What is the protective outer layer of a bud called?

The protective outer layer of a bud is called a bud scale. Bud scales serve an essential role in protecting the delicate tissues of the bud, especially during adverse conditions such as winter or drought. These scales are modified leaves that can cover the developing shoot, helping to retain moisture and protect against physical damage and pests. Bud scales usually fall off as the plant grows and the bud opens, revealing new leaves and stems.

While apical meristem refers to the tissue at the growing tips of roots and shoots responsible for producing new cells as the plant grows, it does not function as a protective layer. The pith is the central part of a stem or trunk, responsible for storage and support, but does not provide external protection. Cork cambium is a tissue that produces the outer protective covering, known as cork, in woody plants but is different from the specialized structures that cover buds. Therefore, bud scale is the most fitting answer for the protective outer layer of a bud.