The thick, celerylike stalks of this buckwheat-family member can reach up to 2 feet long. They're the only edible portion of the plant--the leaves contain OXALIC ACID and can therefore be toxic. Though rhubarb is generally eaten as a fruit, it's botanically a vegetable. There are many varieties of this extremely tart food, most of which fall into two basic types-hothouse and field grown. Hothouse rhubarb is distinguished by its pink to pale red stalks and yellow-green leaves, whereas field-grown plants (which are more pronounced in flavor) have cherry red stalks and green leaves. Hothouse rhubarb is available in some regions almost year-round. The field-grown plant can usually be found from late winter to early summer, with a peak from April to June. Choose crisp stalks that are brightly hued. The leaves should be fresh-looking and blemish-free. Highly perishable, fresh rhubarb should be refrigerated, tightly wrapped in a plastic bag, for up to 3 days. Wash and remove leaves just before using. Because of its intense tartness, rhubarb is usually combined with a considerable amount of sugar. It makes delicious sauces, jams and desserts and in some regions is also known as pie-plant because of its popularity for that purpose. In America, a traditional flavor combination is rhubarb and strawberries; in Britain, rhubarb and ginger. Rhubarb contains a fair amount of vitamin A.