Tuesday, 5 April 2016

The somewhat immortal House Leek at work 24/7


In leaf this plant looks so ordinary, so familiar, we barely notice it. But when it sends up this chunky stalk topped with an umbrella of, if not showy, at least curious flowers, you take a second look. I did.

What we have here is a succulent called Sempervivum, probably Sempervivum tectorum. You see it, or one of its relative, all over the place. It's in the family Crassulaceae, which includes fleshy leaved plants, often with leaves tightly packed in a rosette, like this one. The flowers most often rise above the rosette on some kind of stalk.


Sometime ago I promised to blog about Crassulacean Acid Metabolism, or CAM. Many, if not all, members of the Crassulaceae exhibit this special way of converting sunlight into food (photosynthesis). So too do other plants such as fleshy orchids and bromeliads - it's an adaptation to living in places or climates where water is scarce, such as deserts and up in trees.

The basic concept is that the (usually fleshy) leaves and stems act like a battery, capturing carbon dioxide at night when it's safe to open the pores (stomata) in the leaf. The carbon dioxide is combined into a chemical call malate, which can be stored inside the cell (actually in the 'vacuole'). The plant then strips carbon dioxide from the malate during the day - when the sun's energy is available - to build the sugars needed by the plant to live and grow.

The benefit to the plant of this time shift in the two parts of photosynthesis (other plants take up and fix carbon dioxide during the day) is less water lost during the day. Opening your pores to take in carbon dioxide is always a compromise in a hot, dry environment since water will be lost through evaporation.

So our Sempervivum is adapted to life with limited water. In our garden it gets plenty but it does mean that we can forget to water it occasionally, or take a holiday and return to find it alive. This accounts for one of its common names, Live Forever. In fact it's botanical name means the same thing: always alive.

Yet while the plant as a whole may always be alive, once a rosette flowers, after a year or two, it dies. By that time there will be plenty of offsets around the base so the thing lives on*.


If this species is indeed Sempervivum tectorum, we have access to more obscure common name. Tectorum means 'of roofs', and according to Wikipedia, this species (and quite possibly others) were apparently planted on the top of houses to 'ward off fire and lightening strikes'. This is celebrated in the common name House Leek (note the spelling, and therefore reference to the onion relative it might resemble a little in flower rather than holes in the roof).  There are also other names such as Jupiter's Beard that may relate to the plants connection with thunder and storms, in a good way.

If you look closely at the flowers in my pictures you can see they in their 'male phase' with active stamens producing pollen but the female bits curled up and non-receptive. This way the plant is more likely to avoid self-pollination, a topic I seem to return to quite often in this blog (e.g. the Monkey Flower). You can also see there are lots of floral parts - a dozen or so petals, about twice as many stamens and who I reckon a dozen female bits. Plenty going on, day or night.


*Or so the theory goes. As it happens, a month or two after taking these pictures, our entire clump has succumbed. Bad botany or bad plant husbandry? 

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