— 111 — 



XIII. — Anabaena catenula îsa summer-form, but is appa- 

 reil dy adversely affected by exposure to prolonged summer- 

 sunshine. 



XIV. — The time of dominance of Euglena virklis in the 

 summer dépends on the enrichment oi' the water with the orga- 

 nic substance i'onued by the decay of the filamentous algal flora. 

 There is also évidence that it is favoured by bright sunshine. 



XV. — The two species of Trachelomonas présent in the 

 pond exhibit différent periodicity, T. hispida showing only one 

 (an autumn) maximum, T. volrocina. showing two (spring and 

 autumn) maxima. No deflnite relation to meteorological condi- 

 tions is apparent. 



XVI. — Peridinium tabulatum is favoured by warmer 

 températures and possibly also requires relatively clilute water. 



XVII. — The Diatoms attain their cliief abundance during 

 the winter-months. The amount of the epipliytic Diatoms is 

 determined by the amount of available substratum. There is 

 some évidence that Synedra radians and Eunotia lunaris 

 prefor Cou ferra and Microspora respecti vely as hosts. The 

 causes of the periodicity of the (mainly small) species of Navi- 

 cula hâve not been elucidated, but there is some indication that 

 tliey are intluenced by the amount of organic substance in the 

 water. 



XVIII. — In small ponds irregular factorsplay a mucli great- 

 er part than in large tracts of water, whose periodicity is chiefly 

 determined by seasonal factors. Several examples of correlated 

 factors hâve been recognized in the study of the periodicity of 

 Barton'spond. 



XIX. — The principle of limiting factors plays a great part 

 in the periodicity of freshwater algal végétation, as is shown 

 by the enumeration of a large number of instances. In most 

 cases only one limiting factor appears to corne into play in 

 nature, but there are examples of two factors being concerned. 



XX. — Exceptionally favourable conditions at the time of ini- 

 tiation of the maximum may lead to persistence for some time 

 during subséquent unfavourable circumstances. On the other 

 hand previous particularly unfavourable conditions may lead to 

 a poor development under subséquent favourable ones. 



XXI. — The relation between sunshine and the sexual repro- 

 ductive process is very plain in Barton's pond, and Klebs' conclu- 



