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 salt. 

 Indeed, sodium chloride is generally deleterious, except, 



