That's a great answer! Almost as good as Calvin's "yakka foob mog chumble spuzz" when asked to explain something "in his own words" (Calvin & Hobbes is great stuff for gifted kids, by the way). You and your son would probably enjoy the oft-forwarded "barometer legend":
http://www.snopes.com/college/exam/barometer.asp.
To me as a teacher, the great thing about your son's answer is that it shows an ability to internalize knowledge and use it in a creative way. Most children your son's age won't think about the nature of plants in a writing assignment, but the first thing that came to his mind was "plants grow over time". That means he's absorbed this bit of scientific information into his thought processes, rather than just "remembering" it as most kids would, and he can now apply it to whatever he does. That's important.
Can I ask whether you've considered moving your son to another school? Obviously how easy this is varies with your situation. But I have to say it sounds like those people aren't going to give him what he needs.
And if that classroom teacher really thinks she has "lots of really gifted kids" in her class, she's an idiot. Gifted means rare. Those kids who are doing so well on all her assignments are doing that well because, intentionally or not, she's teaching to the bottom of the class.
