326 Algae. 



»' 



competition with Microspora and Oedogoiiiurn. Ophiocythmi attains 

 its raaximum at the time when the filamentous algal flora becomes 

 abundant. 



Chaetophora pisiformis is a spring-form, influenced in its abun- 

 dance by the concentration of the water. 



The Protococcales favour the warmer months of the year, but 

 really high summer-temperatures affect them adversely. Ineffigiata 

 requires higher temperatures for the attainment of its maximum 

 than do the other genera of Protococcales occurring in Bar ton 's 

 pond. The Protococcales are also influenced by the degree of con- 

 centration of the water, some {Pandorina, Sphaerocystis and possibly 

 Ineffigiata) requiring relatively dilute, others {Eudorina, Gloeocystis) 

 relatively concentraied water. 



Anabaena catenula is a summer-form, but is apparently adver- 

 sely affected by exposure to prolonged summer-sunshine. 



The time of dominance of Eiiglena viridis in the summer de- 

 pends on the enrichment of the water with the organic substance 

 formed by the decay of the filamentous algal flora. There is also 

 evidence that it is favoured by bright sunshine. 



The two species of Trachelomorias present in the pond exhibit 

 different periodicity, T. hispida showing oniy one (an autumn) 

 maximum, T. volvöcina showing two (spring and autumn) maxima. 

 No definite relation to meteorological conditions is apparent. 



Peridinium tabulatum is favoured by warmer temperatures and 

 possibly also requires relatively dilute water. 



The Diatoms attain their'chief abundance during the winter- 

 months. The amount of the epiphytic Diatoms is determined by the 

 amount of available substratum. There is some evidence that Sy- 

 nedra radians and Eunotia lunaris prefer Conferva and Microspora 

 respectively as hosts. The causes of the periodicity of the (mainly 

 small) species of Navicida have not been elucidated, but there is 

 some indication that they are influenced by the amount of organic 

 substance in the water. 



In small ponds irregulär factors play a much greater part than 

 in large tracts of water, whose periodicity is chiefly determined by 

 seasonal factors. Several examples of correlated factors have been 

 recognised in the study of the periodicity of Bar ton 's pond. 



The principle of limiting factors plays a great part in the perio- 

 dicity of freshwater algal Vegetation, as is shovi^n by the enumera- 

 tion of a large number of instances. In most cases only one limiting 

 factor appears to come into play in nature, but there are examples 

 of two factors being concerned. 



Exceptionally favourable conditions at the time of initiation of 

 the maximum may lead to persistence for some time during subse- 

 quent unfavourable circumstances. On the other band previous 

 particularly unfavourable conditions may lead to a poor develop- 

 ment under subsequent favourable ones. 



The relation between sunshine and the sexual reproductive pro- 

 cess is very piain in Barton's pond, and Klebs' conclusions are 

 thus confirmed by direct Observation in nature. There is also evi- 

 dence that unusual concentration of the water exhibits sexual repro- 

 duction in many forms. 



In most cases sexual reproduction begins as soon as the maxi- 

 mum is attained, but this is not invariably the case. Many forms 

 (but not all) die off almost completely after sexual reproduction 

 is over. 



