SYMBIOSIS 95 



There are other cases which can hardly be considered as 

 instances of true parasitism, for the two organisms involved 

 mutually benefit each other. Organisms thus living together 

 in a mutually dependent manner are termed " symbiotic." 

 One of the most remarkable and best known of these 

 examples is afforded by Lichens. Lichens are composed of 

 Fungi and Algae living together. Some of the Algae involved 

 are unicellular plants, others are multicellular or groups of 

 cells united together in colonies. The cells of the fungus 

 grow round the Algae. The Algae multiply and follow this 

 growth, enclosed by the fungus. The bodies thus composed 

 of fungus and Algae combined are known by the name of 

 Lichens. " The two processes of vegetation mutually support 

 and supplement each other. The Alga can live alone as a 

 plant containing chlorophyll, though it may be open to 

 question whether it can do so in all cases, but its vegetation 

 is not hindered by the fungus ; it can often be shown to be 

 permanently promoted by it. The fungus as a strictly 

 obligate parasite is dependent for its growth upon the Alga, 

 without which it cannot attain to its full development, and 

 in most cases, cannot get beyond the first stages of germina- 

 tion." * In many Lichens small brood-buds are formed which 

 separate spontaneously. These contain both fungus and 

 Alga, and under favourable conditions may develop into 

 another Lichen similar to the parent structure. We thus 

 have a widely distributed form, well known to every one, 

 which consists of two entirely different organisms living 

 symbiotically, which in many cases is capable of continuing 

 its existence and becoming distributed by brood-buds con- 

 taining both organisms. 



A still more remarkable case is afforded by a very low 

 form of worm (Convoluta roscoffensis) which lives symbioti- 

 cally with green Algae! 2 The Algae appear to form a special 

 assimilation tissue within the worm, enabling it to live like a 



1 De Bary, A., Comparative Morphology and Biology of the Fungi, English 

 translation, Oxford, 1887. 



2 Haberlandt, quoted by Von Graff in Die Acoela, Leipzig, 1892. 



