151 



On the Nutritive Conditions Determining the Growth of certain 

 Fresh-water and Soil Protista. 



By H. G. Thornton (New College) and Geoffrey Smith, Fellow of New 



College, Oxford. 



(Communicated by Prof. G. C. Bourne, F.B.S. Received April 20, — Read 



May 14, 1914.) 



[Plate 12.] 



It is well known that in ponds and lakes cycles of development occur, 

 in which various kinds of animals and plants replace one another in 

 succession, but the conditions are usually so complex that the succession 

 rarely repeats itself with regularity from year to year, and it is impossible 

 to assign, with any certainty, the successive phases to their determining 

 causes. The same kind of cyclical development occurs in artificially made 

 organic infusions, where bacteria, algae, flagellates and ciliates replace one 

 another in irregular sequence. 



The object of this paper is to indicate certain lines of experiment upon 

 which it may be possible to attack this problem. 



Woodruff has contributed some data for studying the underlying causes of 

 these successive events, and the work of numerous authors has added to 

 our knowledge of the factors regulating the growth of algae and diatoms. 

 Amongst these may be mentioned the work of Oswald Richter* on the 

 nutrition of fresh-water algae, and that of MiqueLf and more recently of 

 Allen and Nelson,! on the culture of diatoms. The work of these authors 

 tends to show that, even in the case of algae and diatoms in which 

 nutrition appears to be holophytic, the presence of some organic matter 

 in the culture medium is of great assistance to the growth of the organisms. 



The experiments with Euglena viridis were carried out with the object of 

 investigating the nature of this organic matter which exerts a beneficial 

 influence on the growth of apparently holophytic protista. 



The method employed is to use a culture medium containing a constant 

 proportion of the inorganic salts necessary for the nourishment of a 

 holophytic organism, and to supply the organic matter in the form of 



* Oswald Eichter, 'Die Ernahrung der Algen,' 1911. 

 t Miquel, 'Le Diatomiste,' 1892. 



| E. J. Allen and E. W. Nelson, "On the Artificial Culture of Marine Plankton 

 Organisms," ' Journ. Marine Biol. Assoc.,' vol. 8, No. 5, 1910. 



