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stituent of plants, especially in marine forms. The alga Valonia 

 contains in its cell sap one-fifth the concentration of sodium 

 found in the sea water where it lives. 



The list of elements occurring in plants and animals, and 

 therefore possibly necessary to life, has lengthened rapidly in 

 recent years. The agricultural chemist McHargue has made 

 many startling additions. He finds that small amounts of 

 manganese, copper, zinc, boron, barium, strontium, lithium, and 

 rubidium are normal constituents of plants grown under natural 

 conditions and that there are traces of nickel and cobalt. Most 

 of these metals are also constituents of animal tissues. Calves' 

 liver contains a surprisingly large quantity of copper (345 parts 

 per million) and a small amount of manganese and zinc. The 

 pancreas presumably contains traces of nickel and cobalt. 

 Among the most recent additions to the list of extraordinary 

 elements in living things is that of radium, discovered in 

 duckweed. 



A number of interesting questions arise in regard to the unex- 

 pected presence of these metals in living tissues. Why have 

 we been so late in realizing that they are there? How does the 

 organism tolerate those that are highly toxic; and are they all 

 necessary? The first question is readily answered by the state- 

 ment that the quantities are too small to be readily detected. 

 Living cells deal with concentrations infinitely lower than those 

 with which the chemist is ordinarily concerned. The presence of 

 radium in duckweed was ascertained, but the concentration of 

 this element in the pond water where the plant grows and from 

 which it must get its supply of radium was too low to be detected. 

 The spectroscope, the same instrument used in analyzing the 

 chemical composition of stars, has made possible the detection 

 of very minute quantities of elements. Lundegardh of Stock- 

 holm has been actively engaged in such work. Simply put, the 

 method consists in burning the stems, leaves, roots, and seeds of 

 a plant and then passing light through the ash and into a spectro- 

 scope; the latter separates the hght into its spectrum. Each 

 chemical element has its own particular spectral lines. It is, 

 therefore, possible to measure the amount of the element present 

 by observing the intensity of the lines. The spectrum is recorded 

 on a photographic plate. In the case of infinitesimal amounts 

 of minerals, a photoelectric cell is used as the light-measuring 



