CHAPTER IV 



THE FUNCTIONS OF TRACE ELEMENTS 



IN PLANTS 



Some years ago R. W. Thatcher (1934) published a short 

 paper in which he put forward a classification of the chemical 

 elements based on their functions in plant nutrition. In the 

 first place he pointed out that nearly all the elements which are 

 known or have been suggested to have a function in plant 

 nutrition are included in the first four periods or orbits of the 

 periodic classification. These first four periods, with the atomic 

 numbers 1 of the elements, are shown in Table VII. Of the 



Table VII. The first four periods of the periodic classification 



Group 



elements shown in this table we may dismiss the inert gases of 

 group O. In the other groups there are a number of elements 

 which so far have not been found essential for plants, but all 

 the elements known to be necessary for plants are included in 

 these first four periods with the one exception of molybdenum 

 (period 5. group VI, atomic number 42). The heavier metals do 

 not figure among elements necessary for plants. 



Thatcher's classification of the elements in relation to their 

 functions in the plant was based on the conception that "green 

 plants are the energy-absorbing and energy-storing agents of 



1 The atomic number is the number of free positive charges in the 

 nucleus of the atom. Except for hydrogen it is very roughly half the 

 atomic weight. 



