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THE QUARTERLY REVIEW OF BIOLOGY 



ganic salts. Still better results were ob- 

 tained by using solutions in jars, so that 

 the roots were not in contact with any 

 solid substratum at all. In this case it is 

 necessary to supply the roots with oxygen 

 by bubbling air through the culture solu- 

 tion from time to time. It was also found 

 that the composition of the solution 

 changes rapidly owing to the absorption of 

 some substances from it more rapidly than 

 others, and the liberating of substances, 

 especially carbon dioxide from the root. 

 This difficulty may be overcome by an 

 elaborate system of growing the plants in 

 a flowing solution. 



But even this most refined method of 

 studying the balanced rations of plants 

 has not given as conclusive results as are 

 desired. One of the difficulties consists 

 in the long period of time required to raise 

 the plants to maturity, even in these solu- 

 tion cultures. So many things may occur 

 during the course of several weeks which 

 may have an effect upon the final results, 

 that it becomes somewhat uncertain just 

 what to attribute to the effect of the one 

 element which is being studied. In order 

 to raise plants to maturity there must be 

 present in the solution at least seven 

 mineral elements. This means at least 

 three or four salts. To discover the 

 specific effect of each of these elements 

 then becomes a problem, because of the 

 interactions which are known to occur 

 between them. Finally there is the diffi- 

 culty of finding an exact criterion of the 

 degree of development of an organism such 

 as one of the higher plants. As noted 

 above increase in size, weight, dry weight, 

 etc. are the resultant of so many processes, 

 and are in addition affected by so many 

 external conditions, such as disease, insect 

 activity, light, etc. that their value as a 

 a basis for judging the well-being of the 

 plant is seriously impaired. 

 There is need then for a new method of 



investigating the effect of the respective 

 nutrient elements as well as the toxic and 

 stimulating factors in the environment of 

 a plant. It seems that a study of cell 

 enlargement of root hairs affords a logical 

 opportunity to do this, inasmuch as they 

 are in part, at least, the absorptive organs 

 of the plant. As noted above, the history 

 of the study of the balanced rations for 

 plants represents a series of progressive 

 simplifications of conditions surrounding 

 the experiment. First plants were studied 

 as they occur in nature. Then they were 

 placed as nearly as possible under the 

 same conditions as far as soil was con- 

 cerned. Next wind and many animals 

 were shut out, and the temperature and 

 humidity of the air were held more con- 

 stant. The next step was to exclude the 

 organic matter of the soil; and following 

 that the entire solid matter of the soil. 

 Finally the gaseous and mineral content 

 of the medium was controlled and kept 

 rather constant. It is now desirable to 

 simplify the experiment still farther by 

 confining the study to just one of the 

 phases of growth, such as cell enlarge- 

 ment, studying as far as possible each of 

 the nutrient elements individually, before 

 putting them together to form a balanced 

 solution. Furthermore there is the de- 

 sirability of studying the plant during a 

 shorter period of time, during which 

 light will be unnecessary, and the tem- 

 perature, and acidity or alkalinity of the 

 medium can be kept more constant. The 

 study of the rate of growth of root hairs 

 affords this opportunity. 



We can determine first the rate of elonga- 

 tion of root hairs in different concentra- 

 tions of calcium hydroxide. Inasmuch 

 as this is a basic compound, increasing the 

 concentration means also increasing the 

 alkalinity of the solution, so that, when 

 we have studied different concentrations 

 of calcium hydroxide we have also studied 



