The common feature of plants grouped here is extreme differentiation between temporary food-forming organs (green leaves or shoots) formed during periods favourable for growth, and permanent. non-green, heavily protected water and food stores at or below ground level that enable the plant to endure long periods of desiccation. But the definition cannot be made more precise: if one fact emerges from a study of living organisms it is that they cannot be made to fit exactly into man-made categories. In the advance into drier and drier habitats, it was of little consequence whether water was stored in swollen leaves, stems or roots or partly in all three, and the grouping together of plants having a caudex is useful only in illustrating one of the three most successful strategies, and as an index to cultivation. It would be a mistake to insist on greater precision, as, for instance, by creating a class for caudiciform plants in a show schedule.
The caudiciform habit has evolved in varying degrees in many Families of flowering plants, and is not typical of any one. For that reason, no key is attempted here, and the choice of what to include in a succulent collection remains very much a personal affair. As a life form it is especially well adapted to flat, scrubby, semi-desert regions susceptible to periodic fires. Plants are modified in various ways to survive these, and the concentrating of their stored water and food at or below ground level is one such strategy.
Since 1946, when the word was redefined in the modern sense, something of a fashion for these weird and out-of-the-way plants has grown up and the thirst for more and yet more novelties remains unslaked. The result has been a source of consternation to cataloguers and show organizers, who now find it more difficult than ever to decide: "What is a succulent?" A large element of the world's flora in all climatic zones develops enlarged underground organs that can be revealed by planting above ground level in cultivation. Others can be induced to become temporarily caudiciform by giving them the "bonsai treatment": small pots, poor soil, little water and judicious pruning. About the only candidates that seem to be
Right (21 I J: Pachycormus discolor, endemic to Baja Calilornia. in a lunar landscape that dwarfs the botanist (Reid Moran] exploring it. Columnar Lophocereus and agaves can be seen in the foreground
ineligible are hardy types such as the bryonies (Bryonia dioica, Tamus communis), which in my experience are not xerophytic at all, and languish if the storage organ is planted above ground.
This fairly large, mainly tropical Family includes the mango. Mangifera, and some hardy ornamental shrubs such as the sumach, Rhus. In the drier regions of Lower California. Pachycormus discolor gives an unearthly air of starkness and passivity to the landscape (21.1), with its swollen, gnarled trunks and tiny pinnate leaves that drop during the dry season. Small specimens (21.11), when obtainable, are beloved of bonsai collectors.
Two South African species of Pachy-podium (Chapter 18) are best treated here because, like an iceberg, the greater part is hidden from view: a massive turnip-like caudex that penetrates deeply into the rocky substrate and is most difficult to dig out. The tangle of spiny shoots above ground is relatively inconspicuous. P. succulentum (21.4) has narrow-tubed flowers; in P. bispinosum. the most free-flowering of those I have grown, they are broad bells.
To the stem succulents described in Chapter 19 one must add at least four genera with caudices. Ceropegias are of diverse habit, but all merit attention for
Right (21.2): Fockea crispa showing the massive basal caudex which is reported to reach 3m (Wit) in diameter in habitat. The foliage is attractively crisped at the margins
Bglow (21.3): Ceropegia conrathii. a rare and intractable South African species reported from the Johannesburg area, but also found in Natal by the author In 1971.
Above (21 4): Pachypodium succulentum has the massive turnip-like caudex hidden below Ihe soil in nature, and the twiggy branches are conspicuous only when the Ilowers open.
Below (215) Brachystelma barberae is the most showy ol its genus, with a ball up lo 12Hcm (Sin) in diameter ol many curious lantern-like flowers. The scent is foul.
the tubular fly-trap flowers (21.3). In some the slender shoots die back to the caudex so that nothing is seen above ground during the resting period; in others they are more or less evergreen. Hardiest and most popular are C. woodii (4.14), with thick, rounded, grey and purple, short-stalked, ivy-like leaves, and its subspecies debilis. with linear leaves. These form new tubers along the creeping stems, which make them ideally easy to propagate. Large tubers are also excellent rootstockson which tograft rare,delicate and cristate Stapelieae.
Bruchystelmas are all caudex-forming, and differentiated from Ceropegia only by the shorter tube to the flower, which is commonly flat or saucer-shaped with free corolla lobes, not united at the tips. An exception is the most spectacular species. B. barberae (21.5), which produces a great ball. 8-12cm (3-5in) across, of sinister purple blooms with a smell that defies description in any polite terms.
Fockea and Raphionacme live to a great age and the underground caudex may weigh many kilograms. F. crispa (21.2) bears a tangle of twiggy, trailing shoots with small, thick, glossy green leaves attractively curled at the margins. It can be multiplied only by seed, but many of the others can be struck quite successfully from cuttings.
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