Today's Botany Photo of the Day comes once again from the album of Ian Gillam, one of our Friends of the Garden. Our thanks go to Ian for another great image and write-up.

Umbilicus rupestris, photographed this past June in the E.H. Lohbrunner Alpine Garden, is a short-lived perennial member of the Crassula family (Crassulaceae). The family thrives in dry environments that range from frank desert to rock faces, and from gravel slopes to walls and even rooftops. It is represented most plentifully in southern Africa, though several members grow in temperate regions as well. Umbilicus rupestris occurs across southwestern Europe in suitable locations, which are usually dry (though not excessively so). The fleshy leaves are circular, about 2 cm. in diameter, and they are attached to the stem at the centre (i.e., peltate). A depression there suggests the name, Umbilicus, which means 'navel'. The flowering spikes grow to about 15 cm. and set copious seed, after which point the plants tend to die. As indicated by the name Crassula (coming from the diminutive of the Latin word for thick or fat), family members typically have thickened leaves that reduce water loss by decreasing surface area to volume ratio, and they often have waxy cuticles with sunken pores that further reduce water loss. Detached stems and leaves are resistant to wilting, and, once fallen, they are capable of rooting and developing into new plants.

Green plants use the energy of sunlight to fix atmospheric carbon dioxide in the production of sugars; thence all of their other organic components. In order to reach the cellular sites of this process, air must be admitted into the spaces within the leaf through pores that open or close on demand. In dry environments this exchange of air during the brightest and hottest part of the day too readily leads to dehydration. Members of Crassulaceae exploit a variant procedure, opening their pores during the cooler conditions of night. Carbon dioxide is then captured in organic acids of low molecular weight (at the cost of some stored energy). When sunlight returns, the pores close and the acids are degraded to release carbon dioxide in close proximity to the fixation apparatus. The process was first elucidated in this plant family and is therefore called Crassulacean Acid Metabolism (CAM). It permits photosynthesis to proceed, though at some cost in efficiency, while simultaneously minimizing water loss. CAM has evolved independently in a number of other families, giving advantages in growth under conditions of water stress.