Yield Comparisons 



As indicated in Table 16, evaluations of the recommended 

 macrophyte medium were possible through comparisons of 

 plant yields in that medium, in modifications of it, and 

 in the original medium. 



For Elodea , the recommended medium (x macrophyte) produced 

 as much yield as the original solution in two experiments. 

 Dilution of the recommended medium (x/2 macrophyte) re- 

 duced the higher yield. Apparently the supply of one or 

 several elements had become limiting. Doubling the concen- 

 trations of all constituents (2x macrophyte) reduced yield 

 slightly. 



The proposed macrophyte solution was also equal to, or 

 somewhat superior to, the original medium in the culture 

 of Myriophyllum . Dilution to half-strength did not reduce 

 yields with Myriophyllum , probably because its require- 

 ments for nitrogen and phosphorus are less than the re- 

 quirements of Elodea . 



The recommended medium has not been quite as effective for 

 the culture of Ceratophyllum as was the original medium. 

 This is verified by data from two experiments reported 

 in Table 16. The reason is unclear. Ceratophyllum does 

 have a higher potassium critical concentration than Elodea . 

 However, increased solution concentrations of potassium 

 did not increase yields. Higher concentrations of indivi- 

 dual trace elements also were without effect. Cerato - 

 phyllum demursum in general has been a less satisfactory 

 experimental organism than Elodea occidentalis or Myrio - 

 phyllum spicatum , primarily because of sudden leaf drop 

 from the stems and branches after several weeks of culture. 

 This accounts for the relatively low yields of Cerato - 

 phyllum in Table 16. 



Probably the most unexpected result in Table 16 was the 

 positive response of Elodea and Myriophyllum to the addi- 

 tion of Na2CC>3 (120 mg per liter) to the medium. The 

 effect was particularly evident as more rapid growth in 

 the early stages of the cultures. It is understandable, 

 therefore, that the response of various species to Na2CC>3 

 and of the same species in different experiments might 

 vary. Macrophyte yields were not increased by the addi- 

 tion of Na2Si03« 5H20. This suggests the beneficial effect 

 of Na2CC>3 was on carbonate-bicarbonate ratios in the medium 

 rather than on pH. 



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