Prickly pear, Opuntia macrocentra, Cactaceae

A large prickly pear cactus, like this Opuntia macrocentra that comes from the Pecos region of Texas, is a thing of beauty when in full flower - although those long sharp spines can also inflict acute pain. 

These magnificent flowers only last for a day and change colour to a more orange hue after pollination, which is apparently performed by bees. You can read an account of the pollination ecology of the plant here.

The fruits that form after pollination are edible in some species of Opuntia, notably O. ficus-indica, and marketed commercially as tunas that are popular in Mexico and occasionally appear on British supermarket shelves.

The stamens shed pollen soon after the flowers open in the morning but the stigma only becomes receptive in the afternoon and must be cross pollinated with pollen from a different individual plant before the flower will set seeds. The stamens are touch-sensitive, curling towards the stigma if you give them a gentle poke, mimicking the action of a visiting bee.

Flattened, jointed, photosynthetic stems called cladodes take over the function of leaves in prickly pears and most species defend themselves with long, sharp spines against grazing animals. These often have clusters of tiny, barbed hairs called glochids around their base and these are acutely irritating if the stick in soft flesh - like lips, for example. In a few species, like ...

...... this O.rufida the long spines are entirely replaced by vast numbers of glochids arranged in dense clusters called areoles. The glochids are ....

... easily detached and break off at the slightest touch, so gently brushing against one of these glochid-covered cladodes can be a very unpleasant experience. For a really close look at these nasty microscopic harpoons, click here.


Both species illustrated here are growing in the desert house in Durham University Botanic Garden.

Tephrocactus articulatus var. papyracanthus, Paper-spined Cholla

This must surely be one of the most intimidating of all plants - the Paper-spined Cholla Tephrocactus articulatus var. papyracanthus. It comes from Argentina and those sabre-shaped spines that look so lethal are about eight centimetres long, but they're not as dangerous as they look. They really are like paper and simply bend under the slightest pressure; you'd be hard-put to draw blood with them. The tufts of tiny bristle-like hairs called glochids that you can see at their base are far more of a problem. They have barbed tips and are intensely irritating when they embed themselves in your fingers.The easiest way to remove them from flesh is to use a piece of adhesvive tape to pull them out.

Paper-spine cholla ( or the 'Edward Scissorhands' cactus, as my kids used to call it) is a slow-growing relative of the prickly pear cactus and is easily propagated by breaking off one of the jointed stem segments and rooting it in well-drained, gritty compost - but be sure to watch out for those nasty little glochids.

Parodia erubescens (Cactaceae)

If I could only grow one cactus this would probably be the one. It was labelled Notocactus schlosseri when I bought it but now it’s been renamed Parodia erubescens. The specimen in the photo was about 10 years old but it flowers just three years after seed germination and blooms annually thereafter, becoming more showy with every passing year. Older plants like the one above produce a magnificent , tightly packed spiral of flowers that bloom for a couple of weeks, then freely set seed which germinates very quickly. They also produce offsets that can be broken off and rooted, which is useful because old plants become top-heavy once they are about 20 centimetres tall and tend to topple over rather easily.