This distinctive and isolated Family, endemic to southwest Madagascar, makes up for its small size by many features of botanical interest. Like the Cactaceae. to which it runs parallel in many ways, for a long time it resisted efforts to fit it into any system of classification, seeming to have no close allies. Now, however, anatomical features and the possession of the pigment beta-cyanin have sited it in Caryophyllales next to the Cactaceae, with which successful grafts have been made—an index of biochemical affinity.
The four genera are all heavily armed, thick-stemmed trees or shrubs of unique appearance, recalling cacti, tree euphor-bias and Fouquieria (1.8) — another instance of convergent evolution. As in cacti, the spines are interpreted as modified leaves. Foliage leaves are also present, except in one species. Alluaudia dumosa (17.2). They are flat, scarcely succulent and simple in outline, and fall during the dry season. The flowers are tiny and unisexual, borne in bunches, but are rarely produced outside the habitat. Heidelberg University took the lead in the 1950s in introducing the riches of the Madagascan flora to Europe, and Prof. W. Rauh. the leading authority on the Family, reports three flowering species there.
Alluaudia produces long, thick stems with usually ascending branches up to 15m (49ft) tall, and has six species (17.2). The stems are covered in solitary or paired spines and small, leathery, almost circular leaves, sometimes turned into a vertical plane rather than borne horizontally.
Alluaudiopsis forms smaller shrubs up to 2m (6&ft) tall. There are two, very local species.
Below (17 lJ. Decaryia madagascariensis brings novelly lo any collection with ils quaintly zig-zagging growth with a pair of spines at each joint Cuttings will root.
D. madagascariensis (17.1), which in youth forms a compact shrub of singular appearance, the zigzag branches bearing a small leaf and two diverging spines at each node. In nature it eventually grows into a tree 8m (26ft) tall.
Finally, Didierea is the wonder of them all. Here the similarity to cacti is greater than in any other plant, the spines being borne in clusters from horny areoles at the tips of long tubercles (17.2). In D.
trunk with a head of thick erect branches that somewhat recalls the saguaro of Arizona. The other species, D. trollii (17.3), is similar in old age, but for the early years looks quite different, with the branches growing prostrate on the ground.
Didiereaceae can be raised from seeds and from cuttings, although it must be
Below (I 7.21: A selection ot Didiereaceae From letl to right. Alluaudia dumosa. Acomosa (back). Decaryia madagascariensis. Didierea trollii and D. madagascariensis.
added that neither, at present, is easy to obtain. They revel in warmth, and I have had Alluaudia procera for years on a sunny windowsill over a radiator, where it is watered throughout the year, although much less is given when the leaves drop—a wise precaution.
It may be asked why so tiny a Family — no more than 11 species in four genera — is given the honour of a chapter to itself. Their unique appearance—"the cacti of the Old World", as Werner Rauh puts it—and their sudden emergence from obscurity have excited much interest among both botanists and plantsmen. Plants are eagerly sought for; like pachy-podiums, they have become status symbols for the connoisseur's collection and the exhibition table. But there is another reason why the Didiereaceae deserve extra publicity. They are all very limited in range in the wild and their habitats are rapidly being destroyed, so their preservation, at least in cultivation, is of special concer
Right (17.3): Didierea trollii begins lile prostrate but eventually throws tall vertical shoots like those ol D.madagascariensis. Liki all the Family, it must have warmth.
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