Generations of botanists and ecologists devoted their time to the study of succulent plants, and so did generations of amateur collectors and plant enthusiasts when cultivating these plants are both impressed and inspired by the enormous morphological and taxonomic diversity.
Whether a plant is succulent or not, depends on the definition of succulen, which may emphasize only the juicy tissue, the surface volume ratio, or else the ecophysiological function of water-storing tissues to mitigate the effects of dry periods.
During the long-time evolutionary history of vascular plants, a diversity of forms of water storage has been developed in different organs, parts of organs.
In particular tissues or even specialized cells (idioblasts) as part of a tissue, contributing to the rich spectrum of morphological forms within the stem, leaf, and root succulents.
As well as the group of pachycaul and caudiciform plants.
Their common feature is the storage of water in specialized tissues to survive more or less regularly occurring dry periods. However, whether a certain species is succulent or not, is often disputable.
The transition from succulent to not succulent is continuous and even quite normal non-succulent plants are able to survive some loss of their water content due to transpiration.
Moreover, several halophytes have a succulent appearance and there are transitional forms between true succulents (eu-succulents) and succulent like halophytes (halo-succulents), too.
If one looks into the huge amount of succulent plant literature, he will find that some groups with definitively succulent organs or tissues like many orchids and bromeliads are not, or only marginally, treated there.
The most recent and comprehensive account of succulent plants used a pragmatic approach and dealt with all these plants, which are succulent or cultivated in succulent plant collections, hence increasing the succulent plant group by not really succulent, but xerophytic taxa as e.g. several Yucca species.
To count the number of succulent plant species (or genera and families) is hence not an easy task, moreover complicated by the ongoing taxonomic discussions about the acceptance of particular taxa and the continuous series of new discoveries and descriptions.
Nevertheless, there are estimations, how many succulents live on earth. According to EGGLI (2007), there are about 12,380 succulent plant species, orchids, bromeliads, and many borderline species.
Recent estimations of the number of seed plants range from c. 220,000 up to more than 446,000 species, dependent from authors and approaches (GOVAERTS 2003, JOPPA & al. 2010).
If we take a number of 262,697 plant species as quoted by (STEVENS 2001), succulent plants would encompass 4.7% of worldwide flowering plant species diversity. It is probablerange that additional succulent species may exist in families not already treated in these books.
Some examples of plants, which are also referred to be succulent or are partly succulent, are Dregeochloa pumila, Pentastemona, succulent monopodial herbs from Sumatra, Hakea clavata (Proteaceae, Proteales), an Australian shrub with succulent leaves, or some species of Clusia, whose leaves are esteemed as succulent (LÜTTGE & DUARTE 2007).
The number of succulents would grow even larger if water storage at the level of specialized water-storing tissues (often in the epidermis or hypodermis of leaves) or water storage within single specialized cells are added.
Piperaceae) up to the of most recent Apiales (with only three known succulent species in the Araliaceae and Apiaceae).
Most rich in succulent species are the order Asparagales (with Orchidaceae, Asphodelaceae, Agavaceae, and others) and the Caryophyllales (with Aizoaceae, Cactaceae, and others) (Fig. 1).
The diversity of succulent plants differs between the families. Some families like Cactaceae and Aizoaceae are entirely made up of succulents, whereas in other families succulent forms only a more or less small part of their species diversity.
One of the largest families, the Poaceae with more than 10,000 species, has only one succulent representative. From a geographical point of view, succulents and their families range on earth.
Hotspots of succulent plant diversity are the arid or semiarid areas of southern Africa and the American continent with thousands of succulent species.
Whereas other areas are comparatively low in succulent species like Australia with a total of about perhaps 400 species (KAPITANY 2007). Consistently the uneven distribution of succulents provoked to study the causes of these patterns (ELLENBERG 1981).
Today, with the comprehensive species accounts of succulent plant families and based on methodological innovations substantial adjustments of phylogenetical concepts and classifications became possible.
Current trends and new insights in systematics provided by these synopses.
As well as the complexity of ecological features and interactions of succulent plant and their environments were main topics of the IOS congress, organized at the Biocentre Klein Flottbek of the University of Hamburg in 2004.
The interesting and exciting contributions presented there summarised the most recent results of succulent plant research, which enables us to understand the causes and patterns of succulent plant diversity and evolution more in detail than before.
Already at that time arose the idea to provide modern synopses about the evolution and diversity of certain succulent plant families to a broader audience.