Growth Measurements 



181 



Fig. 50. Excised tomato roots grown in solutions containing, 

 from left to right, the following concentrations of Fe 2 (S0 4 ) 3 : none, 

 0.675, 1.35, 2.025, 2.7, 3.375, 4.05, 4.725, 5.4, 6.077, 6.75, 7.425, 8.1, 

 8.775, 9.45, and 10.125 x 10" 6 M. The linear increments of roots 

 grown in iron concentrations from 1.35 to 5.4 x lO"* 3 M are greater 

 than at higher concentrations, but the branches are short and the 

 general habit poor. Experience has shown that roots grow more 

 satisfactorily and for longer periods if the iron concentration is 

 from 6.0 to 7.0 x 10 -6 M. The extreme sensitivity of these roots to 

 slight variations in concentration of ions which are present in as 

 minute amounts as is iron is one of the striking characteristics of 

 such cultures. 



