Which of the following terms refers to the thin outer covering of a leaf?

The term that refers to the thin outer covering of a leaf is the epidermis. The epidermis serves several important functions in plant anatomy. It acts as a protective barrier against environmental stresses, prevents water loss through transpiration, and can also aid in gas exchange through structures called stomata, which are embedded in the epidermis.

The epidermis is typically one cell layer thick and is found on both the upper and lower surfaces of the leaf. It can be specialized in various ways depending on the plant species, such as being covered with a waxy layer to further reduce water loss. This protective covering is essential for the overall health and functioning of the leaf, making it a crucial component of the plant's structure.

In contrast, the other terms relate to different parts of a leaf: the petiole is the stalk that attaches the leaf to the stem; stipules are small leaf-like structures at the base of the petiole; and the blade is the broad, flat part of the leaf that is primarily responsible for photosynthesis. Each of these components plays a distinct role in supporting the plant, but it is the epidermis that specifically refers to the outer protective layer.