A relatively constant concentration of an element through- 

 out the range of yield response to that element is a 

 desirable feature of an index region and was another 

 factor considered in the selection of the second one- 

 inch segment for nitrogen assay. The nitrogen content 

 of the third one-inch segment also varied only slightly 

 (Table 2) . However, this segment was rejected as an index 

 region because a large proportion of the lateral brances 

 on Elodea occidentalis were less than three inches in length, 

 In the collection of field samples, it would probably be 

 difficult to obtain sufficient tissue for analysis from 

 the third one-inch segment. 



On the basis of the above considerations, the critical 

 nitrogen concentration for Elodea occidentalis was esta- 

 blished as 1.60 percent in the second one-inch segment. 

 Yield and nitrogen values for duplicate cultures are 

 included in Table 2 to indicate the variation between 

 replicates in experiments of this type. The nitrogen 

 concentration in the second one inch varied over a consider- 

 able range, from 1.14 to 4.32 percent. 



The results in Table 3 and Figure 3 are from an experi- 

 ment to establish the critical concentration of nitrogen 

 for Elodea occidentalis using the tray procedure. In 

 one set of trays, the N0 3 -N concentration in the nutrient 

 medium was 42 ppm; in another set, the concentration was 

 only 4.2 ppm. The total nitrogen concentration in the 

 second one-inch segment of nitrogen deficient plants from 

 the low nitrogen trays at harvest was 1.23 percent. This 

 is slightly below the 1.60 percent critical concentra- 

 tion established by the batch procedure. However, as 

 indicated in Figure 3, the Elodea plants probably were 

 deficient in nitrogen several days before a decision to 

 harvest the plants seemed justified. The dry weight of 

 the low nitrogen culture plants was only 39 3 mg per tray 

 in comparison with 504 mg under high nitrogen. The 14.0 

 ppm external nitrogen treatment in Table 2 represents a 

 comparable relative yield decrease due to nitrogen 

 deficiency. The nitrogen concentration in the second 

 one-inch segment from that treatment was 1.4 3 percent 

 which is in reasonable agreement with the 1.23 percent 

 value from the tray experiment. 



16 



