INTRODUCTION / 



many plants, and may be indispensable for some. Those elements to 

 which particular attention has been directed include cobalt, silicon, 

 sodium and vanadium. Cobalt is an essential element for animals, and 

 various micro-organisms must be supplied with Vitamin B12 of 

 which cobalt is a constituent. A small amount of cobalt is said to be 

 required by blue-green algae, and the growth of higher plant cells is 

 also affected by its absence. Early investigators established that 

 silicon is not a major nutrient, but since it is extremely difficult to 

 free solutions from the last traces of this element, evidence that it is 

 not a micronutrient is still inconclusive. Silicon is certainly essential 

 for the normal growth of those algae which possess silicious cell 

 walls, but although it comprises a large part of the total ash in 

 some monocotyledons (see Table 2), higher plants can apparently 

 be grown in its virtual absence without ill effects. Sodium has often 

 been found to promote the growth of plants under conditions of 

 partial potassium deficiency, and this has led to the idea that 

 sodium can substitute for potassium to some extent. An obligate 

 requirement for both sodium and potassium has been claimed for 

 some blue-green algae, and sodium cannot be replaced by any other 

 element without detrimental effects on the growth of marine algae. 

 Vanadium is apparently an essential element for the green alga, 

 Scenedesmus obliquus, and it can replace molybdenum in some 

 species of nitrogen-fixing bacteria {Clostridium spp.). 



For optimum growth, the essential elements must be supplied 

 in soluble form, and only at certain fairly definite concentrations. 

 An excessive amount of one element, and particularly of a micro- 

 nutrient, results in the appearance of toxicity symptoms. In the 

 case of boron and copper, growth of many plants is inhibited when 

 concentrations greater than 1 part per million are supplied, whereas 

 deficiency symptoms develop at concentrations lower than about 

 0-05 parts per million. Chloride is the only micronutrient which 

 can be presented to many plants at a relatively high concentration 

 without ill effects (see Chapter 9). 



The functions of most of the essential elements are well under- 

 stood. Nitrogen and sulphur, for example, occur in proteins, and in 

 many other cell constituents. Phosphorus enters into the composition 

 of nucleic acids, phospholipids, and a number of coenzymes, 

 including the pyridine nucleotides and nucleoside phosphates. 

 AXihoVigh. potassium is required in considerable quantity for optimum 



