192 VEGETABLE PHYSIOLOGY 



Iron can be absorbed with advantage apparently in any 

 soluble combination. 



The third group of elements contains 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 3). 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, 

 perhaps, to the plants of the sea-shore, in which it brings 

 about their peculiar succulence. 



If we compare the influence of potassium, sodium, and 

 calcium, on the development of a crop of herbage plants, 

 we find that the presence of potassium leads to a develop- 

 ment of stems, flowers, and fruit, or to what may be regarded 

 as the maturing of the plants, while in the absence of 

 sufficient potassium and the presence of calcium and sodium 



