228 Renewal of Culture Solutions [42 S 



It is apparent that the yield is better the more frequently 

 the solution is changed. If, after an initial period, the fre- 

 quency of changing is increased the yield is improved. It is 

 important, practically, that there is small difference between 

 the cultures changed every 3 days and those changed every 

 week. Daily change produces substantial improvement. Al- 

 lowing the solution to remain unchanged for so long as 2 

 weeks is markedly injurious. 



The above cultures were grown on a rotating table. An 

 additional set was grown in the same greenhouse at the same 

 time but not on the rotating table. The results follow: 



Dry Weight. 



grams. 

 Continuous flow of solution through culture jar at 



rate of about 1 liter daily 1.678 



Changed every 3 days 1.222 



Not changed at all 0.666 



This experiment is not strictly comparable with the one done 

 >n the rotating table, but it seems probable that continuous 

 flow of the solution must be regarded as more beneficial even 

 than daily change. 



Parallel with the experiments on the rotating table, one 

 set of three cultures was treated by removing the solution 

 weekly Lnd shaking it with bone black. The solution was 

 then filtered and restored to the culture jars. These cul- 

 tures gave an average yield of 0.780 gram, as compared with 

 0.621 gram for the unchanged culture not treated with bone, 

 black. Evidently the bone black treatment improved the 

 solution slightly but did not correct in important degree the 

 harmful effects of infrequent changing. It was noticed inci- 

 dentally that the magnesium injury that is characteristic of 

 this solution, for wheat, appeared more frequently and se- 

 verely when the changing was frequent than when it was 

 not. The color of the plants was greener in the more fre- 

 quently changed solutions. 



