250 V. Alexander et al. 



Nitrogen may also limit production of the plants, but the evidence is 

 indirect. One type of evidence is the extremely low concentration of 

 ammonia within the sediments of the plant beds compared with the very 

 high amounts outside the plant beds (0.05 to 0.07 vs. 3 mg NH 3-N liter '). 

 Also, the half-saturation values for ammonia uptake by the roots were 2 to 

 3 times the concentrations so the transport systems are undersaturated. 

 The plants are able to operate very effectively with these low 

 concentrations of ammonia and we conclude that there is a close balance 

 between need and rates of supply. It was not possible to decide whether or 

 not there was an actual limitation by nitrogen, but it appears that the 

 plants are operating at close to the maximum rates of supply of ammonia. 



Greenhouse experiments with Carex aquatilis show that there are 

 genetic differences in metabolism and nutrient uptake between 

 populations located only a few decimeters apart (in pond, ridge, trough, or 

 meadow). These ecotypes have developed by competitive interaction which 

 took place as the ice wedges and polygonal ground grew and caused the 

 microhabitats to develop. 



