18 



THE FOUNDATION 



Part I 



of electrolytes either within or outside them. The sensitiveness of the ani- 

 mal organism to hydrogen ions is apparent in scores of cases; in a large num- 

 ber of animals the control of respiration is through the hydrogen-ion concen- 

 tration of the blood. Hydrogen-ion concentration (symbol pH) of substances 

 in their surroundings is also of greatest importance to living organisms; the 

 range of many aquatic animals, certain protozoans, insects, and fishes is 

 limited by it; so is the range of earthworms. 



Tracers. The use of radioactive isotopes as tracers for investigating life 

 processes is probably one of the most significant developments in modern bio- 

 logical work. Such a possibility had been recognized for some years but was 

 limited by the fact that all the work had to be done with heavy elements such 

 as lead, bismuth, and mercury. The isotopes chosen are labeled by exposure 

 to radiations from a radioactive element. After this treatment they give off 

 radiations for a longer or shorter period. The ease of this modern technique 

 is comparable to locating a white penny among ordinary copper ones. They 

 are introduced into plants and animals in various ways (Fig. 2.8). For exam- 



ple. 2.8. The presence of radioactive tracers shown by radioautographs in slices 

 of tomato, especially in the seeds. The vine from which the tomatoes were taken 

 was grown in a solution containing radioactive zinc (Zn^^). This was taken up 

 throughout the plant and affected the photographic plates like light. (Courtesy, 

 P. R. Stout, University of California.) 



