How does natural selection fit into the evolution of a tree? Or a blade of grass?
Addressing this part directly, I'll leave the rest to others.
Natural Selection fits into the evolution of plants in the exact same way as it does for animals; there is no difference. The plant/animal receives a mutation that is either harmful or beneficial for the environment it lives in. A harmful mutation reduces its chances of reproducing, while a beneficial mutation increases its chances of reproducing. Alternatively, the environment might change, and previously neutral/harmful/beneficial features of the plant/animal could become harmful or beneficial or neutral depending on the new environment, changing how successful it is at reproducing.
Why would you assume it is any different than Natural Selection for animals?
rockv is in the Emergency Room.
Gah, this is why I need to read the whole thread before making posts, but I already typed it, so it's getting posted. :/