10 THE JOUKNAL OF BOTANY 



of shelter, and food-supply in the form of waste products of the 

 'host, 1 but also of diminished light-supply if within a plant, or of 

 speedy digestion if inside an animal. Tims in the densely populated 

 environment of the sea, it may be taken for granted that for benthic 



organism, anything- may grow on anything else; and if the host be 

 penetrable, anything may get inside — in every case with certain 

 inevitable consequences. The general facts of intrusive organism 

 thus afford a view of further possibilities of commensalism or 

 symbiosis, as so far the commonplace of the sea; though appearing 

 more unusual, and hence a phenomenon attracting greater attention, 

 in the case of higher plant-forms and the very specialized vegetation 

 of the land; more particularly as we first become familiar with it in 

 the impoverished versions of northern latitudes. Before emphasizing 

 or exaggerating the unique nature of the Lichen-symbiosis, it may be 

 well to consider the biology of the more general phenomena of 

 'symbiotic' life in the sea, from which undoubtedly the Lichen Fungi 

 have been at some time derived. Leaving on one side the case of 

 suggested Bacterial s} T mbiosis, as in forms presumably assisting 

 nutrition by the decomposition of celluloses in the alimentary canal of 

 even higher animals, and the effect of similar Bacteria in decomposing 

 the humus-complex in soil, and so aiding the partial saprophytism 

 of all higher land-vegetation — every grade of association is possible, 

 from harmless and casual intrusion to facultative and obligate 

 association, to s3 r mbiotic union closed by the digestion of the in- 

 truder, to the state of complete belotism with loss of any somatic 

 individuality in the intrusive units, or to a condition of complete 

 dependence in the part of the host. Hence in marine biology the 

 question of intrusive organism plays an important part, and soon 

 attracted the attention of early land-biologists; the possibility of 

 intrusion being based primarily on the opportunity for penetration 

 of the peripheral membranes or tissues of the ' host,' and all such 

 phenomena again being commonly indicated as symbiosis in a loose 

 sense. Kecognition of green chloroplasts and algal units in many 

 widely distributed and clearly animal forms was unavoidable, as seen 

 conspicuously even in fresh-water organism as green amoebae, green 

 ciliate Protozoa as Stentor, the fresh-water sponge (Sponyi/Ia), the 

 green Hydra, as also in the green marine worm Convoluta. The 

 algae were isolated as ZoocMorella^, and the photosynthetic formation 

 of starch was demonstrated in several cases 2 ; the value of such carbo- 

 Irydrate to the host being expressed by keeping green Spongilla 

 exposed to light in filtered water for a month, and Hydra viridis for 

 live weeks 3 . Similar phenomena are associated with the presence of 

 Brown Flagellates, classed generally as Zooxanihellce, also of widest 

 distribution in the sea. Thus. Brandt (1883) 4 gives a long list of 

 marine organism in which such intrusive 'yellow ceils" may he 

 observed; though these do not form starch as a photosynthetic 



1 Beyerinck (1890), Bot. Zeit. 48, p. 725. 



2 Carter (1875) for Spongilla ; Ami. Mag. Nat. Hist. p. 187. Brandt (18S3), 

 Mitt. Zool. Stat. Naples, p. 2li:». 



3 Brandt, loe. cit. p. 2G5. 



4 Brandt, loc. cit. p. 191. Famintzin (1889). Mem. Acad. St. Petersburg, 

 xxxvi. 



