A Fantasy on Ageing 187 



1953) or carbohydrate or even occasionally substances like 

 haematochrome (Droop, 1954). Too much food of the reserve 

 type, however, may per se limit nuclear growth and repro- 

 duction. The unicellular green plant Hcematococcus pluvialis 

 forms cysts when the supply of nitrogen is too small to 

 support growth. The cysts remain small and green except in 

 the presence of light or acetate when they become large and 

 coloured due to the accumulation of fat and haematochrome. 

 The fat cysts germinate much less readily than the small 

 green ones. Tokophrya infusionum again (a suctorian proto- 

 zoon) can be fed exclusively on Tetrahymena geleii and can 

 be grown with it as the sole source of nutriment (Rudzinska, 

 1953). Tokophrya differs from most protozoa in that each 

 cell has a limited span of life which can be made long or short 

 by the level of nutrition. Reproduction is effected by budding 

 off" small motile daughter cells, not by fission into two cells of 

 equal size and potentialities. If the supply of Tetrahymena 

 is moderate and just balances the requirements of the Toko- 

 phrya for their energy expenditure, cytoplasmic and nuclear 

 growth, and cell division, all goes well. If, however, the 

 numbers of Tetrahymena are unlimited the supply of readily 

 oxidisable food outstrips the requirements of Tokophrya for 

 growth and energy expenditure and the cells begin to lay 

 down large amounts of storage material. After a time they 

 cease to reproduce in the normal way, turn into giant cells 

 and die before their normal life-span has been completed. 

 Lilly (1953) has found that the formation of giants can be 

 prevented by adding a mixture of guanylic, adenylic, and 

 cy tidy lie acids and of uracil (in other words of nuclear 

 material) to the medium and it would appear that the 

 suctorian, which has no control over its appetite, cannot 

 obtain enough nuclear material from its standard diet to 

 enable it to synthesize proteins and enzymes (Gale and 

 Folkes, 1954) as rapidly as it can build up reserve materials 

 in the cytoplasm. 



Thus, when unicellular organisms become overnourished, 

 nuclear growth and division may slow down. This will per se 



