Rhubarb, in its sour, vegetable, nonmedicinal guise, came into common kitchen and garden use only after plantations made sugar widely available and affordable in the past few centuries—the sweetener is what made rhubarb fit for pie. The part of rhubarb we eat, the stalks—white, pink, crimson, grasshopper green, whitish green, or red, depending on the variety—are actually rhubarb leaf bases called petioles. Other petioles we commonly eat are celery, Swiss chard, Napa cabbage and endive, and, arguably, asparagus. Stick to the petioles of rhubarb—the leaves, which contain toxic oxalic acid, can be lethal to those with sensitivities to it or to rhubarb-leaf gluttons (five kilos would be enough to poison most people). In my own Minnesota neighborhood I've seen kids chasing each other with rhubarb leaves as if they're murder weapons that could kill on contact.