Symbiosis 141 



perished. Techet also found ('08) in the genera Halimeda, Udotea, Valonia, 

 and Acetabularia, that plants in water of weak salinity grew more rapidly and 

 were less branched than in more strongly saline solutions. 



The adaptation of certain fresh water Algae to a life in salt water depends 

 upon the power to absorb salts from outside and thus permanently acquire an 

 increased osmotic pressure within the living cell 1 . Some Green Algae are 

 apparently able to bring about such osmotic changes in a very slow and 

 gradual manner, others rather more quickly, but the great majority not at all. 

 It is in this way that certain types of Green Algae have been able to migrate 

 from fresh water into brackish water and finally into the sea. The inability 

 of most Green Algae to adapt themselves to a considerable change in the 

 osmotic strength of the cell-sap accounts for the fact that comparatively few 

 of them are inhabitants of the sea. 



A large number of the Green Algae are minute forms occurring in bogs, 

 pools and lakes among the leaves of submerged aquatic macrophytes, and 

 often adherent to them by means of mucus. A considerable number are 

 constituents of the freshwater plankton, not infrequently dominating it during 

 the warm period of the year. 



A few of the Chlorophyceae have become constituents of the thalli of many 

 Lichens, having entered into a symbiotic relationship with some fungus, 

 although, as before explained (p. 37), this phase of symbiosis may in many 

 cases be one of helotism. It is mostly one or two of the unicellular and 

 colonial members of the Protococcaceae which are thus found. These Algae 

 are for the most part indeterminable, the effect of their prolonged association 

 with the fungus having so modified them that even cultures very often do not 

 afford a clue to their exact identity. In the aquatic species of Verrucaria 

 the algal constituent is also one of the Protococcaceae. 



Apart from members of the Protococcacese, which occur in large numbers of crustaceous, 

 foliaceous and fruticose lichens, species of Trentepohlia occur in ARTHRONIA, CCENOGONIUM, 

 GRAPHIS, GYALECTA, and RACODIUM ; and Phycopeltis expansa occurs in STRIGULA 



COMPLANATA. 



Even species of Cladophora and Vaucheria are stated thus to associate themselves 

 with a fungus ; and the simplest, and probably the most primitive, of all lichens, 

 BOTRYDINA VULGARIS, contains the Alga Coccomyxa subellipsoidea Acton ('09). 



1 Livingston ('00) showed that the stimulus which caused the alteration of form in the 

 developmental stages of a species of Stigeoclonium (Myxonema) was the change in the osmotic 

 pressure of the medium in which the Alga was growing. He showed that a high osmotic pressure 

 (1) decreased vegetative activity, (2) inhibited the production of zoogonidia, (3) caused cylindrical 

 cells to become spherical, and (4) freed the Alga from certain limitations as to the orientation 

 of planes of cell-division. On the contrary a low osmotic pressure (1) increased vegetative 

 activity, (2) accelerated the production of zoogonidia, (3) caused developing cells to become 

 cylindrical, and (4) determined the orientation of the planes of cell-division. The inability of 

 most Green Algse to change more than very slightly the osmotic pressure within the living cell 

 may therefore be in part responsible for the relatively small amount of polymorphism in the 

 Chlorophyceae (see p. 145). 



