228 Renewal of Culture Solutions [426 
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 mjurious. 
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 .......-.. 020 - cee cee eee eee 1.222 
INGt Changed a6 a oss sisitie dtentett a ave rcomaes suedaaiers paueia 0.666 
This experiment is not strictly comparable with the one done 
on the rotating table, but it seems probable that continuous 
flow of the solution must be regarded as mare 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 and 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 conipared 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. 
