Modifications to Cactus Epidermis

Modifications to Cactus

Epidermis

Several initial steps in the evolution of cacti to being stem-photosynthetic, stem-succulent plants involved the epidermis.

The epidermis is persistent.

In most plants, epidermis is an ephemeral tissue that functions for only a few months. This is obviously true of epidermis on leaves that live only from spring until autumn; it is true as well as the stem epidermis of annual plants: no epidermis cell of Arabidopsis thaliana, for example, lives longer than a month and a half or so. And even the stems of perennial plants typically replace their epidermis with bark in either the first or second year of life. But in cacti the epidermis is a long-lived persistent tissue: as long as a cactus stem is green, it still has its epidermis. All cactus bark is tan-color or dark, so it is easy to tell when the shoot loses its epidermis. Giant columnar cacti and huge barrel cacti live for many decades, perhaps even for hundreds of years (plant longevity is an area that needs study), with at most a bit of bark on their lowermost, oldest regions.


The stem of this Pereskia diaz-romeroana (a cactus) is green because it is young and still has its epidermis; bark has not yet formed. Notice the leaves on this cactus. Click on any photo for a larger image.	The stem of this Pereskia guamacho is tan colored because it has already formed bark; unlike most cacti, the epidermis is not persistent in pereskias. Without epidermis, light cannot penetrate well, and of course there are no stomata so absorption of carbon dioxide is almost impossible.	These stems of Trichocereus terscheckii are still green despite being decades old: the green color indicates they still have their epidermis.


Chollas like this Cylindropuntia imbricata live for many years, retaining their epidermis. Only the very base of the trunk will have bark.	The base of this stem of Echinocereus coccineus is many years old and has old persistent epidermis but the flower petals -- and their epidermis -- are ephemeral. The petals and the petal epidermis need to function only briefly.	Although short, this Echinocactus horizonthalonius is very old; its epidermis cells have had to maintain their water-proof nature for years. Do they synthesize new cutin and wax every year?

Converting epidermis from an ephemeral to a persistent tissue must have involved many substantial changes, certainly more than merely postponing programmed cell death or delaying bark formation. In all cacti, shoot bark is produced by a cork cambium that arises by reactivation of shoot epidermis cells themselves: after decades or centuries of prolonged cell cycle arrest, cactus shoot epidermis cells can be directed to become mitotically active. After prolonged functioning as epidermis cells, they begin to divide with periclinal walls, with the exterior cell of each division differentiating as either a cork cell or a sclereid (most cactus bark consists of alternating thick bands of cork cells and thin bands of sclereids). In a few species, there is even a production of phelloderm to the inner side of the cork cambium. The ability of epidermis cells to function mitotically indicates a variety of remarkable things that could easily be overlooked: their DNA has survived decades of exposure to intense UV irradiation coming through clear desert skies, and all their organelles are healthy and functional. It is also remarkable that epidermis cell protoplasts survive for such a long time while being separated from extraordinarily dry desert air by only a few micrometers of cutinized wall.

Another aspect of the evolutionary conversion of epidermis into a persistent tissue almost certainly altered epidermis development. In an ephemeral epidermis, for example that of a leaf or flower petal, the cuticle and waxes produced while the organ is young are sufficient to control water movement for the rest of the life of the organ. But in a long-lived cactus shoot epidermis, the waxes and cuticle produced while the epidermis cells are young and still near the shoot apex probably become weathered away after a year or two and need to be replaced. This is another area that needs research, but it would seem as if cactus shoot epidermis cells must be in a state of “prolonged development,” needing to synthesize cutin and waxes even during the many years they are functionally mature.

The stem's epidermis has stomata.

An obvious feature that the persistent epidermis of a stem-photosynthetic, leafless succulent must have is stomata. Although stem epidermis in many species do have stomata, Urs Eggli in Zurich discovered that stems of the early cacti may have lacked stomata in their epidermis: plants of Pereskia almost completely lack stomata in their stem epidermis (Pereskia is the cactus genus whose members – rather ordinary woody, leafy trees – retain the greatest number of relictual features). Maurizio Sajeva, however, found that in contrast to Pereskia, the stems of all ordinary cacti (that is, the members of subfamilies Cactoideae and Opuntioideae – the cacti that look obviously like cacti) do have stomata and at densities almost as high as in the lower epidermis of Pereskia leaves. If the early evolution of cacti involved obtaining the ability to produce stomata where they had not occurred before, that may have been a type of homeotic evolution: mutations in the promoter regions of stomatal complex morphogenesis genes could have allowed those genes to be activated in stem epidermis as well as in leaf epidermis. Stem epidermis could have more or less instantaneously obtained not only the ability to produce guard cells and subsidiary cells but also the morphogenetic metabolism necessary to control their density and spacing relative to each other.


This cross section of stem of Pereskia humboldtii is unusual for actually having a stoma; most Pereskia stems lack them, although Pereskia leaves do have them. Click on any photo for a larger image.	The stem epidermis of Lepismium, like that of all cacti that are obviously cacti (that is, the ones that are not pereskias) has numerous stomata (only one visible here). Note the large substomatal chamber in the stem cortex.	This epidermis has been peeled from the stem of an Opuntia-like cactus (Consolea). The white oblongs are stomata at a density almost as high as that found in leaf lower epidermis. The small circles with dark centers are druses of calcium oxalate.

Mauseth, J. D. 1998. Ontogenetic mechanisms and the evolution of Cactaceae. in: Proceedings of the IV Congreso Latinoamericano de Botánica. Editors: R. Fortunato and N. Bacigalupo. Missouri Botanical Garden Press. pp. 355 – 362.

Terrazas Salgado, T, and J. D. Mauseth. 2002. Chapter 2. Shoot anatomy and morphology. in Cacti: biology and uses. (pp 23-40) Edited by P. S. Nobel. University of California Press.

Mauseth, J. D. 2004. Cacti and other succulents: stem anatomy of “other succulents” has little in common with that of cacti. Bradleya 22: 131-140.

Mauseth, J. D. in press. Blossfeldia lacks cortical bundles and persistent epidermis; is it basal within Cactoideae? Accepted by Bradleya.

Mauseth, J. D. 2004. The structure of photosynthetic, succulent stems in plants other than cacti. The International Journal of Plant Sciences 165: 1-9.

Mauseth, J. D. 2000. Enfoques anatómicos para el estudio de la biodiversidad: La diversificación de las Cactaceae. In Enfoques contemporáneos para el estudio de la biodiversidad. Eds. H. J Hernández, A. N. García Aldrete, F. Álvarez and M. Ulloa. Instituto de Biología, UNAM, México, México.

Mauseth, J. D. 1995. Ontogenetic mechanisms and the evolution of Cactaceae. Giornale Botanico Italiano 129: 429 – 435.

[End of Epidermis page]