In its broadest sense, polarity can be described as asymmetry. And, if you look at a plant, you will immediately notice a variety of asymmetric features at different levels of complexity. There is a root at the bottom and a shoot at the top. Leaves are attached to the shoot at one end and are free at the other. The upper side of the leaf may have hairs on it, while the underside is smooth. When you take a closer look at a hair, you might see that it is asymmetrically branched. If you happen to have a microscope handy, a cross-section of the plant's stem will reveal that the cells at the core look different from those further out. Even better equipped, you will recognize polarity at the cellular level, which manifests itself in the directions of vesicle movement and in the asymmetric distribution of organelles,cytoskeletal strands, and even...

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