There are five broad groups of cucumber varieties. Middle Eastern and Asian types are relatively small and thin. American pickling varieties are either smaller or slower-growing than “slicing” varieties, and have a thin skin to ease the penetration of brine. The standard American slicing cucumbers have been bred for the rigors of field production and long-distance distribution. They tend to be short and thick, with a relatively tough skin, dry flesh, prominent seeds, a strong cucumber flavor, and some bitterness at the stem end and just under the skin, thanks to the presence of defensive chemicals called cucurbitacins that discourage pests. European varieties, which are mainly grown in the controlled environment of greenhouses, are typically long and slender, with a thin tender skin, moist flesh, unformed seeds due to an absence of pollinating insects, a milder cucumber flavor, and no bitterness (breeders have eliminated the cucurbitacins). American cucumbers are often waxed to slow moisture loss, and so are almost always peeled before use, while European varieties are wrapped in plastic to accomplish the same purpose without compromising the edibility of their skin.