24 MINERAL SALTS ABSORPTION IN PLANTS 



of a variety of ions. For details of the technique, particularly as 

 apphed to analyses of plant materials, see Riches (1948), 



c. Flame photometry has gained widespread popularity for the 

 estimation of some cations, particularly sodium and potassium ions. 

 The apparatus required is inexpensive, while the method is rapid 

 and reasonably accurate. An aqueous solution of salt is injected 

 into an air-coal gas, or air-butane, flame under controlled conditions 

 and the intensity of radiation at a selected wavelength is determined 

 photometrically. With the assistance of calibration curves based on 

 analyses of solutions of known concentration, the amount of salt 

 in an unknown solution can be measured. Selective optical filters 

 are available which enable determinations of one ion to be made 

 in the presence of another, for example potassium can be estimated 

 in presence of sodium, and vice versa. The flame spectrophotometer 

 is a more elaborate instrument than the flame photometer, incor- 

 porating a monochromator, and exhibiting greater sensitivity and 

 versatility. 



d. Radioactive isotopes. The availability of a variety of radio- 

 active isotopes since about 1945 has made possible the introduction 

 of a new and powerful technique for the measurement of ion 

 movements. Some of the isotopes which have been used are listed 

 in Table 6. Gamma-radiation easily penetrates a thin layer of glass 

 and counters, incorporating a Geiger-Muller (G.M) tube within a 

 glass envelope have been designed for measuring the radioactivity 

 of labelled salt solutions. Beta-radiation, which does not penetrate 

 glass to an appreciable extent, is usually measured by means of an 

 end- window, G.M tube, in conjunction with electronic counting 

 equipment. An aliquot of the radioactive solution or tissue extract 

 is dried on a planchette, which is placed at a fixed distance from the 

 mica window of the G.M. tube. Sometimes the radioactivity of a 

 slice of tissue can be determined accurately enough, merely by drying 

 the material on to the planchette and holding it flat in front of the 

 tube. If the tissue has appreciable thickness, a correction for self- 

 absorption of some of the radioactivity by the specimen must be 

 made. When the half-life of an isotope is short in relation to the 

 length of the experiment, it is also necessary to make an allowance 

 for natural decay of isotope during the experimental period. 



Combinations of radioactive isotopes can be used to determine 

 the absorption of both ions of a dissociated salt. This may be done 



