DISCUSSION 



Two aspects of this study seem critical in developing 

 plant analysis as a practical nutrient assay for aquatic 

 environments. First, critical concentrations were 

 established in Elodea occidentalis for most of the essential 

 elements rather than just nitrogen and phosphorus. This 

 greatly increases the usefulness of the technique. It 

 also has made this initial effort with aquatic environ- 

 ments one of the most complete studies of plant analysis 

 with any species. Secondly, it is considered important 

 that the technique has been based on critical concentra- 

 tions in index segments. This should minimize errors 

 resulting from sampling either terminal tissues, which 

 might be disproportionately high in some elements even 

 under deficiency conditions, or older senescent tissues, 

 which could have a low content of the elements even under 

 adequate nutrient supplies. 



Interpretation of the plant yield-tissue content response 

 curves is subjective to some degree. The curves often 

 do not agree with the theoretically ideal curve (Ulrich, 

 1952) . There has also been some disagreement among 

 investigators on the point on the response curve at which 

 the critical level should be established. The approach 

 in this study was to consider the critical concentration 

 the tissue content at a point approximately 5 percent 

 below the maximum yield represented by the leveling off 

 of the response curve. This conforms with the inter- 

 pretation of numerous investigators when applying the 

 tissue analysis technique to the culture of agricultural 

 and horticultural crops. 



The reproducibility and reliability of the reported 

 critical contents are of some concern. The degree of 

 variation in yield and in tissue content of nitrogen in 

 duplicate cultures of the same experiment was indicated 

 in Table 2. This is typical of the best experiments in 

 this study. Better agreement in replicated cultures 

 cannot be expected as long as the plants are propagated 

 vegetatively. Even with the most careful selection of 

 plant segments when subculturing, it is impossible to 

 obtain uniform initiation of growth from terminal and 

 lateral buds. 



The results from this and a related study (Gerloff and 

 Krombholz, 1966) allow comparison of critical values for 

 the same element established in separate experiments and 

 by different techniques. In a particular species it 



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