The story of the evolution of plant life on this planet is the story of the conquest of dry land. From microscopic beginnings beneath the sea, of which the seaweeds (algae) are modern survivors, plants evolved the means to become independent of a continuous water supply for at least part of their life cycle, and were thus able to colonize the land. Mosses and ferns are still usually plants of moist shady places: conifers show a marked advance, as well as possession of a woody skeleton. Finally came the flowering plants, latest and most successful of all major groups, with improved adaptability and efficient reproduction by seed. The coal that we mine is a reminder of some of the less successful competitors in the struggle to survive.
The missing evidence. Our only direct evidence on which to reconstruct the past comes from fossils, which put the origin of flowering plants somewhere in the early Cretaceous period. 115,000,000 to 100.000,000 years ago. before the continents as we know them today had completely separated from the one united landmass. A rapid increase in the late Cretaceous made them the dominant land plants. Unfortunately the fossil record for flowering plants is very scanty, and for succulents nothing at all. Hot, dry climates are not, apparently, conducive to the formation of fossils. Among the earliest flowering plants that can be recognized are magnolias and catkin-bearing trees (all mesophytes), and it is logical toassume that xerophytes, including succulents, were among the latest developments. But the occurrence of related succulents in areas widely separated today, such as Sedum in North America and Asia, and Portulaca in South America and Africa, suggests a common origin before the continental landmasses began to drift apart. So we need a lot more evidence before attempting to construct an accurate chronology of events in the evolution of succulent plants.
Evolutionary trends. The chart on pages 16-17 reflects one person's ideas on which Orders are primitive and which are more specialized in plants living today. But one should not read too much into such family trees. Anyone can arrange the Mesembryanthemaceae. for instance, into a series showing progressive xeromor-phism, beginning with a leafy shrub such as Aptenia, scarcely succulent at all, and progressing through intermediate stages of condensation and surface reduction — shrubby Lampranlhus (11.2) and Ruschia (11.9), stemless Cylindrophyl-lum and Bergeranthus—to the ultimates of compaction such as Pleiospilos (11.4), Argyroderma (11.24) and Conophytum
Below: Convergent evolution in succulents. The" Medusa head" habit evolved in a South American cactus. Opuntia tephrocactoides llelt, 5.5) and in a South Atrican spurge Euphorbia caput-medusae (right, 5.6}.
(11.26). 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. Succulents show this in a high degree. An American cactus may look superficially so much like an African Euphorbia, Pachypodium or Hoodia that one has to look closely to spot the difference (5.5,6). It is 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 overand overagain.
The direction of evolutionary trends is often difficult toascertain. 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? Doubtless both trends have occurred; we may never be sure. But we can be certain of one thing: organs evolve 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 of time. 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 (16.4) stands out as primitive in having thin, deciduous leaves, little succulence and unspecialized flowers and fruits. Yet it has fully formed areoles—a highly advanced feature.
Occasionally the direction of an evolutionary trend is indisputable. For example, it would be hard to imagine the spineless astrophytums (16.31) 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.
In general, therefore, it is dangerous to be dogmatic about the ancestry of succulents. A study of only living plants tells us no more about their past history than a study of a crowd in a football stadium would help us to sort out the blood relations of the spectators. 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.
How species evolve 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 of time. Sudden origin comes from a chromosomal change that is passed from cell to cell.
and eventually to succeeding generations via sexual reproduction. The best-known and most easily definable examples are those 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. But a problem arises at meiosis: 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.
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 its 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. Col-chicum. This retards the growth of a new cell wall following 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 neighbouring 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 centres 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 favourable 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
Right (5.7): Survival in nature depends upon continual regeneration The giant saguaro ot Arizona (Carnegiea gigantea) here shows all stages trom seedlings to a dead adult plant.
Below (5.8): Islands separated trom the mainland are often centres of active evolution. Aeonium. shown here has evolved thirty-one species in the comparatively small area of the
may become the basis of new species. Seasonal variations in range have been studied for the giant saguaro in Arizona, where fluctuating minimum temperatures in winterare the limiting factor to survival (5.7). Lack of summer rainfall in California as compared to Arizona is another factor that limits the spread of the saguaro westwards over the Colorado River3.
A clear example of an active centre 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. Here we find 31 endemic species of Aeonium (5.8) as well as numerous interspecific hybrids between them. 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 increasing in proportion to the distance from the nearest continent. The evolution of unique, fat-stemmed succulents on remote oceanic islands has been alluded to on page 36.
Nature or nurture ? In all that I have said so farabout genetics and evolution, my concern has been only with what is inherited. There are in addition changes brought about as a result of nutrition, climate or disease —
fluctuations, we call them (6.2). Redleaves may result from phosphate deficiency, yellow ones from lack of iron, stunting from general starvation. But such changes affect only the individual and are not i inherited. If returned to normal growth conditions the plant will recover. The transformation that occurs in some im- I ported succulents when they are grown in I mild, moist conditions can be so startling
Below left(5.9]: Mimicry'' in trie Asclepiadaceae Hoodia, 20cm (8in) high, seen against its natural rock background in South West Africa Such plants can become Quite inconspicuous at a short distance
Below right (5.10): "Mimicry" in the Mesembryanthemaceae a many-headed Lithops in habitat in South West Atrica Note how different the pattern of camouflage is from the popular idea of matching pebbles as to lead the observer to believe that he has a new species.
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