The Plant Monsters are the result of a genetic modification experiment on plants, creating mutant man-eating plants that will attack when approached.
There are various different monsters:
Mr. Snuggles: An enormous venus-flytrap-like plant. It appears twice throughout the quest. In its first appearance, you drive it away with steam from the steam pipes. In its second appearance, you kill it with herbicide. It was nicknamed "Mr. Snuggles" by Hazmat Hermit, a creator of Poptropica.
Grabbing Vines: These vines grab Sprocket. You kill them by snapping them with the window shutters that pop out.
Tiny Plants: These plants look like tiny green animals and jump about. You can kill them with the weed whacker or the toxic blaster.
Bolas Plant: This plant throws exploding fruit in your direction. You can kill it with the weed whacker if you get close enough.
Giant Mouth Plant: This cycloptic plant has a long tongue with a tiny mouth that snaps at you. You can kill it with the weed whacker.
Acid Bubble Plant: This plant hangs from the ceiling and drops acid bubbles from its mouth. It is the only plant you cannot kill, due to it being in a part of Steamworks Island which is inaccessible for the Mech.
Humanoid Plants: These plants are produced near the boss area. They keep on running away.
Flying Fruit Plant: These plants fly by using their leaves as propellors. They whack you with their fruit.