ALGAE — MEIER 421 



to serve as points of attachment for other plants. The ellipsoidal 

 pebble, really belonging to the vegetable kingdom, to the casual 

 observer is in nowise different from an ordinary rounded pebble. 



Marl deposits at the bottom of shallow lakes are thought to be 

 almost wholly due to the action of the blue-green algae. 



Just as the Eskimos can live and prosper in the frozen north, 

 so nature has equipped species of algae to live in the ice and snow. 

 In the Swiss Alps, the Pyrenees, the Carpathians, the Urals, the 

 Sierra Nevadas, the Andes, and in the mountains of Scandinavia 

 and Greenland, where large areas are covered with eternal snow, 

 these hardy little plants may be found coloring the old snow with 

 a rosy tint of great beauty, or sometimes in such abundance that 

 they look like blood stains. The motile stage of the algae is con- 

 fined to the thawing surface. Generally they are on rather hard, 

 more or less permanent snow of which the surface is somewhat 

 wavy during the thawing period, and the algae are found in the 

 wave troughs, which they undoubtedly accentuate because of their 

 ability to absorb the heat rays. Sometimes the algae are only 

 faintlj'^ visible through the upper layers of snow where they are 

 about an inch or so under the surface. They are especially con- 

 spicuous in large fields of soft snow where their faint tinge leaves 

 bloody traces when walked upon. Mineral dusts, pollens, or seeds 

 help to increase the absorption of the heat rays and prepare better 

 conditions of life than exist on the quite bare surface of the snow. 

 Sphaerella nivalis and varieties of Cystococcus are the algae most 

 commonly known as " red snow." A temperature above 39° F. is 

 harmful to these particular varieties. 



There are other varieties that color the snow yellow-green or 

 green, though it is generally supposed that the young cells are 

 yellow-green and green, and that as they grow older the haemato- 

 chrome or red pigment develops and masks the green pigment. 

 Various hypotheses concerning the physiological adaptations en- 

 abling snow algae to grow at low temperatures have been based 

 upon the storage of reserve food as fats and upon the haemato- 

 chrome functioning as an absorber of heat rays. In experiments at 

 the Smithsonian Institution I found that the red pigment formed 

 in Uaeniatococcus pluvialis when the alga was exposed to intense 

 continuous illumination for a month, whereas in intermittent light 

 the pigment remained dark green, and in contmuous darkness red- 

 dish brown. The greatest amount of growth took place in the 

 intermittent light, and the least amount in continuous darkness. 



The marine algae or seaweed receive the earliest mention in our his- 

 tories. Humboldt relates how the Phoenician mariners came to a place 

 where the sea was covered with rushes and seaweed. The seaweed is 



