Go to the part where Brock suggests that they should use Ash's Sun Stone to evolve the Sunkern. Ash looks at his Pokedex and it says something like:

'A Sun Stone can evolve a Sunkern to Sunflora, and a Gloom to Vileplume'.

A Sun stone actually evolves a Gloom to a Bellossom. A Leaf stone evolves a Gloom into a Vileplume.