Succulents are a type of plant that is able to retain and conserve the moisture in its body. This is a defense mechanism evolved in order to survive in arid, dry climates and poor soil conditions. Fleshy leaves, needled trunks to deter animals and insects that would prey upon them, and altered metabolic rates are just a few of these mechanisms. Though cacti immediately spring to mind, these are by no means the only types of succulent plants.
Agavaceae is a family of plants found throughout most tropical and desert regions of the world. They may or may not be succulents. This is distinguished by the presence or absence of thick, water-bearing leaves and deep roots that contain the vast majority of the plant's water supply. Unlike cacti, the main body of the plant is squat and has long, overlapping barbed leaves. Examples of this family are yucca trees, agave plants, tuberoses, cabuyo and Joshua tree.
The Crassulaceae family is more commonly known as the orpine family. It is composed of over 1,400 species found throughout much of the world but concentrated primarily in Africa and the Northern Hemisphere in the form of bushes and trees. Orpines are characterized by having copious succulent leaves. The thick, fleshy masses often resemble the flesh of an apple in consistency. The interior is a thick infrastructure of plant cells. These cells swell up many times their original size with moisture and retain this moisture until the plant requires it for metabolic function. Some examples of orpines are gold drop, pig's ear, rosularia, jade tree and bear's paw.
The Apocynaceae family is also known as the dogbane family. It encompasses 1,500 species native to the Mediterranean, Africa and the Middle East. They come in the form of shrubs, trees, plants and vines. All share the same characteristic of having thick, woody stems and milky sap. This helps reduce the amount of fluid that leaves the plant by dry air and by broken stems. Some examples of this family include plumeria, oleander, periwinkle, Indian devil tree, golden trumpet and the poison arrow plant.