41 6 PHYSIOLOGY 



elements themselves, over thirty out of the present total of seventy- 

 eight ha\'ing been recorded. Yet of this large number only a few seem 

 to be indispensable. These are usually reckoned as calcium, potassium, 

 magnesium, and iron; while chlorin and sodium are present in all and 

 may be necessary. !Many attempts have been made to determine the 

 precise role of each of these indispensable elements, with rather conflict- 

 ing results. It does not seem possible by cultures which omit a par- 

 ticular element to reach rehable conclusions; nor is it at all likely that 

 the role of any particular element is simple, and the withdrawal of one 

 may permit others to act in a wholly different way. Thus if plants be 

 grown in solutions of calcium chlorid or of magnesium chlorid of a cer- 

 tain concentration, they wiU die; but if the two be mixed in the same 

 concentration, the plants will grow well. Singly both are injurious, 

 together they are not, though no reaction occurs between them. 



A\Tien therefore it is said that a definite amount of each " indispen- 

 sable " element is needed by a plant, and that the minimum determines 

 the crop (" law of the minimum "); that on potassium depends the for- 

 mation of new organs at the growing point; that calcium is required 

 for the transfer of starch, and so on, all such statements must be con- 

 sidered as extremely doubtful and liable to complete reversal when a 

 deeper insight is gained into the processes concerned. 



