NUTRITIVE FUNCTIONS OF SALTS 103 



plants will grow only in a basic soil, and calcium salts are 

 commonly added to bring about the proper soil reaction. Such 

 plants as well as those which grow wild in calcium areas are known 

 as calciphiles or calcicoles. Others demanding an acid soil where 

 little calcium is present are known as calciphobes. Since calci- 

 phobes, like calciphiles, require lime in their nutrition but an acid 

 soil, a better name would be acidophile. 



Iron. — Iron occurs universally in plants in small amounts, but is 

 more abundant in physiologically active regions such as leaves 

 and flowers. It is added to the culture solution in very minute 

 amounts, but seems to be necessary for every living cell. Accord- 

 ing to Warburg, iron acts as an oxygen carrier and is, therefore, 

 essential in respiration (Chap. XXV). In its absence chlorophyll 

 will not form, and plants without it are pale and chlorotic even 

 when grown in the light. The first leaves which develop from the 

 germinating grain will be green but, as the iron stored in the seed 

 becomes exhausted, the succeeding leaves become paler and paler. 

 If a trace of ferric chloride is then added to the nutrient solution, 

 in two or three days all the leaves will be green. As previously 

 suggested, it is thought that the iron aids catalytically in the 

 formation of the pyrrol ring, which plays such a prominent part 

 in the chemical constitution of chlorophyll. In the case of chlorosis 

 of fruit trees like the pear, a similar transformation is quickly 

 effected by boring into the sap wood and inserting a small crystal 

 of ferric sulphate; and spraying chlorotic trees of Pinus bank- 

 siana with FeS0 4 has been found to reduce the chlorosis from 75 

 to 5%. Most soils contain sufficient iron for the crops grown upon 

 them, but in Hawaii and Porto Rico pineapples can grow only 

 when iron sulphate is added to them. This is commonly done by 

 spraying during growth. 



Magnesium. — This element enters into the chemical composi- 

 tion of the chlorophyll molecule and is hence necessary for its 

 formation; in its absence, also, chlorosis results. It is more abun- 

 dant in parts undergoing development and is hence thought to 

 be necessary for the formation of nucleoproteins (the proteins of 

 nuclei), which may explain why nodules in leguminous plants do 

 not develop well in its absence. Magnesium is also found abun- 

 dantly in seeds and in those parts rich in fat. In Vaucheria, fat 

 globules were not found unless magnesium was present. It also 

 seems to be necessary for the transportation of phosphorus, and, 



