426 THE ALGAE 



to water. Changes in osmotic pressure affect respiration in Lamin- 

 aria^ Hormosira hanksii and Fucus serratus (rate rises on dilution) 

 but not in Enteromorpha, Porphyra or F. vesiculosus. Photosyn- 

 thesis is affected in Fucus and Ulva^ being more than doubled when 

 sea water is diluted by a third. 



Bio-electric phenomena 



The large cells of certain Siphonales, the Charales and Hydro- 

 dictyon are very suitable for studies of this phenomenon. It has 

 been found that potential differences exist between the interior and 

 the exterior of the cell, usually the exterior being positive, e.g. 

 Chara, Nitella, Halicystisy Bryopsis, Hydrodictyon, though in 

 Valonia^ Ernodesmis and Chamaedoris the exterior is negative. It 

 seems likely that the gradient of potassium ions is responsible for 

 this potential in Nitella and Hydrodictyon. In the other algae it 

 seems likely that the potential is a result of an internal concentra- 

 tion gradient within the protoplasmic lining itself. This gradient 

 would be the simi of all the salts of organic acids, amino-acids, 

 proteins and mineral salts contained in the protoplasm. 



* Symbiosis 



The most striking and well-known examples of symbiosis in- 

 volving algae are provided in those cases where the plants are asso- 

 ciated with animals, especially Coelenterates, or with fungi, as in 

 the common Hchens. Apart from these examples, however, there 

 are other cases which are not so well known, largely because they 

 are not so common. Gloeochaete, for example, is a colourless genus 

 of the Tetrasporaceae which possesses blue green bodies that look 

 like chromatophores, though they are really a symbiotic blue-green 

 alga. Glaucocystis is a colourless genus of the Chlorococcales in 

 which a symbiotic member of the Myxophyceae also forms the 

 blue-green *chromatophores' that appear as a number of curved 

 bands grouped in a radiating manner around the nucleus. In this 

 case the illusion is further enhanced because they break up into 

 short rods at cell division. It has so far proved impossible to grow 

 the blue-green alga separately and it may thus have lost its power 

 of independent growth. Geosiphon, which is variously regarded as a 

 siphonaceous alga or as a Phycomycete, possesses small colonies of 



