No animals get nitrogen from eating plants or other animals that contain nitrogen and plants get nitrogen from the soil by absorption through their roots as amino acids, nitrate ions, nitrite ions, or ammonium ions.
        
             
        
        
        
<span>Unidirectional flow in the heart is ensured because the heart contains valves that prevent backflow.
These valves keep the flow of the heart going in one direction (hence the name "unidirectional").</span>
        
             
        
        
        
Answer:
Box 1: AA
Box 2: Aa
Box 3: AA
Explanation:
In order to figure out the pedigree, you first have to do a punnet square- kind of like cross-multiplying. EX: For the first box: Aa x AA
For box one, we know that it is Aa, and not AA, because box 2 has to be AA. 
We know that because Aa X Aa would give us some "aa" offspring which we do not see in the last generation (see 2nd pic). The only way to get no "aa" offspring is to have an AA X Aa cross.