Weepinbell will evolve when you use a leaf stone on it.
Bellsprout > lvl 21 > Weepinbell > leaf stone> Victreebel
It evolves into a Weepinbell at level 21.
Bellsprout evolves into Weepinbell at level 21.
it evolves with a leaf stone
with a leaf stone at any level
Bellsprout evolves into a Weepinbell at level 21 in Pokemon Crystal.
You go and level it to Level 21, where it evolves to Weepinbell.
bellsprout will evolve at lv.21 and again with a leaf stone it will evolve into victreebell
In Pokemon Blue, Bellsprout evolves into Weepinbell at level 21 and Victreebell when exposed to a Leaf Stone
http://www.dltk-kids.com/pokEmon/adoptions/bellsprout-p.htm (two evolutions)
Spearow evolves at level 20 in Pokemon Yellow.