Live Long and Prosper

By Joe Merkelbach (March 2010)

One of the interesting things about stem succulents – whether they are cacti, euphorbias or others – is that they conduct photosynthesis with cells on the bodies of the plants.

Since there are no leaves to be shed and replaced, and the chloroplasts have to have unimpeded exposure to light with no shedding of epidermal cells, the surface stem cells have very long lives. The surface cells have to maintain their protective coating of cutin and wax over many years, not just a single season, as with ephemeral annuals and deciduous leaves.

The seeming explanation is that the epidermal cells of cacti have prolonged lives due to a retarded and extended developmental cycle. The other stem succulents are not as well understood as cacti, for I could not find an explanation for them, so they may have evolved similar lifestyles, or perhaps they are quite different.

The cactus bodies also have small openings called stomata that allow the exchange of oxygen and carbon dioxide during photosynthesis. These openings are usually found on the bottom surfaces of leaves, not an option for almost all cacti. The modified versions of photosynthesis that most cacti employ keep these openings closed during the heat of the day in arid environments and open at night only in order to minimize moisture losses.

The cacti of the subfamily Cactoideae have another unique feature: cortical bundles running off the central cortex that function very similarly to veins in leaves. These bundles carry water out to the stem surface and photosynthetically produced sugars back to the cortex. The bundles allow the large girth of cactus stems.

It is noteworthy that Blossfeldia liliputana is the only member of the subfamily without bundles, and its miniature size is obviously a related feature. None of the other stem succulents – euphorbias, pachyodiums, etc. – have these bundles, so they never grow to the same thickness as some cacti.

Bark as it develops in cacti reflects not so much a change in texture, as per woody trees, but a change in color to tan or brown. After some years, the oldest epidermal surface begins to grow cork cells and sclereids, and the stem surface ceases to function photosynthetically.

The developments that allow the successful function of stem succulents in their hot or cold, but always dry environments are quite sophisticated and number far beyond spines. They truly foster the ability to live long and prosper.

Source:
University of Texas Section of Integrative Biology – http://www.sbs.utexas.edu/mauseth/researchoncacti/