In trees, this, which is made up of dead phloem, protects the phloem beneath it.

Probably the best answer would be bark, or tree bark.  The cells that compose the phloem become part of what is known as a yearly ring of tree growth.  These rings of vascular tissue are pushed in an outward direction from the cambium, which is the woody material in the center of the plant.  Phloem are used to transport the products of photosynthesis in plants to other parts of the plant for use and storage.  As the plant ages, new layers of phloem are produced and progress in an outward fashion from the center of the plant.  As these new rings are added, the old phloem dies and becomes part of the outer covering of the plant, generally referred to as bark.  The bark serves as a protective outer layer to secure the vulnerable inner cells from degenerative disease in the environment.