"Nature's first green is gold, her hardest hue to hold" what does gold possibly mean in this quote?

"Her early leaf's a flower; But only so an hour"

Why do leaf and flower only last an hour?
"Then leaf subsides to leaf so Eden sank to grief"
What is Eden and how did it sink to grief?

Look at this dictionary entry: realize (ree-uh-lyz) v. to become completely aware of (something) as a fact; understand without a doubt to make (something desired or anticipated) occur to give real, definite or physical form to a plan, dream, or goal to earn (money or a profit) from a deal which sentence uses the word realize as it is described in the fourth definition? a. mike realized the gravity of the situation only when dalia explained it to him. b. only hard work and perseverance can people realize their dreams. c. he felt that his vision was realized when he finished reading the final draft of the narrative. d. fay's glass paintings are expected to realize a handsome profit. e. dan will realize his goal of going to an ivy league school only if he works hard.