How come your gloom will not evolve into Bellossom in Pokemon firered?

There are two ways to evolve a Gloom, you can either evolve it into Vileplume or you can evolve it into Bellossom. To get Vileplume all you need to do is use a leaf stone on it. Likewise to get Bellossom you have to use a stone on Gloom, but this time it is a Sun Stone.