INTRODUCTION. 7 



and saltmarsh ditches, lagoons, and pieces of water 

 only separated from the sea by sand-bars, are usually 

 good hunting-grounds. Some species are capable of 

 existing in water at widely different degrees of tem- 

 perature. Of G. fragilis, which is the most hardy 

 and ubiquitous of them all, Dr. T. F. Allen remarks that 

 it is " found in every country and clime, in ice water 

 at the north and in the hot springs of the Yellow- 

 stone." It is also recorded from hot springs in 

 Iceland, in water the temperature of which was such 

 as to boil an ewQ> in four minutes ! 



oo 



Though for the most part plants of the lowlands, 

 some species occur in mountain-lakes and a few have 

 been found at great elevations. Braun and Nordstedt 

 record Gliara vulgaris from 7,000 feet on the Swiss 

 Alps, and the same species from 9,000 and 10,000 feet 

 in Chile, and, as well as Nitella clavata, from 14,000 feet 

 on the Cordilleras of Peru, G.laltico at about 12,800 feet 

 in Bolivia, and G. delicotula on the Sierra Nevada in 

 California at 10,000 feet, 



Charophytes usually grow in rather shallow water, 

 ranging from a few inches to a few feet deep, but 

 sometimes, especially in the clear water of lakes, are 

 found at much greater depths, the factor of light 

 being a most important one. We are indebted to 

 Messrs. W. H. & W. Harold Pearsall, M.Sc., for the 

 following very interesting contribution to the ecology 

 of Nitella opaca. 



" The results appended have been confirmed by 

 many thousands of soundings extending over the past 

 seven years. Esthwaite Water alone, the smallest lake 

 included, occupied four weeks' continuous work in 

 1914 and two in 1915. 



" Nitella opaca occurs in mixed communities at 



