74 TEXTBOOK OF PLANT PHYSIOLOGY 



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 physiological function evidently is 

 similar to that of potassium. Besides, magnesium is present in 

 some of the most important metallorganic compounds, 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 demands of calcium by the plant, one must 

 note that for the lower organisms without chlorophyll, such as 

 the fungi and bacteria and which in general have the same 

 requirements for cations as the higher plants, calcium is not abso- 

 lutely necessary and that 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 the neu- 

 tralization of the poisonous oxalic acid, which is formed in many 

 plants as a byproduct of metabolism. Hence, in plant tissues 

 crystals of calcium oxalate as druses, raphides, and other forma- 

 tions are found. A small amount of calcium is, however, abso- 

 lutely 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. 



Iron is necessary for the formation of chlorophyll. In its 

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

 iron in the life of the plant, however, as it is equally necessary for 

 organisms without chlorophyll. Iron is generally accepted to be a 

 catalyzer in the oxidation processes closely connected with growth, 

 but especially in respiration. Manganese seems to have a similar 

 function. 



The significance for the plant of those slight traces of zinc, 

 boron, silica, and perhaps some other elements, the necessity of 

 which for the successful growth of plants in water cultures has been 

 revealed after the use of reagents purified in the highest degree, 

 remains unknown so far. Evidently these ions must be added to 

 the number of catalysts necessary for the plant. 



26. Toxic Properties of Single Salts. Balanced Solutions. — 

 The study of the specific activity of separate cations is highly com- 

 plicated by another very important factor, namely, the toxicity 



