COLOUR OF WATER AND VEGETATION 53 



is sometimes green or red. This is due to one of the simplest of our 

 Algae, the Protococcus, an organism consisting of one cell, in the 

 protoplasm of which red and green colouring matter is suspended. 



It is another Alga, very similar in structure to Protococcus, 

 that makes snow red, as travellers have often noted. The red 

 colouring may extend to the depth of two inches, and appears to 

 occur on most mountains with perpetual snow. This phenomenon 

 has been observed both in the Arctic and Antarctic zones ; on 

 the Alps, Scandinavian mountains, and on the Andes. Ice has 

 sometimes a superficial brown layer, due to those organisms which 

 can thrive on fine dust. 



The colour of the water in lakes has been the subject of some 

 experiment. As every one knows, many lakes have a distinct 

 colour of their own, independently of the tints due to changes in 

 the sky. Some are intensely blue, others green or yellowish, 

 some almost colourless. The natural colour of pure water in 

 sufficient quantity is blue ; the crevasses in glaciers are blue ; 

 why, then, should the water of some lakes be green ? It is sug- 

 gested that blue is turned into green by minute quantities of 

 organic matter in solution. The blue water of the Lake of Geneva 

 was turned into a green colour, similar to that of the Lake of 

 Lucerne, by adding to it an infusion of peat. Forel, who tried 

 these experiments, instances in support of his theory the two 

 lakes of Achensee and Tegernsee in the Tyrol. The former is 

 blue, the latter green ; the basin of the former is free from peat, 

 that of the latter is covered with peat mosses. 



This does not account for the greenish tint of sea-water near 

 the shore. Shallow water over yellowish sand would look green 

 from the reflection of the yellow light at the bottom of the water. 

 The constantly varying tints of river, lake, and sea are, however, 

 due above all to the sun, and to the changes in the intensity of the 

 rays of light. 



SUGGESTIONS FOR PRACTICAL WORK ON AQUATIC PLANTS 



I. Note the Plant associations 



(a) In bogs. There are at least three well-marked associ- 

 ations (p. 35). 



