But why did NASA send a botanist to Mars?

Went to see The Martian last night.  It was as good as various people have said, so thanks for the recommendations! Full of competent people competenting which I like.

But I still don't know why a botanist on Mars with no seeds.  Anyone know?

For a bonus point, why potato growing in the gravel at a NASA training academy place?  Although, to be fair, that seems very much in character for a potato.  Opportunist vegetables.