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Zygocactus truncatus

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The most striking feature of the Cactaceae in general, is their sheer structural simplicity.  With very few exceptions, they have no leaves nor any stem/trunk, just roots and a ‘plant body’ which perform all functions.  Those that have evolved in arid environments often have a roughly spherical or barrel-shaped plant body, which is a very effective way of minimising surface area to volume ratio – vital in moisture conservation.

Of course, not all cacti evolved in an arid environment, some are forest epiphytes, i.e. they grow in the trees of the tropical and sub-tropical forests of Central and South America.  Although their roots may penetrate the bark of the supporting trees they are epiphytic rather than parasitic, i.e. they draw their nutrients from leaf litter, moss and decomposing organic matter around them, but not from the tree itself.  They have also adapted to an environment in which water is fairly plentiful, but nutrients and sunlight are relatively scarce.

Zygocactus truncatus is an epiphyte which originated in the upland forests of South Eastern Brazil.  It was introduced to the UK in 1816 by Kew, and classified by Salm-Dyck in 1834 as Cereus truncatus altensteinii.  Since then it has acquired many aliases, the most common of which have been Epiphyllum bridgesii, Zygocactus bridgesii & Zygocactus delicatus.  The genus Zygocactus was created in 1890 and the species became either Z. truncatus or Z. altensteinii depending on whose authority you want to take.  Later it was re-classified as Schlumbergera truncata, the genus Schlumbergera having been defined by the French botanist Charles Lemaire in 1858.  Schlumbergera truncata is now its official designation, though Zygocactus is still widely used.

There have been several periods in which this plant has enjoyed peaks of popularity, particularly the late 19th century, and the 1980s/90s.  At these times there was a good deal of hybridisation with related wild species, so that many of the plants we accept as Z. truncatus today are probably hybrids.  Indeed in the wild, the flowers tend to be red rather than the magenta common in the cultivated varieties.  This may be what led one Dr. Rose to announce in 1915 that he had discovered what he described as the ‘true’ Z. truncatus, when he found a red flowered specimen in the Organ Mountains near Rio de Janeiro.

The plant is made up of flat stem segments (they are not leaves), each with a large elongated terminal areole at its distal end and two, four or six small lateral areoles set in notches along the segment margin.  It is the possession of areoles that define it as a cactus.  New segments and flowers develop from the terminal areole, so the entire plant consists of branching ribbons of more or less identical segments (Fig. 1, left).  With age, some of the segments close to the base of the plant tend to become rounded.

It is a short day plant, which means that flowering is triggered when the day length falls below a certain level, in this case about 10 hours daylight/14 hours dark.  In the UK that's around late October.  The fully developed flowers open in the dead of winter - hence its common name ‘Christmas Cactus’.  This is the secret of its success as an ornamental plant; anything which produces bright colours in the middle of a UK winter is almost guaranteed to catch on.

The fully opened flower (Fig.2 right) is zygomorphic, i.e. it has a plane of asymmetry – in the case in the horizontal plane.  It has been suggested that this is to make access easier for larger insects.  A similar inter-pretation could be put on the re-curved petals, although the fact that the stigma and anthers hang well clear of the petals suggests that it is susceptible to wind as well as insect pollination.

The magenta pigmentation extends to the stigma and the anthers, but not the pollen grains.

Although Z. truncatus doesn't generally set seed readily, it can be propagated very easily by breaking off a strip of stem segments and rooting them in moist compost.  Under dry conditions the plant will sometimes spontaneously shed segments or even whole branches, which then re-root themselves when conditions improve.  This 'controlled disintegration' approach is a survival mechanism in a number of cactus species.

To flower reliably it needs a summer rest period: put it outside in a shady place, and let it have just the water that nature chooses to supply.  Move it to the greenhouse in late September, or earlier if there's a possibility of frost.  Feeding and re-potting are best done in the early spring, after flowering, but well before it goes out for the summer.

Click here to see micrographs of open anther and stigma surface.