Succulent plants, also known as succulents, are plants adapted to arid environments as they are equipped with special cellular tissues, called "succulent tissues", which are actually aquifer parenchymas, capable of storing big quantities of water. The organs designed to store water become fleshy and swollen, assuming particular, typical forms. These swollen, fat organs, are the only common, unfailing feature of every succulent plant: otherwise, succulents are really different from each other.
This feature is defined from the latin word "Crassus", which means "swollen", "fat", which is still found today in the succulent family name "Crassulaceae". The succulent tissue, called aquifer parenchymas, can be located in leaves, such as in the family Agavaceae, in stems, such as in Cactaceae (cacti) and Euphorbiaceae, or in the roots.
Succulent plants are often confused with cactus, but this is an error, because the botanical term "Cactus" refers to the family "Cactaceae". So cacti are actually succulents, but succulents are not necessarily cacti: the word "succulent" refers only to the capacity of water storage of a group of plant which includes many botanical families, such as Agavaceae, Aizoiaceae, Apocynaceae, and also Cactaceae. So, even if the world "cactis" is often used to define succulents, it actually defines only plants belonging to the botanical family "Cactaceae". So "Succulent" is a wider and more generic definition and refers to plants which belong to many different families.
Here below are the families with the highest number of succulent species and the organs designed to water storage:
• Agavaceae - 300 species – Leaves
• Aizoaceae - 2000 species - Leaves
• Apocynaceae - 500 species - Stems
• Asphodelaceae - 500 species -Leaves
• Cactaceae - 1600 species - Stems
• Crassulaceae - 1300 species - Leaves
• Didiereaceae - 11 species - Stems
• Euphorbiaceae - >1000 species - Stems
Plants with succulent leaves live generally in climate zones with not so long droughts and rainy periods. During these rainy periods they store water in leaf cells, which become swollen and dilated, and then use it during dry periods, when leaves wither and often fall as an extreme defence against drought.
Leaves can be flattened, as in the genus Echeveria, or cilyndric as in many Sedums, or either spherical, as in the peculiar species "Senecio rowleyanus”. Sometimes these plants don't have any stem, as in the genus Lithops, whose habitats are so dry that its leaves barely come off the ground, being partially buried to protect the plant from water loss and from the too intense sunlight.
Plants with succulents stems, as the majority of Cactaceaes, have in most cases early deciduous leaves, very small or turned into spines, to reduce as more as possible water losses through perspiration. Stems are particularly enlarged and they can assume a columnar (as it happens in Carnegiea gigantea, the "Saguaro") shape, or globose shape, as in Echinocactus. These modified stems are always green to allow these plant, even without leaves, to photosyntesis. So here chlorophyll is stored in stems and the leaves loose their function. All this is to minimize water loss: plant, in fact, eject water through microscopic holes in their surface, called stomatas, as we humans do through the skin. That's why the perfect shape to minimize water losses would have a maximum surface/volume ratio: there are, in fact, many globose-shaped cacti (spherical forms have the minimum surface/volume ratio). Cacti's forms tend to minimize the surface and increase the volume. That's why they get rid of the leaves: leaves, in fact, with their high surface/volume ratio (they generally have a flattened shape) increase the evapotranspirant surface, and so increase water losses through evapotranspiration.
Among succulent stem plants are the so-called "Caudex plant", which have a pecular basal bulge of their stem, called "Caudex". This is also to maximize water storage and it goes with a enlarged root system which often comes partially out of the soil.
Plants with succulent roots live in desertic areas. They have generally a fibrous or tuberous root system that expands in width just below soil surface: in this way, the plant can store water and protect itself also from predators and from the strong winds, typical of deserts. Stems and leaves of these plants dry up when the dry season arrives, leaving the succulent roots alive until the next rainy (or just more humid) period, when the root gives birth to a new plant.
Succulents don't just live in the deserts! These plants are actually adapted to a large variety of environments and have differents habitats.
In real deserts, such as Sahara, where years can pass without raining, there aren't succulents: no species is capable to store water for so long periods. Succulents live in subdesert areas where dry periods last several months, after which there are brief rainy periods that allow them to gather water reserves. These habitats are, for example, plateaus of Mexico and USA, where several cacti form colonies, iin a few cases similar to forests, as it happens, for example, for Saguaro's forests (Carnegiea gigantea) in Arizona. Also these kind of habitats are in Chile, Perú and several areas of South Africa.
Many species grow in high-altitude mountain areas. During the day, they are exposed to intense sunlight and strong winds. Overnight, they bear temperature fallings also far below 0ºC. These plants are adapted to a rocky, poor, not soo deep soil, incapable to retain water. This is the case of many species of the genus Mammillaria, in Mexican mountains, and of the genus Lobivia, in the Andes. In the temperate belt of the Northern hemisphere, also in Europe, many species of the genuses Sedum and Sempervivum grow in the mountains or even in the walls.
Many succulents grow also in rainforests: this is the case of the epiphytes succulents: plants which grow upon the trunk or stem of other plants. This species generally show a climbing or falling aspect. These plants usually grow upon soils rich in organic matter, in humid and hot climates, typical of rainforests. Unlike other succulents, these species are used to partial shade. The species that prefer this kind of habitat are included in the genuses Aporocactus, Rhipsalis (Central and South America), Hoya (asian and australian jungles) Epiphyllum, Schlumbergera.
The extreme conditions in which succulent plants live have forced them to develop many mechanisms to survive, as well as the succulent tissue, such as the special, flashy, big flowers of many of them, the spines, special kind of seeds, and many other features.
Most succulents developed peculiar features to escape predators, such as the presence of unpleasant or toxic substances (for example the latex of many Euphorbiaceas), the substitution of leaves into spines, or a mimetic aspect (for example in the genus "Argyroderma").
Flowers, in succulents, are flashy and coloured to attract pollinators, but they last for a short time. Many species have nocturnal blossomings, to attract bats or butterflies: others, such as Stapelias, develop flowers with a smell of rottenness, to attract necrophilic insects that lay eggs inside their corolla and pollinate them. Also, seeds have a high germination capacity and they can exploit really short humid periods to give birth to new plants. But succulent plants are also capable to propagate without seeds: through basal offsets, little rosettes of leaves, bulbs, creating roots from pieces of leaves or stems. This feature is widely exploited by nurseries to propagate succulents.
As well as the succulent tissues, succulent plants have developed several tricks to resist to drought. Several of them have have a leathery and glossy leaf surface: that's beacuse they have a waxy, hydrophobic strate. Others developed a thick hair, silky or woollen, with the function of retaining the low atmospherical humidity overnight. Big, rugged spines convey humidity towards the base of the plant, while the more light, hairy spines, serve as a protection from solar rays.
Another interesting feature of many succulents, which makes them very tough and resistant to extreme environments, is their mechanism of metabolism: the CAM metabolism (Crassulacean Acid Metabolism).
CAM metabolism is a typical mechanism of carbon fixation that enables succulents, to optimize photosynthesis in extreme environments, such as deserts. CAM plants are able to realize the photosynthesis process while keeping their stomatas close. Stomatas are little, invisible holes in leaves surface used by any plant to absorbe Carbon dioxide (essential for the photosynthesis) from the atmosphere and to expel water in the form of steam when it's hot. Stomatas, in plants, remain normally open all day and night long. Cactis,instead, so as not to loose water, close their stomatas during the day, when the temperature normally reach its maximum value, and open them during the night, when it's cooler and water loss through evaporation is reduced. Overnight, carbon dioxide is stored through several chemical reaction of fixation in the plant cells, to be used for the photosynthesis during the day, when stomatas are closed.
The peculiarity of this mechanism is that it is genetically determined but it's triggered by enviromental condition. So, during the rare humid days in the deserts, for example after a storm, these plants can "decide" to maintain their stomatas open.