The buds of many woody plants, especially in temperate or cold climates, are protected by a covering of modified leaves called scales which tightly enclose the more delicate parts of the bud. Many bud scales are covered by a gummy substance which serves as added protection. When the bud develops, the scales may enlarge somewhat but usually just drop off, leaving on the surface of the growing stem a series of horizontally-elongated scars. By means of these scars one can determine the age of any young branch, since each year's growth ends in the formation of a bud, the formation of which produces an additional group of bud scale scars. Continued growth of the branch causes these scars to be obliterated after a few years so that the total age of older branches cannot be determined by this means.

Since buds are formed in the axils of leaves, their distribution on the stem is the same as that of leaves. There are alternate, opposite, and whorled buds, as well as the terminal bud at the tip of the stem. In many plants buds appear in unexpected places: these are known as adventitious buds.

Often it is possible to find a bud in a remarkable series of gradations of bud scales. In the buckeye, for example, one may see a complete gradation from the small brown outer scale through larger scales which on unfolding become somewhat green to the inner scales of the bud, which are remarkably leaf-like. Such a series suggests that the scales of the bud are in truth leaves, modified to protect the more delicate parts of the plant during unfavorable periods.

Types of buds

Buds are often useful in the identification of plants, specially for woody plants in winter when leaves have fallen. Buds may be classified and described according to different criteria : location, status, morphology, function. Botanists commonly use the following terms :

• for location,
• terminal, when located at the tip of a stem (apical is equivalent but rather reserved for the one at the top of the plant),
• axillary, when located in the axil of a leaf (lateral is equivalent but some adventitious buds may be lateral too),
• adventitious, when occurring elsewhere, for example on trunk or on roots (some adventitious buds may be former axillary ones reduced and hidden under the bark, other adventitious buds are completely new formed ones),

• for status,
• accessory, for secondary buds formed besides a principal bud (axillary or terminal),
• dormant, for buds whose growth has been delayed for a rather long time (the term is usable for buds resting during winter or dry season, but is rather employed for buds waiting undeveloped for years),
• pseudoterminal, for an axillary bud taking over the function of a terminal bud (characteristic of species whose growth is sympodial : terminal bud dies and is replaced by the closer axillary bud, for examples beech, persimmon, Platanus have sympodial growth),