Anc/irti multi/fida, and Nardia 

 em-arginata. The occurrence of some species where 

 water is rapidly moving indicates that their mucila- 

 ginous investment possesses considerable tenacity. 

 Some species are associated with Batrachosp&rmum 

 nil/inn in peaty waters, and in moor pools with 

 Jungermannia injl<it ; others are attached to the 

 leaves of Jxoi'trx lacustris, Lohfliit Dortmanna, and 

 Eriocaulon ; the peduncles and petioles of Nymphaea 

 and Nuphar ; and the submerged parts of the sterns 

 and leaves of Oalliti-i<_'1t<>, MyriophyUum, and Nr/Yymx 

 jjititttitx. Utricularia minor and U. int<'i-tn<'<lin are always 

 submerged and are often clothed with a rich coat of 



O 



Desmids. Many rare species occur on the leaves of 

 the submerged form of Littorella lacustris, and a few 

 on species of Nitella and ('Inmi- in the beds of rocky 

 lakes. Desmids are also often associated with floating 

 filamentous alga? of various genera. Many species 

 occur in quiet pools among such mosses and Hepatica? 

 as Sphagnum nniturfitut, N. cuspidatum v&r. plumosum, 

 Amblysteguim scorpioides, and A. exannulatum ; others 

 occur in mountain springs where Philonotis /<>ntn<i, 

 Jungermannia cordifolia, Saxifraga stellaris, and Epilo- 

 l>iurn alpiiiiini abound. 



Some Desmids are found in the Plankton of large 

 lakes, occurring in the surface-water far away from the 

 shores. Staurastrum anatinum., N. pelagicum, N. ji^-Kiln- 



