A "Caudex" is a swollen stem or trunk of many desert-dweller plants, used to store water as a mechanism of adaptment to the severe droughts of their habitats.
Plants with caudex are called "Caudiciforms", but also, sometimes, "fat plants", because of their swollen and lumpy trunk, which makes them take on slightly weird shapes. Anyway, this oddity makes them very appreciated by succulent collectors and in the world of nursery and ornamental gardening.
Cacti are not caudiciforms. They also store water inside their stem or trunk, but what makes them different from Caudiciforms is that their stem has also a photosynthetical function: it is consequently green, doesn't usually swell up or take strange shapes, and it doesn't have bark either.
Almost all caudiciforms are semi-desert dwellers: they come from climate regions with a rainy period during the year, the so-called "seasonal rains", during which the great quantity of water is stored into the caudex to be used after several months during the dry season, when it may not rain for months. Most commercially grown caudiciformes come from semi-desert areas in Africa and Central America (Mexico), or either South America.
Caudiciforms are found within several families, totally unrelated from each other, showing that the caudex is a form of adaptation to certain types of habitats and not a genetically determined character of one particular family. In fact, arid habitats are found in very different parts of the planet, just as the different species of caudiciformes are found all over the world: this makes us understand how efficient the caudex is as a form of adaptation of the plant and shows once again the incredible facets of the plant world, able to adapt to nature in a very intelligent and fashinating way.
To better understand the phylogenetic variability of the caudiciformes, it's sufficient to show the examples of the big baobab next to the peculiar Cycas and next to the well known Adenium obesum (small caudiciform of the family Apocynaceae, very appreciated in the world of ornamental gardening): these plants all have the caudex, despite their great diversity both morphological and phylogenetic.
Here are are some families that contain caudiciformes: we have Cycadaceae, Apocynaceae, Asparagaceae, Dioscoreaceae, Euphorbiaceae, Vitaceae, Passifloraceae, Ruscaceae, etc...
There are over 100 genera of plants that have species that can be described as caudiciforms.
The most popular are Euphorbia, Cycas, Bowiea, Dioscorea, Adenia, Pachypodium, Boophone, Jathropa, Beaucarnea.
So, as already said, the definition "Caudiciform" is extrermely general and includes anything with a caudex, going from giant trees to small herbs. That's why morphological classification is preferred to phylogenetic classification to describe them. According to this kind of classification, Caudiciform plants can be divided into 4 general groups depending on their form:
Caudiciforms belonging to the wide "Phanerophytes" group are plants with a swollen part of their trunk or stem which comes prominently up above the ground. Consequently their growing centre (the point from whom new cells are produced in all directions) is far above the ground, approximately 25 centimeters or more.
Many examples of Caudiciform Phanerophytes plants can be found in the genuses Adenia, Pachypodium, Cyphostemmas, Beaucarnea. Within these genuses there are many unfailing examples, such as, for example:
• Adenium obesa, the "desert rose", as it's called for its wonderful flowers and the astonishing shape of its trunk
• Cyphostemmas juttae, another marvelous succulent with this turtle-shaped woody sphere as a basal caudex and the small ivy-like leaves
• The tiny, irresistible Pachypodium brevicaule, with its bright yellow flowers and its little, barely coming off the ground, woody caudex
These plants, differently from Phanerophytes, have a swollen caudex which may or not may prominently emerge from the ground and it's more like a root than a keg. This is the case of Dioscoreas: the genus Dioscorea is actually very wide, including also edible species such as Dioscorea trifida ("Yam").
In this kind of plants the caudex remain below the ground, but the growing centre remain above the ground and so the caudex is observable also above the ground. The unfailing example is Ceratozamia zergozae, with its giant, ball-shaped caudex from which a little green part develops like an untidy clump o grass, so small if compared to the enormous caudex.
Also Boophone disticha is grouped together with Hemicryptophytes, although its caudex is actually classified as a bulb in the official botanical nomenclature.
These plants form their caudex and either their growing point above the ground. Plants belonging to the genus Ibervillea and Trochomeria are a good example of Geophytes.