Definition of a Succulent

Classifying plants as succulent or nonsucculent is problematic. Regional floras and popular books on succulents are all vague at defining what makes a plant a succulent. For example, Rowley (1978) concluded only that many plants are difficult to categorize as to succulence. Popular publications on succulents often ignore clearly succulent plants such as many orchids and bromeliads simply because most succulent collectors don't grow them (e.g., Eggli 2001). Plant physiologists and systematists tend to be similarly noncommittal. Some authors use the term semisucculent for those plants with less obvious succulent characteristics, but this still leaves the separation between semisucculent and nonsucculent undefined.

Von Willert et al. (1992) represents the only source we know of that attempts a concise description. They define a succulent as any plant that possesses a succulent tissue, and further specified a succulent tissue as "... a living tissue that... serves and guarantees a ...temporary storage of utilizable water, which makes the plant... temporarily independent from external water supply...". The authors recognized a subcategory of xerophytic succulents, which excludes halophytic succulents (salt-tolerant plants that often grow in saline wetlands) and most geophytes (plants with their perennating organs below ground, e.g., potato, Jatropha macrorhiza, and most plants that are colloquially called "bulbs" in horticulture).

Examples of bulbous geophytes. (The Freesia is technically a corm; the others are true bulbs.)

The fleshy parts of most "bulbs" serve more for food storage than water storage; they produce above-ground growth only after the soil is moistened. However, some bulbous geophytes sprout before the beginning of the rainy season or maintain green foliage well into the dry season; we would classify these as succulents. The succulent tissues of halophytes and of most geophytes serve functions other than to support growth when soil moisture is unavailable. This definition of the term xerophytic succulent still leaves the status of a number of plants in question.

Some questionably succulent species
(click on images for additional information and more photos)

matacandelilla, giant cane milkweed Asclepias albicans	tescalama, rock fig Ficus palmeri
torote prieto Bursera hindsiana	ocotillo macho Fouquieria macdougalii
calabacilla, coyote gourds Cucurbita spp.	matacora Jatropha cuneata
liga Euphorbia xantii	mistletoe Psittacanthus sonorae

We classify all of the above except Psittacanthus as succulents; this parasite is rather fleshy, but it has a dependable supply of water as long as its host is alive.

Calabacilla and many other cucurbits have large tuberous roots that have considerable moisture as well as copious starch reserves. A sample of Cucurbita foetidissima root was 81% water. They produce leafy shoots, flowers, and fruits well in advance of seasonal rains. This trait is itself insufficient to separate calabacilla from clearly nonsucculent plants such as manzanita (Arctostaphylos spp.), which sprout from woody crowns immediately after dry-season fires. Metabolizing stored starch in manzanita crowns and roots generates enough water to support growth in the fall before the winter rains begin. Though tuberous-rooted cucurbits may also produce some of their water from starch breakdown, the high free water content along with their ability to produce growth even after a year without rain leads us to classify them as xerophytic root succulents.

Cucurbita foetidissima root with section cut out to show succulent tissue	Manzanita (Arctostaphylos sp.) sprouting from the root crown 2 months after a fire

The growthform of tescalama (desert rock fig, Ficus petiolaris palmeri) is intermediate between a woody tree and a stem succulent. The fact that seedlings can establish on exposed rock faces (saxicole) in the desert indicates that this species has adaptations that typical woody trees lack. The caudex of a young tescalama contained 68% water, somewhat more than stems of nonsucculent trees such as foothill palo verde (Parkinsonia microphylla, 53% water). Tescalama does not appear to have CAM. We tentatively classify it as a succulent based on its marginally elevated water content and lifestyle. (A closely related species, F. petiolaris petiolaris, occurs in tropical deciduous forest, a community comprised of so many similarly semisucculent trees of numerous species that the forest cannot support a fire.)

Ficus palmeri (rock fig), in cultivation	Parkinsonia microphylla (foothill palo verde)	Ficus petiolaris (rock fig)

Most species of Fouquieria exhibit a woody shrub growthform, albeit a strange one. But they have a clearly succulent lifestyle: very shallow roots and the capacity to produce functioning leaves within 2 days after a light rainfall (ca. 7 mm). The thin subcutaneous layer of moist tissue in these plants is succulent in nature (Henrickson, 1969a and b, 1972). The rapid leaf production indicates the presence of an undescribed non-CAM idling metabolism (Dimmitt 2000).

Fouquieria on a sand dune with shallow roots exposed by wind erosion	Fouquieria leaves, 2 days after a rain	The broken spine reveals a subcutaneous layer of moist tissue

REFERENCES

Dimmitt, Mark A. 2000. Flowering plants of the Sonoran Desert. In: Phillips, Steven J. & Patricia W. Comus (eds.). A Natural History of the Sonoran Desert. University of California Press.

Eggli, U. (ed.) 2001. Illustrated Handbook of Succulent Plants: Monocotyledons. Springer.

Henrickson, J. 1969a. An introduction to the Fouquieriaceae. Cactus and Succulent Journal (U.S.) 41:97-106.

Henrickson, J. 1969b. The succulent Fouquierias. Cactus and Succulent Journal (U.S.) 41:178-184.

Henrickson, J. 1972. A taxonomic revision of the Fouquieriaceae. Aliso 7:439-537.

Rowley, G. 1978. The Illustrated Encyclopedia of Succulents. Leisure Books (publ. by Salamander Books Ltd.).

Von Willert, D. J., B. M. Eller, M. J. A. Werger, E. Brinckmann, and H.-D. Ihlenfeldt. 1992. Life Strategies of Succulents in Deserts with Special Reference to the Namib Desert. Cambridge University Press, London.