184 VEGETABLE PHYSIOLOGY 
the colouring matter. The influence both of the metal and 
of light in this particular may consequently be similar. 
That it is associated with the plastid does not appear im- 
probable in view of some observations of Macallum’s that 
iron is always found in direct relationship with the chromatin 
of the nucleus, of which it appears to be an integral part. 
There is here evidence of a close association between the 
metal and some forms of living substance. 
Iron can be absorbed with advantage apparently in any 
soluble combination. 
The third group of elements comprises several that are 
of importance to particular plants, but are not universally 
necessary. Others usually included here are not known 
to be functionally important at all, except that they have 
a certain power of replacing to some extent the more 
important metals which have been already spoken of. 
Of the metals of this group, sodium is the most widely 
distributed. It exists in all soils, and it is capable of 
absorption in considerable quantities. Experiments by 
means of water-culture show, however, that its beneficial 
influence is extremely slight. It can be omitted from the 
culture-fluid without entailing any harm to the plant, and 
its presence in any quantity will not compensate for the 
absence of potassium (fig. 89, 1 and 8). If compounds of 
sodium and potassium are present together in sufficient 
quantity, the latter is always absorbed in far the largest 
amount, indeed almost exclusively by many plants. So- 
dium seems able, however, to effect a certain economy 
in the use of potassium. If a cereal plant is supplied 
with too little potassium, and with a certain amount of 
sodium, development is normal, and an examination of 
the distribution of the two metals in its tissues shows 
that the potassium is accumulated in the flowers and seeds, 
while the sodium replaces it in the vegetative parts. It is 
absorbed in the same combinations as potassium, but the 
chloride is not, as in the latter case, a valuable galt. 
Indeed, sodium chloride is generally deleterious, except, 
