holes were drilled in each glass cover. Surgical rubber 

 tubing inserted through one hole brought air (filtered and 

 1.0 percent C0 2 enriched) into the culture. Four capillary 

 air outlets provided vigorous aeration of the nutrient 

 medium. The second hole was plugged with cotton and 

 permitted air to escape. The inoculum for a tray was a 

 1-1/2 - 2 inch terminal segment of an Elodea main shoot 

 or lateral. 



In a specific experiment, some trays contained the com- 

 plete medium; others contained amounts of nitrogen or phos- 

 phorus which would support a maximum rate of growth only 

 for a limited period. Plant growth in the trays was followed 

 by daily measurements of the total length of the plants, 

 both main and lateral shoots. These measurements were made 

 without removing the cover from a tray. From continuous 

 plots of growth in the deficient and the complete cultures, 

 the point at which the nitrogen or phosphorus content of 

 plants in the deficient cultures had been sufficiently 

 reduced to affect the rate of growth could be determined. 

 At this point, plants in all trays were harvested, divided 

 into various segments, oven-dried, and analyzed. 



RESULTS 



Critical Nitrogen Concentration in Elodea 



The data in Table 2 and the curves in Figure 2 are from an 

 experiment to establish the critical level of nitrogen in 

 the most suitable index segment of Elodea occidentalis . 

 The data on total plant yields in Table 2 show that growth 

 was limited by the nitrogen supply in solutions containing 

 10.5 and 14.0 ppm nitrogen and reached a maximum of 2.34 g 

 at 21.0 ppm nitrogen. The large increase in yield between 

 14.0 and 21.0 ppm suggests that the critical concentration 

 is approximately the amount of nitrogen in plants of the 

 21.0 ppm nitrogen cultures. The nitrogen contents of 

 plants from the 31.5 and 42.0 ppm treatments represent 

 luxury consumption of nitrogen, that is, increasing 

 nitrogen content which did not result in further yield 

 increases. 



The second one-inch segment of the main shoots and laterals 

 was selected as the most satisfactory index part for nitro- 

 gen. The possible re-export of nitrogen from older to 

 younger tissues under conditions of nitrogen deficiency, 



13 



