Plant and Garden

Succulent Plant Evolution

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 succulent. This may indicate the juicy tissue, the surface volume ratio, or the ecophysiological function of water-storing tissues to mitigate the effects of dry periods.

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During the long-time evolutionary history of vascular plants, a diversity of designs of water storage has been developed in different parts of organs.

In special tissues or even specialized cells (idioblasts) as part of a tissue.

This contributes to the rich spectrum of morphological forms within the stem, leaf, and root succulents.

Example of plant convergent.

Their characteristic 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.

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The transition from succulent to not succulent is continuous and even 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), also.

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The most recent and comprehensive account of succulent plants used a pragmatic approach. This dealt with all these plants, which are succulent or cultivated in succulent plant collections. Thus, increasing the succulent plant group by not actual succulents, but xerophytic taxa as e.g. several Yucca species.

To count the number of succulent plant species (or genera and families) is not an easy task. This is further complicated by the ongoing taxonomic considerations about the acceptance of particular taxa and the continuous series of new discoveries and descriptions.

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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 on 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 a probable range that additional succulent species may exist in families not already treated in these books.

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Some examples of plants, which are also referred to be succulent or are partly succulent, are Dregeochloa pumila, Pentastemona

There are also succulent monopodial herbs from Sumatra such as Hakea clavata (Proteaceae and Proteales), an Australian shrub with succulent leaves, As well as, some species of Clusia, whose leaves are esteemed as succulent (LÜTTGE & DUARTE 2007).

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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.

However, these are observations at the margin of the concept, which may not change the pattern outlined in Fig. 1 substantially. During the evolutionary process of succulent plants have been developed independently in several lineages of angiosperms. From the oldest succulents comprising lineage Piperales, with the many succulent species of Peperomia Piperaceae, up to the of most recent Apiales.

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Most rich in succulent species are the order Asparagales (with Orchidaceae, Asphodelaceae, Agavaceae, and others) and the Caryophyllales (with Aizoaceae, Cactaceae, and others).

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.

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One of the largest families is known as Poaceae. With more than 10,000 species, it has only one succulent representative. From a geographical point of view, succulents and their families extend covering the earth.

Hotspots of succulent plant diversity are the arid or semiarid areas of the world. Most so in southern Africa and the American continent where thousands of succulent species can be found.

Whereas other areas are comparatively low in succulent species like Australia with a total of about perhaps 400 species (KAPITANY 2007).

Today, with the comprehensive species accounts of succulent plant families and based on methodological innovations substantial adjustments of phylogenetical concepts and classifications became possible.

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Current trends and new insights in systematics provided by these synopses.

As well as the complexity of ecological features and interactions of succulent plants and their environments were the 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. This work enables us to understand the causes and patterns of succulent plant diversity and evolution more in detail than before.

But to suggest that one evolved from the other would be quite unwarranted. For one thing, their fruits are diverse in structure and show them to belong to different subtribes of the Family. For another, we must take into account the phenomenon of convergence.

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Succulents show this to a high degree. An American cactus may look an African Euphorbia, Pachypodium or Hoodia that one has to look closely to spot the difference. It is a small wonder that the layman calls them all cacti! Yet we know all these to be poles apart in ancestry, and their flowers and fruits show them to belong to different Orders.

Many similar examples of convergent evolution could be cited. It seems as if conditions in regions of intense sunshine and periodic drought are so rigorous that very few life forms can endure them, and the cactiform habit has arisen independently over and over again.

The direction of evolutionary trends is often difficult to determine. Are the ribs on a cactus derived from the fusion of rows of tubercles, or did ribs come first and divide up into tubercles later? Certainly, both trends have occurred, so we may never be sure. But organs have evolved independently of one another in response to different stimuli. One organ may advance to a high level of specialization while another remains little changed over long periods.

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Thus, all cacti show extreme advancement of the vegetative body but retain primitive, spirally arranged flowers that have been little modified above the level of the magnolia or water-lily. Among the cacti, Pereskia stands out as primitive in having thin, deciduous leaves, little succulence and unspecialized flowers and fruits. Yet it has fully formed areoles which are a highly advanced feature.

Occasionally the direction of an evolutionary trend is indisputable. For example, it would be hard to imagine the spineless astrophytums as suddenly growing spines where none existed before: it is far simpler to regard them as secondary derivatives from spiny cacti by the suppression of armature. This is supported by the observation of fine spines on the seedlings. Seedlings tend to be conservative and to retain features not present in the adult plant. Thus, leafy seed leaves may be noted in species where adult plants develop no expanded leaves at all.

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In general, therefore, it is dangerous to be confident about the ancestry of succulents. A study of only living plants tells us no more about their history. As far as drawing up a genealogical tree of succulents is concerned, we see only the tips of the branches: the stems and trunk are forever hidden from view.

Although the origin of succulents remains veiled in the past, the origin of species is more open to direct analysis. We can create species to order. In nature, new species arise from old ones either suddenly or gradually over long periods. Sudden origin comes from a chromosomal change that is passed from cell to cell.

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Eventually succeeding generations via sexual reproduction with best-known and most easily definable examples being associated with hybridization followed by doubling of the chromosome number.

Let us suppose that two related diploid species have genetic constitutions AA and BB. The letters refer to the two haploid sets of chromosomes possessed by each. The Fi hybrid will be AB. because it has received one A set from one parent and one B set from the other, regardless of which was pollen donor or egg donor. The two sets work happily side by side in each cell throughout the life of the plant, producing something that usually shows characteristics of both parents or is intermediate between the two.

However, a problem arises at meiosis when the A set of chromosomes cannot pair with the B set. As explained earlier, there is a breakdown and functional pollen and egg cells cannot form.

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Suppose now that by some accident of cell division a plant arises with a doubled chromosome constitution: a tetraploid with AABB. Here, at meiosis. each A chromosome can pair with it’s own A partner, and each B with a B. Fertility is restored, and the mechanism can repeat itself. Further, the seed breeds true and the plant is isolated from its parents.
because backcrosses would result in sterile triploids AAB or ABB. By definition, a new species has been born.

But how does such chromosome doubling occur? In the laboratory, it can be brought about by interfering with the normal division of cells in the growing tip of a stem or germinating seed. The classical method uses colchicines very poisonous and expensive alkaloid extracted from the autumn crocus. Colchicum.

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This retards the growth of a new cell wall following the division of a nucleus so that a tetraploid nucleus results. An example is Kalanchoe vadensis, a new man-made species produced by doubling the chromosome number of the diploid cross K. hlossfeldiana X K. grandiflora2. In nature, such tetraploids can arise spontaneously from seed, or as sports (mutations) on single branches.

The gradual origin of species in the wild takes place undramatically and is harder to study, though no less potent in enriching the flora. A plant population, or part of it, may become isolated, either by migration to new terrain or by developing internal breeding barriers so that it is cut off from gene exchange with its fellows. Its slow divergence in appearance from neighboring populations in response to a new environment may eventually lead a botanist to recognize it as a separate new species. Ultimate isolation occurs when chromosomal divergence creates a barrier and the new species can no longer intercross with its progenitors.

The evolution of a succulent flora Many of the habitats of succulents give indications of being active centers of evolution. These are areas of mountain and valley, with sharp daily or seasonal contrasts of temperature and rainfall, and with different soil types and different exposures. Stimulants to change are many, from freak frosts and droughts to hurricanes, earthquakes and volcanic eruptions. In times favorable for lush growth, ranges of different species may extend and overlap, and hybrid swarms arise at the point of overlap. In the retreat following adversity, isolated survivors.

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Seasonal variations in the range have been studied for the giant saguaro in Arizona, where fluctuating minimum temperatures in winter are the limiting factor to survival. Lack of summer rainfall in California as compared to Arizona is another factor that limits the spread of the saguaro westwards over the Colorado River.

A clear example of an active center of evolution is the Canary Isles, long isolated from the mainland of Africa and with a high concentration of contrasting habitats in a small area where 31 endemic species of Aeonium, as well as numerous interspecific hybrids, are found.

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A similar ferment of active evolution is to be found among the Andean cacti, where species limits are correspondingly difficult to define a situation reflected in the confusion of names and synonyms confronting the grower of these popular succulents.

Much interest attaches to the flora of islands. How did the plants get there in the first place, and from where? And how have they changed in isolation? Darwin noted that islands are usually poor in numbers of species, as compared with equivalent mainland areas, but rich in endemics, the percentage increase in proportion to the distance from the nearest continent.

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