ABSORPTION OF MINERAL ELEMENTS BY PLANTS 249 



The role of the various metals in the life of the plaut will be 

 discussed only in general. Potassium is closely connected with 

 the vital activity of protoplasm, since most of it is found in the 

 meristem and in general in young organs, whose cells are rich in 

 protoplasm. An important role is ascribed to potassium in the 

 general metabolism of the cell, especially in the formation of 

 carbohydrates and proteins. In the active regions of the plant, 

 as much as 50 per cent of K2O may be found in the ash. Of all 

 the necessary cations, potassium is the only radioactive element. 

 Consequently, it may also have some other significance for the 

 plant as yet unexplained. 



The magnesium content of the ash is considerably less than 

 that of potassium. Both seem to be widely distributed in the 

 various organs. Like potassium, magnesium is found mainly 

 in the seeds and young organs, where it constitutes 10 to 15 per 

 cent of the total ash content. Its general physiological function 

 evidently is similar to that of potassium. Besides, magnesium 

 is present in some of the most important metalorganic com- 

 pounds, of which chlorophyll is a conspicuous example. Up to 

 50 per cent of magnesium may be found in the combined state, 

 while potassium is present in the plant almost entirely in the 

 form of free ions. 



Regarding the demand.s for calcium by plants, one must note 

 that for the lower organisms without chlorophyll, such as the 

 fungi and bacteria which in general have the same require- 

 ments for cations as the higher plants, calcium is not 

 absolutely necessary. Calcium is abundant in the older parts 

 of higher green plants, for instance, in the bark of trees, and in 

 old leaves. One of its most important functions seems to be the 

 neutralization of poisonous oxaUc acid, w^hich is formed in many 

 plants as a by-product of metabolism. In many plant tissues, 

 crystals of calcium oxalate occur as druses, raphides, and other 

 formations. A small amount of calcium is, how^ever, absolutely 

 necessary for the growth of young tissues; and if calcium is 

 excluded from the nutritive solution in water cultures, a marked 

 reduction of the root system is observed. A sufficient amount 

 of calcium seems to be very necessary also for proper growth and 

 development of legumes. 



Iron is required for the formation of chlorophyll. In its 

 absence, the plants become chlorotic. This is not the only role 



