10 HISTORICAL INTRODUCTION 



rubidium, bromine, iodine, fluorine, selenium and beryllium. 

 This group was designated C13. The fourth solution also con- 

 tained sodium. All the elements of the A 4, B7 and C13 groups 

 were present in very low concentration, that is, as traces. The 

 four solutions, which may be denoted by I, II, III and IV thus 

 contained respectively the following mineral elements : 



I: N, S, P, K, Ca, Mg, Fe. 

 II: elements of I + B, Zn, Mn, Cu, +C1. 

 Ill: elements of 11 + Mo, Ti, V, Cr, W, Co, Ni. 

 IV: elements of TII + A1, As, Cd, Sr, Hg, Pb, Li, Rb, Br, I, F, Se, Be, 



+ Na. 



The fresh weights in grams of the plants grown in these 

 different solutions are shown in Table I. Thus the great effect 

 of the four well-established micro-nutrients is well demon- 

 strated, but the seven additional elements of solution III 

 produced a further increase in growth of asparagus, while for 

 lettuce their effect was most striking, the yield being increased 

 about ten times as a result of their addition. The subsequent 

 demonstration by Arnon and Stout that molybdenum is neces- 

 sary for the tomato has been mentioned earlier, and this leaves 



Table I. Growth of plants of asparagus and lettuce 

 in four different culture solutions. (Data from Arnon) 



in doubt whether the effectiveness of the seven elements of 

 solution III not included in solution II is due solely to the 

 molybdenum or whether some or all of the others are also in part 

 responsible for this. That no further increase in yield results 

 from the addition of the extra thirteen elements of solution IV 

 at first sight suggests that none of these is essential for the 

 growth of asparagus and lettuce, but it may be that one or more 

 of these is actually necessary but exists in sufficient quantity 

 as impurity of one of the salts used in making up solution III. 



