Wednesday, May 16, 2012

Xerophyta retinervis

Xerophyta retinervis, or “Black Stick Lily,” is a flowering plant from the high-altitude interior of eastern South Africa, with tufts of coarse leaves emerging from a clump of fat stems with a fibrous coat. The stems are often charred by fires in the veld, giving rise to the first part of Xerophyta’s common name. The “lily” part of the name comes from the large cobalt violet flowers that seem to emerge from the stems and open overnight, and always catch me by surprise when they appear in spring. I have hardly ever caught the early stages of flower development, but it is less than a week between no visible flower buds at all and full bloom.
Xerophyta is part of the family Velloziaceae, which is a small, Southern Hemisphere group of monocots (plants like palms or grasses that typically have parallel leaf venation and a single seedling leaf). The Velloziaceae are related to Pandanus, the screwpines, a genus of tropical swamp and coastal trees.
Xerophyta retinervis is poikilohydric, like Myrothamnus, which I wrote about in a previous entry.  That is, it is a “resurrection plant,” which can become completely dehydrated, then soak up water and start growing again when conditions are favorable. In the greenhouse I’ve seen this happen once, accidentally: the leaves curled up and turned yellow, and the plant looked like it was finished, but it perked up again and looked fine a few days after watering.
The desiccation and resurrection process in Xerophyta has been the subject of a certain amount of research, in part because of interest in improving the drought tolerance of crop plants, and it turns out that the leaves actually break down all of their chlorophyll when drying, then rapidly synthesize new photosynthetic pigments when water is available again. In cultivation, it’s probably safest not to test this remarkable ability too much, and keep the plants well-watered during the summer growing period, and barely moist enough to keep the plant from dehydrating in winter. No matter how I treat Xerophyta, it does tend to drop a lot of its foliage in the cooler months.
Tuba, Z. et al. 1994. Planta 192: 414-420.
Clark, W.D. et al. 1993. Annals of the Missouri Botanic Garden 80: 987-998.