NUTRITION 101 



is even less understood in its physiological relations than 

 calcium. As phosphate it may stand in some relation to 

 the formation of protoplasmic matters, although it is not a 

 constituent element of protoplasm. Like calcium, it occurs 

 as a constituent of some of the reserve foods stored hi such 

 oily seeds as Castor Bean (Ricinus) and Brazil Nut (Ber- 

 tholetia ) ; but even here, hi these complex compounds, it is 

 not clear whether it is the magnesium ( or calcium ) itself, or 

 the acid radicle of the salt, which is valuable. 



IRON. Although this element is not known to be a con- 

 stituent of protoplasm or of any compound incorporated 

 into the living protoplasm, it is indispensable to all plants. 

 The minimum needed is, however, smaller than that of any 

 other element. If more than the minimum amount of iron 

 is supplied, growth seems to be proportionally stimulated.* 

 Unless green plants receive enough iron they will not be 

 able to form chlorophyll, although iron is not a constituent 

 element of any chlorophyll pigment. Plants remaining 

 white from lack of iron are said to be chlorotic. Chlorosis 

 may be regarded either as the evidence of an abnormal 

 state of health or as the disease itself. The former seems 

 by far the more probable. 



The plant ordinarily obtains all the iron it needs from 

 the salts of iron dissolved in all natural waters. Sometimes, 

 however, chlorosis occurs in spite of the abundance of iron 

 in the soil. When a shoot grows so rapidly that iron salts 

 do not reach the developing parts rapidly enough, the new 

 leaves will be white instead of green. 



Some of each of the necessary elements found in the ash 

 of plants is necessary to the normal development of the 

 plant. The plant will develop normally only when it can 

 obtain an amount of mineral matter in solution more than 

 equal to the sum of the minimal amounts of each of the 

 ash constituents. In virgin soil, and in the natural forest, 

 the soil waters will contain all the necessary mineral salts 



* Richards, H. M. Die Beeinflussung dee Wachsthums einiger Pilze diirch 

 chemische Reize. Jahrb. f. wise. Botanik, Bd. 30, 1897. The effect of 

 chemical irritation on the economic coefficient of sugar. Bull. Torrey Bot. 

 Club, vol. 26. 1899. 



