374 THE ALGAE 



and the method of attachment. The differences seen above, there- 

 fore, must be explained by the behaviour of the motile reproductive 

 bodies which either come to rest in the depressions or else are swept 

 there by micro-currents in the water. Another interesting feature 

 is the frequent association of Gomphonema with the basal cells of 

 Oedogonium, but so far there is no evidence to suggest whether this 

 is a casual relationship or not. Ponds with muddy bottoms have a 

 reduced number of epiphytes probably because the pH and the 

 gases evolved are toxic, but so far little or no work has been carried 

 out to ascertain the effect of the host plant on the microchemical 

 environment. Summing up, it can be said that the factors influenc- 

 ing the distribution of epiphytes are as follows : 

 (i) Age of substrate. 



(2) Rate of growth of substrate. 



(3) Light intensity. 



(4) Screening. 



(5) Nature of the surface. 



(6) Chemical surroundings. 



Of these (3) is probably the most important, although it is diffi- 

 cult to separate its effects from those of (i) and (4). 



In recent years increasing use has been made of the unicellular 

 green algae, especially Chlorella, Scenedesmus and Chlamydomonas^ 

 as a means of purifying sewage effluent. Most of the pioneer work 

 has been carried out in the United States where large lakes of 

 effluent are innoculated with the algae, and these after a few days 

 increase in such number that the oxygen they produce materially 

 aids the aerobic decomposition of the remaining organic matter. 

 When one of these oxidation lakes is operating, water is allowed to 

 flow through at a rate which takes it about 6-7 days to pass from 

 entry to exit or else it is aUowed to recirculate. At the end of the 

 period when the effluent emerges it is clear of organic matter and 

 the bacterial count of pathogens has also been reduced to a neg- 

 ligible value. The largest oxidation lakes for sewage purification 

 (i 100 acres) are at Auckland in New Zealand. 



Soil Algae 



Terrestrial algae may be classified conveniently as foUows : 

 (i) Aerophytes. These can be subdivided into : 



(a) Epiphyllophytes, algae growing on leaves, e.g. Phyco- 

 peltisy Cephaleuros, 



