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THE QUARTERLY REVIEW OF BIOLOGY 



a second optimum at pH io.o and then at 

 higher concentrations dropping to zero 

 at pH ix. o. The rate at pH io.o is the 

 most rapid, being an average of 97.0 

 microns per hour for all root hairs meas- 

 ured, and in the case of one hair attaining 

 a rate of 114.x microns per hour, which 

 was the most rapid growth obtained for 

 any hair of the 3500 or more so far meas- 

 ured in this and other solutions. 



It is perhaps significant to observe that 

 the simplest solution in which root hairs 

 will grow supports the most rapid growth. 

 While other solutions may be found later 

 which will give still more rapid elonga- 

 tion, yet this indicates the great impor- 

 tance of calcium to root hair elongation, 

 and perhaps the lack of a necessity that 

 any other mineral element be present on 

 the outside. That calcium is utilized di- 

 rectly in root hair elongation is indicated 

 by the fact that it is not the lowest con- 

 centration of calcium hydroxide that 

 supports the most rapid growth. It is 

 apparently the highest concentration 

 which can overbalance the inhibitory 

 effect of the hydroxy 1 ion. This is further 

 indicated by a comparison of root and root 

 hair elongation in calcium hydroxide. 

 Root elongation rises to an optimum at pH 

 8.0 or 8.5, above which it drops off gradu- 

 ally to zero without rising to a second 

 optimum. This indicates that at least 

 in these dilute solutions the calcium does 

 not penetrate into the interior of the root, 

 but is consumed largely or entirely by the 

 superficial cells, especially the root hairs; 

 while the hydroxyl ions do penetrate and 

 in the more alkaline solutions have a 

 retardative effect upon root elongation. 



The bimodal graph for variations in 

 hydrogen ion concentration has been 

 secured for a large number of biological 

 reactions, especially those involving 

 growth. Salter and Mcllvaine obtained 

 it for the growth of wheat seedlings, 



Hixon for the germination and growth of 

 seedlings of various plants, Olsen for the 

 growth of plants to maturity, Hopkins 

 for the development of a fungus, Gibbenlla, 

 Cole for the growth of roots of corn seed- 

 lings, Arrhenius for the extension of leaf 

 areas and growth of plants in water, sand, 

 and soil, Cohen and Clark for the number 

 of viable bacteria of Bacillus dysenterica, 

 Robbins for the growth of fungi and 

 reactions of potato tubers, McSwiney and 

 Newton for reactions of smooth muscles, 

 Hercik for root growth of Pbarbitis, and 

 Hopkins for locomotion of Amoeba. Simi- 

 lar bimodal curves have been obtained with 

 non-living colloids, with regard to their 

 swellability, precipitation, viscosity and 

 other properties. Loeb demonstrated that 

 in the case of certain proteins the median 

 minimum of these curves was located at 

 the hydrogen ion concentration of the 

 isoelectric point of the protein. It has 

 therefore been concluded by Robbins and 

 others that the depression in the pH graph 

 for growth was indicative of and a re- 

 sponse to the isoelectric point of the 

 constituent proteins. Pearsall has found 

 in some cases trimodal and multi-modal 

 graphs for the response of plants to 

 changes in pH, and suggests that the 

 various depressions in the graphs indicate 

 the respective isoelectric points of the 

 constituent proteins. 



It is obvious that a much more extensive 

 study is necessary in order to ascertain 

 the growth rate of root hairs in different 

 solutions of other calcium compounds 

 than the hydroxide. With the hydrox- 

 ide, hydrogen ion concentration is pro- 

 portional to molar concentration, and 

 hence there are only two variables, con- 

 centration and rate of elongation. With 

 simple calcium salts, however, the hydro- 

 gen ion concentration may vary without 

 changing the molar concentration, and 

 vice versa, so that there are three variables : 



