Introduction

Succulents are defined as plants that are able to withstand drought because of the water stored in some of their organs. They constitute a widespread group, being represented in several families of flowering plants and, to some extent, in the Gymnosperms as well. The definition is somewhat problematic because several plants, although not true succulents, are included among their number for aesthetic reasons. The most precise scientific definition is given in Life Strategies of Succulents in Deserts by D.J. von Willert, B.M. Eller, M.J. A. Werger, E. Brinckmann and H.D. Ihlenfeldt:

A succulent (or succophyte) is a plant possessing at least one succulent tissue. A succulent tissue is a living tissue that, besides possible other tasks, serves and guarantees an at least temporary storage of utilizable water, which makes the plant temporarily independent of an external water supply when soil water conditions have so deteriorated that the root is no longer able to provide the necessary water from soil. This definition implies that succulence may be present in one or more plant organs. The specialization of an organ is determined by the particular selective pressure in the species' habitat.

Leaf succulence occurs in several families, and it is usually associated with environments that do not have a long dry season. When the dry season is very long the succulent leaves are highly specialized. Lithops is, perhaps, the best example: the stem is absent and the leaves grow hidden in the soil, thus reducing water loss.

Stem succulence, which is seen in the cactus family, is also familiar to succulent enthusiasts. Succulent stems have leaves that are reduced in size or are caducous when well developed, and photosynthesis is performed by the green stems. Stem succulents may inhabit more arid environments, but the critical point is their size: gigantic species need regular water to support growth, and in very dry environments stems are reduced in size.

In root succulence the organ of reserve is subterranean, which is thus protected from the stresses of wind and predation. Root succulence is often associated with annual stems, which desiccate during the dry season.

In addition to these visible characteristics, succulents have several less evident features that give them strong selective advantages in arid habitats. The number and size of the stomata (the holes that allow air to enter leaves and leaves to capture carbon dioxide) are usually reduced. Not only does this reduce water loss but it also reduces the amount of carbon dioxide taken in. Moreover, the opening of stomata during the day, in the presence of strong sunlight and dry air, would lead to the evaporation of water without a compensatory intake of water in the form of rain. A particular type of photosynthesis, Crassulacean Acid Metabolism (CAM), has evolved in several families. In CAM the stomata of plants open at night, when the temperature is lower and the humidity of the air is higher. These plants store carbon dioxide, using organic acids - namely malic acid - during the night when the stomata are open. Carbon dioxide is released during the day when the stomata are closed but light is available for photosynthesis to transform the carbon dioxide into sugars. CAM is not only efficient in the economic use of waler but is also highly effective in capturing carbon dioxide from the air. CAM photosynthesis evolved independently in several families, and it is also present in some aquatic plants, when the selective advantage arises from the efficiency of capturing carbon dioxide that is not abundant in water.

In arid habitats succulent plants may be the only source of food for wild animals. Several features have been evolved to cope with this predation, the most common being the presence of spines, which make the plants less appetizing, and mimicry, which makes them difficult to see against the soil. Another defence against predators is the presence of toxic or repellent chemicals - once the predator has tasted the plant it will not try to eat a similar specimen.

The different adaptive characteristics are present in various groups of plants. Often species that are distant in geographic or phylogenic terms may look similar. The most striking example of this is probably the columnar or globular euphorbia and cacti. The volume to surface ratio is critical from the point of view of storing water, and it is possible to identify a trend in stem shape, from the cylindrical to the almost spherical, that is the ideal for the volume to surface ratio. These plants may have very similar shapes even though they are native to different continents and have different ancestors. The resemblance is caused by the same selective pressure that causes convergent evolution.

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