4 i6 PHYSIOLOGY 
elements themselves, over thirty out of the present total of seventy- 
eight having 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 reliable 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 will 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. 
When 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 gamed into the processes concerned. 
