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  Tucson Cactus and Succulent Society

Growing Succulents in the Desert Column

(List of Growing Succulents in the Desert columns)

"What is a Succulent, Anyway?" by Mark Dimmitt

Photos by Mark Dimmitt

(This article may only be reprinted with the author's permission.)


If you're reading this article, it's a pretty sure thing that you're a succulent collector. But do you know what a succulent is? You might be surprised to learn that there is no clear definition of the term. I've been trying to answer the question throughout my long career both as a botanist and a horticulturist. I've discussed the issue with many other professionals, and most of them are unclear too. Gordon Rowley, who has written many books on succulents, freely acknowledges that it's often difficult to decide whether any given plant is succulent. When I asked Park Nobel, a molecular biologist renowned for his research on the special metabolism of succulent plants (particularly CAM photosynthesis), he punted. He basically answered that a succulent is whatever people agree to categorize as such. I want a better definition than that. So, after much thought and considerable trepidation, I'll make a stab at one.


The basic definition is simple. In general, the word “succulent” means juicy. A succulent plant is one that stores water in its tissues; hence it's juicy (Figure 1). Sure, but how much water is enough to qualify? That's where the disputes begin. There is no doubt about some plants such as lithops (Figure 2), which are almost all water. But what about, say, yuccas, which are much less juicy. Some try to avoid the question by calling them semi-succulents. But we shouldn't go there. Now you have two gray areas, not just one, to decide on. Is it succulent or semi-succulent, or is it semi-succulent or non-succulent? See where that leads? So let’s try to be more rigorous.

I found one good stab at a technical definition, which includes the plant's function in addition to just having lots of water inside. Von Willert et al. (1992) define a succulent as any plant that possesses a succulent tissue, which is "... a living tissue that... serves and guarantees a ...temporary storage of utilizable water, which makes the plant... temporarily independent from external water supply...". In other words, the water stored in succulents functions to help them survive dry seasons. They further defined these plants as xerophytic succulents, ones that live in dry habitats. There are also succulent halophytes that live in saturated, saline soils. Their succulence serves some function, yet unknown, other than drought adaptation. From now on, I'm talking only about xerophytic succulents.


Now we need some additional traits to distinguish some of the less obviously succulent plants from non-succulent. The main ones are:

1. Nearly all succulents have extensive, very shallow root systems that are adapted to absorbing lots of water after very light rainfalls.

2. Succulents aren't just juicy. Most of them use their stored water to continue metabolic activity when there is no available water in the soil. That is, they may grow during the dry season.

3. In order to be able to do the above two functions, many succulents have a special variant of photosynthesis called CAM. Without going into technical detail, CAM plants are ten times more efficient with water consumption than non-CAM plants. This is very important where water is scarce. It's important for growers to know that CAM requires a big day-night temperature differential to work, at least 15 degrees Fahrenheit. If you keep CAM plants at the same temperature, they will die. Almost all succulents that have succulent leaves or stems have CAM. But succulent-stemmed plants that have non-succulent leaves are not CAM, e.g., Burseras and Adeniums.




So which of the plants we love are succulents and which are not? Here is a partial list. (More than 25 plant families have at least one succulent species.)

Agave family (Agavaceae):
All species of Agave are succulent. Some are CAM, some not, and some can switch depending on how moist the soil is.

Yuccas:
Some are succulent, others not. The non-succulent ones (like soaptree, Y. elata, Figure 15) have thin leaves and deep roots.

Aloes (Aloaceae):
Oh yeah, and CAM (Figure 3).

Bromeliad family (Bromeliaceae):
Dyckias are definitely succulent, and CAM. Succulent collectors tend to ignore the genus Tillandsia, which has numerous succulent species (Figure 4).

Cactus family:
Almost all cacti are succulents. Contrary to popular lore, there are exceptions. Pereskias (Figure 5) are just woody shrubs and trees. Their leaves look succulent, but they are shed during the dry season; they don't help the plant get through droughts. All succulent cacti are CAM.

Cycads:
NO! They have no water storage tissue. Deep roots, no CAM. Figure 6.


Euphorbia family:
This huge group has all growth forms from non-succulent herbs like the spurges that grow in your yard, to woody shrubs and trees like poinsettias and *****, to the fat ones which are definitely succulent (Figure 7). There are many species in between that have slightly juicy stems, and can be difficult to classify. Examples are *****. All succulent euphorbias are probably CAM. Many Jatrophas are also succulent, and some Pedilanthus.

Dogbane family (Apocynaceae, including former milkweed family Asclepiadaceae):
Many growth forms. Oleander, jasmine, and many milkweeds are non-succulents. Many others are quite succulent: the stapeliads (Figure 8), most hoyas, adeniums, pachypodiums. Stapeliads probably CAM; adeniums and pachys not. Plumerias (Figure 9)? Well, they probably qualify, but not CAM.

Geophytes (bulbs):
Many families have bulbous or tuberous plants, and many of these bulbs are pretty juicy. But most don't qualify; they're dormant during the dry season. Some begin growing before the rains come. These might be legitimate succulents; possible examples: Boophane (Figure 10), Synandrospadix.




Mesems (Aizoaceae):
Yep, almost all succulents, and CAM. Lithops, Conophytum, etc.

Ocotillo family (Fouquieriaceae):
Probably all 11 species are succulent, including ocotillo (Figure 11). It doesn't look very fat, but it has all the other traits of a succulent the broad, shallow root system, and the ability to leaf out within a couple of days after a rain. That latter feat is impossible for non-succulents. Reportedly not CAM, but they must have something like it.

Orchid family (Orchidaceae):
This huge family is almost completely ignored by succulent collectors, but there are many succulent orchids. Some of the most xerophytic ones such as Eulophia petersii (Figure 12) are showing up in succulent nurseries.

Stonecrop family (Crassulaceae):
Mostly succulents, and CAM.

Echeverias Torchwood family (Burseraceae):
Includes many succulents, such as Bursera (Figure 13), Boswellia, and Commiphora. These genera also have species that are probably not succulent. This family has no CAM.

Welwitschia:
NO! No water storage, no CAM (Figure 14). If the pot dries out for even a day, it's dead. In habitat, they probably have very deep roots that find a perennially moist layer. In cultivation the roots dive straight down, very unsucculent.

For more technical explanations and references, see the story on the Desert Museum's website: http://desertmuseum.org/programs/succulents_definition.php.










 

List of Growing Succulents in the Desert columns