THE WEB OF LIFE 297 



Besides the Zooxanthellae there are other symbiotic 

 forms, e.g. the green Zoochlorellse. Their occurrence is 

 known in Amoebae (a colony of Amoeba viridis nour- 

 ished for ten years without food), in the green Hydra, in 

 the green freshwater sponge, in some sea-anemones, and 

 in many Alcyonarians. They are usually referred to 

 the family Palmellacese, but are not certainly known to 

 live apart from symbiosis. 



Lichens. One of the most striking instances of partner- 

 ship is that illustrated by lichens, which Schwendener, 

 Bornet and other botanists proved to be compound 

 plants. Each consists of the branching and interlacing 

 threads of a Fungus, enclosing partner Alga cells. The 

 Fungus fixes the plant, absorbs air, water and salts, pro- 

 tects the Alga from drought and injury, and forms spores 

 which are wafted away by wind and water, and may start 

 new lichens if they find their proper partners. The Alga 

 uses the sunlight to build up carbon compounds, and it 

 joins with the Fungus in forming sexual reproductive 

 bodies. By taking proper precautions the Alga can be 

 got to live in water without the Fungus, and the latter 

 can live on sugary media or the like without the Alga. 



The life of many a lichen is rather more complicated 

 than we have indicated. Thus in many of those that 

 grow on trees the Fungoid elements absorb decaying 

 organic matter ; and some tropical forms are actually 

 parasitic, absorbing food from the living tissues of leaf 

 and stem. In some cases the Fungus seems to kill its 

 partners and absorb them. The Algae are sometimes so 

 much shut in from light and air that it is difficult to believe 

 that they can do much in the way of photo-synthesis, 

 and there is strong evidence that in such cases the Algae 



