ALGAE — MEIER 423 



interesting example of symbiosis. Lichens are composed of green 

 algae that live within the colorless filaments of fungi. Possibly 

 neither plant could exist alone on the hot bare surfaces of rocks where 

 we so often find them growing in beautiful rosettes of various colors 

 and on trees. But together each helps supply the needs of the other. 

 Tlie fungi absorb and retain moisture from the air, while the algae 

 by means of their green chlorophyll carry on the photosynthetic 

 activity necessary for the double existence. 



Algae may grow in symbiosis with diatoms, with other algae, and 

 with higher plants. In the case of the alga Anabaena cycadeae^ 

 growing in the roots of the higher plant Cycas^ a true symbiosis 

 exists. The algal cells in the nodules are soon surrounded by nitro- 

 gen-fixing bacteria of the soil. The leaves of the higher plant obtain 

 energy from the sun and manufacture carbohydrate food material 

 not only for the plant itself but to supply the bacteria which make 

 nitrogenous material available to the alga. In some cases of sym- 

 biosis, the two plants, though living in close association, seem to have 

 neither a harmful nor a beneficial effect upon each other. 



Codium hursa, which is found in northern seas as well as warmer 

 ones, appears to harbor a flora and fauna all its own. It has a 

 large spherical thallus with a fluid-filled interior cavity. When 

 the thallus is ruptured, the fluid gushes forth as if under consid- 

 erable pressure. This fluid is more saline than the surrounding sea 

 water. Lining the interior cavity, especially near the base, is a 

 reddish mucus consisting of blue-green algae (mostly filamentous 

 forms), numerous diatoms, microscopic animals, and worms. Some 

 of the species are peculiar to this habitat. Their reddish color is 

 correlated with the feeble violet light at the depth of 14 to 40 meters 

 at which they live. 



Algae also grow in one-celled animals, as the blue-green algae that 

 color amoebae and other Protozoa. They also grow in symbiosis 

 with small marine animals, mollusks, and insects. One species of 

 Rhopalodia grows in thick, yellow-brown, matted masses on the back 

 of a beetle, Limnogeton^ on tropical lake shores. 



Most people are familiar with the term " mossback " as applied to 

 the common snapping turtle, or perhaps have noticed turtles covered 

 with green " moss." In reality, this green moss is a mass of green 

 algae. The shell of the turtle and the mud that settles upon it make 

 an excellent habitation for aquatic green algae, while in return for 

 their ideal living conditions they render the turtles as inconspicuous 

 as old rotten logs. Turtles have been seen with filamentous algae 

 attached to their shells sometimes in tassels fully twice as long as 

 the turtle. As the turtle swims along, the green filaments trail out 

 behind it in a most attractive manner. In general, the algae found 



