50 BUILDING OF THE LIVING MACHINE 



duced by almost any interruption or interference with 

 the operation of the systems which constitute protoplasm. 

 Thus, coagulation, produced by heat, alcohol, or other 

 agent, stopping and preventing chemical change, is fatal. 

 The solution of colloidal gels by acid, alkali, or electricity 

 will cause death. Chemical changes which result in pre- 

 cipitation or solution of protein or other essential con- 

 stituents of the system will stop operation and this 

 constitutes death. 



The Mineral Constituents. — What the botanist 

 has called the " ash constituents " among the food ma- 

 terials of plants, namely, calcium, magnesium, and po- 

 tassium, and the small amounts of iron, correspond to 

 what the animal physiologist speaks of as " dietary acces- 

 sories." These elements are all indispensable to health 

 and normal activity, but they do not all appear as con- 

 stituents of protoplasm. They are taken up from the 

 soil in the form of nitrates, sulphates, phosphates. While 

 sulphur and phosphorus are constituents of protoplasm 

 with nitrogen, carbon, hydrogen, and oxygen, and while 

 magnesium is a constituent of chlorophyll, calcium, po- 

 tassium, and iron have never been shown by analysis to 

 be constituents either of the living protoplasm or of any 

 essential product. Speculation, and even assertion, have 

 been freely inrlulgod in ; but beyond recognizing certain 

 apparent " uses " in certain plants, and some of the physi- 

 cal and chemical properties of these elements wherever 

 they occur, one cannot now go. In certain plants calcium 

 neutralizes the oxalic acid which might otherwise accumu- 

 late in harmful quantity, bringing it down as a crystalline 

 precipitate of calcium oxalate. That this is invariably 

 its function is disproved, for example, by the free oxalic 

 acirl in rhubarb, in wood sorrel, etc. Nor does calcium 

 neutralize the other acids which characteristically accu- 

 mulate in the tissues of plants — citric acid in lemons, 

 malic acid in apples, salicylic acid in cranberries, etc. 



Potassium, shown to be feebly radio-active, may owe 



