326 Mineral Nutrition of Plants 



growth, would it not be possible to indentify either an essential cellular 

 constituent or a crucial biochemical reaction in which the inorganic 

 element participates? For some of the essential elements the answer to 

 this question has been obvious as soon as the chemical constitution of 

 cellular substances was established. There was no difficulty in assigning 

 an indispensable role to carbon, hydrogen, and oxygen solely on the 

 basis of their entering into the composition of all living matter. As for 

 the elements derived from the soil, the indispensability of nitrogen, 

 sulfur, and phosphorus was adduced at an early period from their 

 identification with proteins and nucleoproteins. As far as cations are 

 concerned, the discovery that magnesium is an integral part of the 

 tetrapyrrolic chlorophyll molecule assured an essential status to that 

 element irrespective of what other functions it may perform in the 

 plant. Calcium combines with pectic acid to form calcium pectate in 

 the middle lamella of the cell wall. 



This leaves only two elements on the classical list, potassium and iron, 

 to which no essential status was assigned solely on the basis of then- 

 entering into the chemical composition of cellular constituents. With 

 respect to potassium this state of affairs persists to this day. No organic 

 compounds containing this essential element have been detected among 

 the components of plant cells although suggestions have been made 

 that it may combine with proteins. The case of iron deserves special 

 treatment. Although it is known today to be an essential component of 

 cellular constituents, that discovery followed an initial path rather 

 distinct from analytical biochemistry. It resulted from a series of in- 

 vestigations whose primary objective was the understanding of a phy- 

 siological process. In the case of iron the process investigated was 

 respiration. 



Before embarking on this phase of our discussion, it might be well 

 to state certain premises. A convincing demonstration that a given 

 element is indispensable to some vital process would suffice to establish 

 its essentiality, even in the absence of appropriate growth experiments 

 or corroborative analytical evidence on plant constituents. It is not in- 

 conceivable that for some micronutrient required in exceedingly minute 

 quantity, the previously discussed experimental difficulties would make 

 it impossible to remove completely the element in question either from 



