OF VITAL PHENOMENA 35 



and hence any contained sugar could aid in maintaining the 

 turgor. 



A solution whose osmotic pressure is lower than that of the 

 cell is said to be hypotonic, and one whose pressure is greater, 

 hypertonic. The osmotic pressure of many animal cells is very 

 near that of the medium, or the fluid which bathes them. When 

 the medium is hypotonic the cell swells, when hypertonic it 

 shrinks until equilibrium is established. Since animal cells are 

 not surrounded by very strong cell walls, they burst or swell 

 when placed in pure water. But some animal cells are imper- 

 meable to water, and consequently are not affected by changes 

 in the medium. 



Fresh water protozoa are prevented from bursting by the 

 activity of the contractile vacuole. Zuelzer (1907) acclimatized 

 Ameba verrucosa to sea water and the contractile vacuole beat 

 more and more slowly and finally disappeared in y 2 sea water. 

 The few marine protozoa that have contractile vacuole are 

 parasitic and hence not in pure sea water and the pulsations of 

 the vacuoles are much slower than is the case with fresh water 

 forms. 



Most of the cells in the mammalian, reptilian and avian bodies 

 are bathed by fluids whose osmotic pressure is practically con- 

 stant. One of these fluids is the blood. Its osmotic pressure 

 at body temperature is about 8 atmospheres (6.7 at 0°), its 

 freezing point being about — .56°. This corresponds to about 

 10 per cent (.3 molecular) cane sugar or .95 per cent NaCl solu- 

 tion (.16 mol). See Fig. 30, Chapter XV. 



More minute data on osmotic pressure will be considered in 

 Chapter XV. 



