Spock had some good scenes with Uhura early on - I am guessing this episode influenced the current Trek movies' idea for a Spock/Uhura relationship.
Pretty much, this episode and the later scene where she teases him in Charlie X
. The movies' writers mention them in the commentary track for the first reboot movie as a justification for pairing them up.
I was a little confused on just what Sulu does - he's introduced working on plants in what's clearly marked as Life Sciences. Later he seems to be coordinating security. And only in the final scene do we see him at the helm. I guess he just works any job he feels like that day.
One does get the feeling of Sulu being a bit of a layabout.
Later episodes stated that he's just a guy with eclectic interests. In The Naked Time
he's singing the praises of fencing, and in Shore Leave
he professes an interest in antique handguns. I've always figured his botany here is just another one of those personal interests.