Double fertilization occurs when two sperm cells fertilize the two polar nuclei. the sperm and tube nuclei fertilize the egg. the polar nuclei fuse with the egg after it has fused with the sperm. one sperm fuses with the egg and the other sperm fuses with both polar nuclei.

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the nitrogen cycle provides nitrogen to the ecosystem from the atmosphere, ground and oceans. nitrogen is an important component of complex molecules such as amino acids and nucleotides, which lead to the creation of proteins and dna, the building blocks of all life. plants absorb nitrogen in the form of ammonium, nitrate ion and, on rare occasions, as amino acids. animals receive nitrogen necessary for biological processes from feeding on living or dead organic matter.