External Factors and Size 153 



coccus when placed in various solutions of sodium 

 chloride. The results are indicated in figure 53, each 

 point of the graph representing measurements of 

 diameter made upon over a hundred cells. The curve 

 is of the same shape as for yeast. 



In the green colonial flagellate Gonium, Hartmann 

 ('24b) obtained giant individuals by cultivating in 

 "Knop solution" of 0.02 or 0.05 osmolar concentration 

 instead of the usual 0.001 osmolar. Upon transfer to 

 the more concentrated medium, reproduction ceased 

 for several days while growth proceeded. When a 

 larger size had been attained, then reproduction began 

 in a few colonies; but most continued to grow until 

 each individual had three times the usual diameter. 

 This increase of about 27 times in volume required 

 many days, and the individuals which became largest 

 never reproduced, but died. If large individuals were 

 returned to the dilute medium, they divided very rapid- 

 ly, and thus gradually restored the size of the individ- 

 uals. In this process there was a sudden increase in 

 the number of fissions per colony, for whereas each 

 individual normally divided four times in quick suc- 

 cession, they now divided five times, thus producing 

 colonies of 32 individuals instead of the usual 16 in- 

 dividuals. 



Mechanism. We may compare the amount of 

 shrinkage in solutions of equal osmotic pressures in 

 the several species of unicellular organisms which 

 have been measured. Fortner ('25), whose results 

 were given earlier in figure 23, found that Paramecium 

 lost 73 percent in volume when transferred from water 

 to a solution of sucrose having an osmotic pressure of 

 10 atmospheres. The time required for this degree of 

 shrinkage is not known. In sodium chloride solutions 

 of equal osmotic pressure (0.24 M), yeast lost only 26 

 percent and Pleurococcus 54 percent. The conclusions 



