I read this book a long time ago (+30 years)
The story is about a scientist that arrive on a planet where people (humans) worship a huge tree. There is a priestly class and a worker class on the planet. The scientist finds out that the workers are going inside the tree as food.
When the priests get a seed of the tree to an other planet, the scientist and others find a way to destroy the 'son of tree'.
In the end they will try to destroy the huge tree on the other planet.