INTRODUCTION . J 



undoubtedly be extended as more sensitive methods of detection 

 become available. A plant probably contains at least traces of all 

 the elements present in the environment in which it grows. Absorp- 

 tion is not restricted to those which occur naturally; it has been 

 shown, for example, that plants will readily take up plutonium which 

 is produced artificially in nuclear reactors. 



Table 4. Composition (milliequivalents per litre) of the Sap in Chara 

 ceratophylla, and of the Media in which the Plants were grown 



(CoUander, 1942) 



Individual plant species tend to have a characteristic salt content, 

 which is relatively independent of the composition of the medium 

 in which they are grown — at least as far as physiologically important 

 elements are concerned. Table 4 shows that the concentrations of 

 salts present in the water in which the stonewort, Chara ceratophylla 

 grows has little influence on the potassium, magnesium, or calcium 

 content of the plant, although there are effects on the amounts of 

 sodium and chloride taken up. The shoots of higher plants tend to 

 show greater independence in this respect than do roots. In 

 particular, sodium is present at a low concentration in the above- 

 ground parts of many plants, irrespective of the amount of sodium 

 in the soil, and it is for this reason that the diet of domestic animals 

 (and man) which feed largely on shoots, must be supplemented with 

 common salt. 



C. Functions of Mineral Salts 



The presence of a particular element in a plant does not 

 necessarily imply that it serves a useful function there. In order to 

 prove that an element is essential, it is necessary to show that 



1 M.S.A.P. 



