Question: What is a Succulent Anyway? Answer: A Pretty Clever Plant! By Paula Szilard We all know one when we see one, or so we think! But these plants have come up with more varied adaptive strategies for conserving and storing water than we give them credit for. The phenomenon of succulence cuts across many plant families. Although there are many plant families which boast huge numbers of succulents, like the Euphorbiaceae some families have relatively few, like the Passifloraceae, the passion flower family. These plants have had to adapt to a scarcity of water in their environment or die trying. We know the survivors rose to the occasion. We will never have an appreciation for how many plants died trying! Succulents are plants with at least one type of tissue or organ for water storage. The different adaptations we see are: Leaf Succulents These plants developed thick leaves to store water. We have all grown some of them. Common ones that come to mind are Crassulas (jade plants), Kalanchoes, Lithops, Aloes and Echeverias. Stem Succulents In these plants, it’s the succulent stems that store the water and often carry on photosynthesis. There are many examples in the Euphorbiaceae family (old world analogues of our new world cacti) and the Cactaceae themselves, most of which are stem succulents. Yes, most cacti are succulents, but not all succulents are cacti! Cacti have the identifying feature of areoles, small cushion like structures with hairs or spines, or even barbed bristles (glochids). There are also a number of species of pelargoniums with succulent stems. Plants with Caudexes (Caudices ) Some plants have stems that are widened at the bottom. These stems are called caudexes or caudices. Ones we are all familiar with are the Pony Tail Palms, (Beaucarnea recurvata), Adenium obesum and plants in the genus Adenia (Passion Flower Family). Some people call these fat-bottomed plants. The commonly used adjective to describe them is caudiciform. The fat part of the stem can be underground or above ground, or partly underground. Pachycauls Pachycauls are similar to caudiciform plants. These plants can be herbaceous plants, shrubs, even trees with abnormally thick stems, often fatter on the bottom, like the baobob tree. Some authors group pachycauls together with caudiciform plants in a group called pachyforms, or pachyform plants. Root Succulents Although we ordinarily don’t think of them as succulents, many plants will die back in the dry season and reemerge from a succulent root or tuber. One such example is Pelargonium appendiculatum. Other Adaptations Water storage is not the only helpful adaptation to a lack of water. Many succulents have developed wide networks of surface roots to collect more of the small amount of rainwater that falls in deserts and a large number form waxy skin coatings to keep the water from evaporating during the heat. Others go into a period of hibernation in the hottest months. Many succulents, including agaves, cacti, crassulas, euphorbias and other plants, such as corn and certain orchids have developed a different type of photosynthesis called Crassulacean Acid Metabolism or CAM, which allows plants to conserve water by keeping the stomates closed during the daytime heat. The stomates are pores on leaf surfaces and other tissue that allow the plant to take in carbon dioxide, but while open, they also cause the plant to lose water. Please note: There really is no such substance as crassulaean acid. This type of photosynthesis is called Crassulacean Acid Metabolism because it is acid metabolism in succulents and it was first observed in crassulas. To survive, however, succulents must photosynthesize during the day when they have access to sunlight. They still need carbon dioxide for photosynthesis, and they can’t get it with their stomates closed. These plants open their stomates at night, collect carbon dioxide and store it in the form of malic acid, which is later broken down to carbon dioxide when needed. Pretty amazing! People who have counted the number of stomates on plants (Yes, there are such people!) have found that succulents have far fewer of them than, say, plants with leaves, which are riddled with these openings on the bottom of their leaf surfaces. This is just another strategy for hanging on to water. Botanists have noticed that succulents widely separated by geography are nonetheless similar in form. The most interesting examples of this phenomenon are the euphorbias of the Old World and the cacti of the New World, many of which are similar in shape. There is a volume to surface ratio which is critical for plants that store water. For a given size, the plant must maximize water storage capacity while minimizing surface area. Since there are only so many options for shapes that accomplish this, plants in disparate parts of the globe often look similar. The cylinder seems to be a very efficient shape for water storage. Many cacti have also developed vertical ribs, which make it easy for them to shrink when water storage is low and to expand after a rain. Finally, not all succulents are dry land plants. Witness the many common cacti used as houseplants. Christmas cacti, Easter cacti, epiphyllums and other rainforest cacti are epiphytes living in the tops of trees and occasionally on rocky outcrops in the humid tropics. At some point in their history they were dry land plants because they also developed the ability to photosynthesize utilizing the Crassulacean Acid Metabolism pathway. In their new surroundings, they have access to a lot more water than dryland cacti. In their present environment, in the tops of trees in tropical rainforests, they wouldn’t ordinarily need this special metabolic pathway, but are basically stuck with it. You could say they have become the most indolent members of the cactus family. While other members of the family are hard at work storing and conserving water, epiphytic cacti just lounge around and drink in the rain as it passes by. Not a bad life, really!  

Question: What is a Succulent Anyway?

Answer: A Pretty Clever Plant!

Paula Szilard

We all know one when we see one, or so we think! But these plants have come up with more varied adaptive strategies for conserving and storing water than we give them credit for. The phenomenon of succulence cuts across many plant families. Although there are many plant families which boast huge numbers of succulents, like the Euphorbiaceae some families have relatively few, like the Passifloraceae, the passion flower family.

These plants have had to adapt to a scarcity of water in their environment or die trying. We know the survivors rose to the occasion. We will never have an appreciation for how many plants died trying!

Succulents are plants with at least one type of tissue or organ for water storage. The different adaptations we see are:

Leaf Succulents

These plants developed thick leaves to store water. We have all grown some of them. Common ones that come to mind are Crassulas (jade plants), Kalanchoes, Lithops, Aloes and Echeverias.

Stem Succulents

In these plants, it’s the succulent stems that store the water and often carry on photosynthesis. There are many examples in the Euphorbiaceae family (old world analogues of our new world cacti) and the Cactaceae themselves, most of which are stem succulents. Yes, most cacti are succulents, but not all succulents are cacti! Cacti have the identifying feature of areoles, small cushion like structures with hairs or spines, or even barbed bristles (glochids). There are also a number of species of pelargoniums with succulent stems.

Plants with Caudexes (Caudices )

Some plants have stems that are widened at the bottom. These stems are called caudexes or caudices. Ones we are all familiar with are the Pony Tail Palms, (Beaucarnea recurvata), Adenium obesum and plants in the genus Adenia (Passion Flower Family). Some people call these fat-bottomed plants. The commonly used adjective to describe them is caudiciform. The fat part of the stem can be underground or above ground, or partly underground.

Pachycauls

Pachycauls are similar to caudiciform plants. These plants can be herbaceous plants, shrubs, even trees with abnormally thick stems, often fatter on the bottom, like the baobob tree. Some authors group pachycauls together with caudiciform plants in a group called pachyforms, or pachyform plants.

Root Succulents

Although we ordinarily don’t think of them as succulents, many plants will die back in the dry season and reemerge from a succulent root or tuber. One such example is Pelargonium appendiculatum.

Other Adaptations

Water storage is not the only helpful adaptation to a lack of water. Many succulents have developed wide networks of surface roots to collect more of the small amount of rainwater that falls in deserts and a large number form waxy skin coatings to keep the water from evaporating during the heat. Others go into a period of hibernation in the hottest months. Many succulents, including agaves, cacti, crassulas, euphorbias and other plants, such as corn and certain orchids have developed a different type of photosynthesis called Crassulacean Acid Metabolism or CAM, which allows plants to conserve water by keeping the stomates closed during the daytime heat. The stomates are pores on leaf surfaces and other tissue that allow the plant to take in carbon dioxide, but while open, they also cause the plant to lose water. Please note: There really is no such substance as crassulaean acid. This type of photosynthesis is called Crassulacean Acid Metabolism because it is acid metabolism in succulents and it was first observed in crassulas.

To survive, however, succulents must photosynthesize during the day when they have access to sunlight. They still need carbon dioxide for photosynthesis, and they can’t get it with their stomates closed. These plants open their stomates at night, collect carbon dioxide and store it in the form of malic acid, which is later broken down to carbon dioxide when needed. Pretty amazing!

People who have counted the number of stomates on plants (Yes, there are such people!) have found that succulents have far fewer of them than, say, plants with leaves, which are riddled with these openings on the bottom of their leaf surfaces. This is just another strategy for hanging on to water.

Botanists have noticed that succulents widely separated by geography are nonetheless similar in form. The most interesting examples of this phenomenon are the euphorbias of the Old World and the cacti of the New World, many of which are similar in shape. There is a volume to surface ratio which is critical for plants that store water. For a given size, the plant must maximize water storage capacity while minimizing surface area. Since there are only so many options for shapes that accomplish this, plants in disparate parts of the globe often look similar. The cylinder seems to be a very efficient shape for water storage. Many cacti have also developed vertical ribs, which make it easy for them to shrink when water storage is low and to expand after a rain.

Finally, not all succulents are dry land plants. Witness the many common cacti used as houseplants. Christmas cacti, Easter cacti, epiphyllums and other rainforest cacti are epiphytes living in the tops of trees and occasionally on rocky outcrops in the humid tropics. At some point in their history they were dry land plants because they also developed the ability to photosynthesize utilizing the Crassulacean Acid Metabolism pathway. In their new surroundings, they have access to a lot more water than dryland cacti. In their present environment, in the tops of trees in tropical rainforests, they wouldn’t ordinarily need this special metabolic pathway, but are basically stuck with it. You could say they have become the most indolent members of the cactus family. While other members of the family are hard at work storing and conserving water, epiphytic cacti just lounge around and drink in the rain as it passes by. Not a bad life, really!