GROWING PLANTS WITHOUT SOIL — JOHNSTON 387 



jars used are frequently of the " Mason " type and should be wrapped 

 with heavy paper to exclude most of the light from the roots. 

 This covering prevents algae from growing in the solution. 



Recently a very interesting series of experiments has been com- 

 pleted by Dr. J. E. McMurtrey, of the United States Department of 

 Agriculture, on the mineral deficiency symptoms of the tobacco 

 plant. The nutrient solutions he used were so devised that one 

 element could be omitted without changing the amounts of the other 

 elements. The general appearance of these plants, each of which 

 suffered from an insufficient amount of an essential element, is shown 

 in Plate 4, together with a tobacco plant grown on a complete mineral 

 diet. In connection with these experiments, a key has been prepared 

 which gives promise of having considerable value in the diagnosis of 

 certain malnutritional diseases of the tobacco plant. 



The division of radiation and organisms of the Smithsonian Insti- 

 tution has initiated certain types of research in which extremely ac- 

 curate measurements are being made of several fundamental plant 

 activities. In one experiment the absorption of carbon dioxide by 

 the plant is being studied under accurately controlled conditions. It 

 is highly desirable that the plants be grown in a medium which can 

 be better controlled than soil. In other experiments the effect of 

 various light intensities and wave lengths on the growth of plants 

 is being investigated. Here again soil can not be used because of 

 errors it would introduce into the experiment. For the reasons 

 pointed out in the foregoing discussion the plants used in these 

 accurately controlled experiments are grown in nutrient solutions 

 made by adding the proper inorganic chemical compounds to dis- 

 tilled water. 



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