Plants 101: Succulents

The Sill seeks to educate and inspire all plant lovers. In our plant novice series, Plants 101, we introduce some of our favorite plant types, explore where they come from, and gently advise how to take care. 

What is a succulent?

They look great on a sunny sill or side table. They’re child-friendly since they’re spineless and generally safe in case you brush up against one. Plus, they’re real cute. All succulents are adapted to dry environments with full sun and no shelter. According to a NASA study, succulents have even been found to purify the air of harmful toxins. Succulents are excellent starter plants because they’re low maintenance.

Succulents (from the Latin word “succulentus,” for juice or sap) are defined by their moisture-storing capacity and come from many botanical families. Botanically-speaking, the term succulent refers to ANY plant that has evolved adaptations to survive hot arid environments. It is a term that does not refer to any specific family or clade of plants, and in fact many succulent plants are not related to one another at all.

The definition of succulents in the consumer world is semi-arbitrary, and may exclude many plants that have succulent traits, such as bromeliads and other tropical plants. It has been used by collectors and in the marketing world for so long that the term 'succulent' is simply an accepted colloquialism at this point. Often, one will find collections of ‘succulents’ marketed towards families with children, or as child-friendly options because many succulents are spineless and generally safe to bump into. Cacti, on the other hand, have been kept separate because of the potential danger from the spines, even though they are indeed succulents.

Succulence can include many morphological characteristics: an epidermis with waxy cutin, thickened, fleshy leaves, an alternative mode of photosynthesis, and so on. Many plants would, by botanic definition, be considered succulent even though we do not normally think of them as such. For example, snake plants (Sansevieria) and Ponytail palms (Beaucarnea) have adaptations for surviving in desert conditions. Snake plants have thickened leaves and CAM photosynthesis, and Ponytail palms have a thickened, woody trunk for water storage.

Cacti & Euphorbs

Like we said, all cacti are succulents, but not all succulents are cacti. Cacti are succulents from the family Cactaceae, which only are native to the New World/Western Hemisphere. Euphorbs are succulents from the plant family Euphorbiaceae, and are cosmopolitan in their distribution. The trait of succulence has evolved multiple times throughout plant history, each time as a response to climate shifts to more arid conditions. Euphorbia (Euphorbiaceae) has been evolving since the early Eocene (56-40MYA) to be adapted and speciate in arid environments, whereas Opuntia (Cactaceae), has only been evolving since the Oligocene (~39MYA). Other plants have evolved succulent characteristics, such as Yucca, Agave, and Aloe.

We are Family

Let’s take a look at the families that live under the succulent roof:

The Stonecrop Family (Crassulaceae)

The most commonly thought of and sought after family whenever one thinks of a succulent. Crassulaceae is a truly diverse family, called the stonecrop family. Plants in this family occasionally resemble stones, grow in between stones, and are resilient as...yep, stones. Unlike its namesake, no plant in this family is actually farmed as a crop, but rather propagated for the horticultural and ornamental trade. Most are best and easily propagated via fallen leaves and disheveled plants or cuttings. If you think about it, they are weeds in their native environments.

Common crassula include Crassula (Jade Plants), Echeveria (pictured below), Graptopetalum, Kalanchoe, Sedum, and Sempervivum—one of the few succulents native to Europe and temperate Asia. In ancient Rome, it was grown the roofs of houses, not only to appease Jupiter—it was observed that thunderbolts would never strike these plants, so they were thought to ward off thunderbolts, sorcery and storm damage—but also to cool houses in the hot Mediterranean summers. We now know that it’s likely that the boulders this plant grows on are the real reason why these plants were rarely struck by lightning. 

The Stonecrop is an older family of eudicots (see eudicots: a larger grouping of flowering plants) that evolved about 60-100MYA. Their flowers are more primitive and undifferentiated than other eudicot plants. Their morphology is so diverse and pliable, it’s often hard to get a solid description on this family. In recent genetic studies too, Crassulaceae plant genetics are so pliable that it’s hard to do molecular work or even track the family’s evolution. What we can say is that Crassulean Acid Photosynthesis (CAM) was discovered first in this family and named after it. CAM photosynthesis is an alternative photosynthesis that evolved in hot, dry environments, so that the stomatal pores can be closed during the day. Its purpose is to help the plant better conserve water. It should be noted that this family is quite unique in that every member of this family uses CAM photosynthesis. CAM photosynthesis developed independently in other plant families as well, such as in Poaceae (the grasses) and Orchidaceae (the orchids).

The Cactus Family (Cactaceae)

All cacti are from the same family known as Cactaceae. The family gets its name from the Greek Kaktos (κάκτος), meaning ‘spiny plant’. Cactaceae is a New World family that has evolved relatively recently in the plant world, but has diversified quite a bit. This family has evolved to be completely succulent, with a trend of modifying their leaves to form spines. The branches are reduced to structures called areoles. One of the key identifying factors of cacti is the presence of areoles, as well as multiple spines coming from those areoles. Their flowers have many stamen, which produce lots of pollen, and they are often scented as well for their flying pollinators. Most cacti flower at night and usually for only one night. Night-blooming is an adaptation that helps to conserve water. Since the window for pollination is so short, cacti rely on their short-lived flowers to attract bats, insects and birds, all of find the flowers fragrant. The goal is to attract as many pollinators as quickly as possible. Most cacti reproduce easily by seed, but can also be propagated by segments or pads.

Pereskia is a genus of plants that is a sister to the cacti and is related to the ancestor of cacti. By studying Pereskia, we can get a better grasp of how cacti have evolved so quickly and how they modified their leaves so rapidly. It turns out that Pereskia is already semi-cactus-like in that they have leaves modified into spines, but also retain a few leaves. Only a few mutations are needed to turn all the leaves into spines. Many cacti have used their resources to evolve spines and water-conserving mechanisms, so they have not evolved too many secondary metabolites that are toxic. In other words, many cacti are edible, including species of Opuntia (Prickly Pear), Stenocereus (Dragonfruit), Hylocereus (Dragonfruit), Peyote, Carnegiea (Seguaro).

The Spurge Family (Euphorbiaceae)

Euphorbiaceae is a family of flowering plants whose ancestor was tropical in origin. This is believed to be why out of most succulents, Euphorbs have the most toxic secondary metabolites—tropical plants defend themselves by synthesizing many poisonous compounds. All euphorbs, in fact, produce a latexy sap that is highly toxic to ingest and irritating to those who have a latex allergy. They have reduced flowers and are mostly wind-pollinated. Cultivated euphorb species include those within genus Euphorbia, Croton (pictured below), and Hevaea (true rubber tree). Most reproduce easily through pups or cuttings, although through seeds is also possible.