Shrubs and trees of all sizes, willows may grow up to 30 m tall. Some are even prostrate. With few exceptions, they all have alternate, single (some finely toothed), long, straight leaves. They are lance-shaped or elliptical and have a short stalk in comparison with the length of the blade. At the base of the stalk are two small leaf-like organs, called stipules, that may remain on the tree all summer long. In winter, willows can be identified by their buds, which are covered with a single scale. Their bark has a distinctive, bitter taste. Their twigs are usually slender and flexible and frequently break off. On some species, however, they are long and strong. The colour of the twigs and buds varies, but dull yellow and orange predominate. It is often difficult to distinguish between the various species of willows, because they are generally recognized by their flowers and fruit, which do not remain on the tree for long. Identification is further complicated by the fact that the trees are doecious - male trees do not bear any fruit. There are also a number of hybrids.