COMPETITION FOR COMMON FOOD IN PROTOZOA 93 



a small quantity of sediment originating from the oatmeal was 

 allowed to remain.) The cultivation was made in tubes with a flat 

 bottom (about 1 cm. in diameter and 5-6 cm. high) of the nutritive 

 solution. The tubes were closed by cotton wool stoppers and kept 

 in a moist thermostat at 26°C. Close paramnized cork stoppers were 

 not found convenient because if we use them the population begins 

 to die off immediately after cessation of growth, and the curves take 

 the form described by Myers ('27). At the same time under optimal 

 conditions after the growth of the population has ceased the level of 

 the population is maintained unchanged for a certain time, and only 

 later Paramecia begin to die off. 



In the initial experiments no change of the medium in the course 

 of growth of the population was made, and the increase in the number 

 of individuals was studied according to the average values for the test 

 tubes of a definite age. The contents of the tube was destroyed every 

 time after examination just as in the experiments with yeast. The 

 counting was made under a magnifying glass on a slide plate. Figure 

 18 represents the growth of the number of individuals in pure lines 

 of Paramecium caudatum and Stylonychia mytilus cultivated sepa- 

 rately and in a mixed population. These data are founded on two 

 experiments which gave similar results. At the beginning of the 

 experiment into each tube were placed five Paramecium, or five 

 Stylonychia, or five Paramecium plus five Stylonychia in the case of a 

 mixed population. Stylonychia for inoculation must be taken from 

 young cultures to avoid an inoculation of degenerating individuals. 



(5) The growth curves of the number of individuals in Figure 18 

 are S-shaped and resemble our well known yeast curves. After 

 growth has ceased the level of the saturating population is maintained 

 for a short time, and then begins the dying off of the population which 

 is particularly distinct in Stylonychia. It is evident that this dying 

 off is regulated by factors quite different from those which regulate 

 growth, and that a new system of relations comes into play here. 

 Therefore there is no reason to look for rational equations expressing 

 both the growth and dying off of the populations. 



Figure 18 shows that Stylonychia, and especially Paramecium, in a 

 mixed culture attain lower levels than separately. The calculated 

 coefficients of the struggle for existence have the following values: 

 a (influence of Stylonychia on Paramecium) = 5.5 and /3 (influence of 

 Paramecium on Stylonychia) = 0.12. This means that Stylonychia 



