Fig. 17.1-5a and b. Transverse section. of cactus bark (Acanthocereus sicariguensis). In cacti, the cork cambium always arises from the epidermis: even though the epidermis remains alive and functional as long as the stem is green (hundreds of years in many species), when bark is needed, the epidermis cells become mitotically active and act as the cork cambium. Although by this time the epidermis cells have been vastly more long-lived than in any other group of plants, as cork cambium cells, they must again be long-lived because they are never replaced. Cacti form only this one cork cambium, new ones are not later formed in the cortex or secondary phloem.
Because the cork cambium arises from the epidermis, and because there is always a hypodermis just below the epidermis, the hypodermis acts as a marker: in these micrographs you can see that there are many cells between the cork cells and the hypodermis collenchyma cells. These cells must be new cells (they could not possibly have existed between the epidermis and the adjacent hypodermis). Such new cells must be phelloderm. In the larger micrograph, the phelloderm cells are radially elongate, almost columnar.