Plotties are made of plant tissue, also known as celulose. They don't have a skeleton but their tissue is strong enough to support their weight and keep them upright as long as they have enough water and starch (sugar) running through their system.
Since they lack blood to pump through their bodies, Plotties don't have need for a heart. Instead, their vascular system is filled with water, sugar and other nutrients. When a Plottie is cut, it "bleeds" sap. Depending on the plant, the sap may vary in color from white to different shades of amber or brown to completely clear. Some Plotties have toxic sap that could be poisonous or cause allergic reactions.
Plotties don't have lungs and they don't breathe through their mouth or nose (because they don't have one). They breathe through their skin, and they also sweat. However, Plotties do have brains.