I don't understand what you mean by represented by leaves.
Venusaurs most distinguishing feature is its flower.
Tangela is a mass of vines.
Sunkern is a seed, Sunflora is a flower.
Cherrim is a fruit.
Torterra is distinguished by its tree.
Carnivine is characterized by its Venus Flytrap maw.
Whimsicott is distinguished by its cotton.
Cacturne and Maractus are cactus.
Lilligant, Bellossom, and Vileplume are all flower-focused.
Victreebel by its bell maw.
Exeggute as seeds, Exeggutor as a tree.
Jumpluff by its cotton.
Breloom, Parasect, and Amoongus by mushrooms.
Torterra by its tree/fruit.
Roserade by its flowers.
Abomasnow as a tree.
Ludicolo for its lilypad.
Cradily as vines (even though its not a plant)
If you mean leaf as a symbol, why is Fighting represented by a fist when kicks are used? Why Fire by a flame when lava is also covered?
It's simply a recognizable symbol.
