37o 



THE QUARTERLY REVIEW OF BIOLOGY 



selves to a wide range of conditions. On 

 the contrary, they are extremely sensitive 



Fig. 9. Tri-dimensional Graph Showing the Rate 

 of Growth of Root Hairs of Collards in Con- 

 centrations Ranging from 0.004 M to o.ioo 

 M and in Different Degrees of Alkalinity and 

 Acidity 



to changes in the medium, as was pointed 

 out above in regard to temperature, and 

 as will be shown below. But it does 

 show clearly that the root has a great 

 ability to adjust itself to a wide range of 

 conditions, and to produce root hairs in 

 solutions of many different constitutions. 

 The prime requisites seem to be that cal- 

 cium be present, and that the minimum 

 amount of calcium supplied be greater the 

 greater the acidity of the solution, that 

 the solution shall not be too strongly 

 alkaline or acid, and that the concentra- 

 tion of the salt must not be too high since 

 the resistance to acid and alkali is less 

 at high concentrations. 



It is to be observed that the data referred 

 to above on the rate of elongation of 

 root hairs in different solutions were ob- 

 tained from hairs which appeared on the 

 root ten or more hours after immersion in 

 the solution. A consideration of the 

 growth rate of root hairs which had been 

 growing in air might yield somewhat 

 different results. A limited study was 

 made to ascertain the rate of elongation 

 of these so-called amphibious root hairs 

 several hours after immersion. It was 

 found that only in solutions which support 

 the optimum growth rate did they con- 

 tinue to grow during the first two hours 

 after immersion. In all other solutions 

 they did not grow during the first two or 

 three hours, and then they began growing 

 at a rate coordinate with that obtained 

 for the aquatic hairs in the same solution. 

 However, in especially concentrated solu- 

 tions the older root hairs collapsed, those 

 a little younger ceased growing, but did 

 not change their form, those still younger 

 grew after a period of rest, but changed 

 their form or direction, while the young- 

 est resumed growth apparently normally. 



This method of studying the rate of 

 elongation of root hairs and of plotting 

 tri-dimensional graphs affords a means 



