I need some help in finding an SF book I read back in the 80s.

 

 

 

 

 

 

 

 

The author and title escape me and I can barely remember the plot.

A survey ship from earth is circling a planet. - Now it get hazy - The planet is under huge gravity, so much so that the inhabitants are (I think) 2D and the time differential between them and the survey vessel is huge. Many generation pass on the planet in one life time on the ship.

The story revolves around the evolution of the plants beings and the interaction with the ship.

So can anyone identify the title and author for me - I want to get my son to read it.