Mistletoes of Africa

  • Roger Polhill &
  • Delbert Wiens
Royal Botanic Gardens, Kew: 1998. 370pp. £70
Decoratively minimal: Viscum minimum is fruit and little else.

Mistletoes are rootless plants that plug into the ‘plumbing’ of their host plant's stem to steal water, dissolved minerals and, to varying degrees, organic products. They are a unique yet global group, full of fascinating attributes. But until now there has been no comprehensive book on the African species. Roger Polhill and Delbert Wiens' Mistletoes of Africa gives an up-to-date summary of mistletoe biology, keys to the taxa and some biogeography, as well as describing their haustoria (by which they attach to the host) and flower structure in relation to opening, and providing appetizers for further work, for which they note a strong need.

Mistletoes may be in danger of extinction for several reasons. They are probably prone to excessive herbivory by large African animals like elephants. It has even been suggested that the absence of large animals, such as giraffe, in some reserves leads to a build up of mistletoes on their preferred hosts, thorny acacias. This increase in mistletoes attracts fruit-eating birds which in turn produces an influx of seeds of fleshy-fruited, broad-leaf species that become established beneath the acacia. Eventually, acacia savanna is replaced by dense, broad-leaf woodlands, the phenomenon of bush encroachment. This is perhaps an exaggeration but certainly more work is needed on the relationships between large African mammals and mistletoes.

Some mistletoes may be vulnerable because of their obligate relationships with mutualists. Dioecious fleshy-fruited species, like Viscum continuum, depend on animals for both pollination and dispersal. People are also potentially disastrous for mistletoes. ‘Wood roses’, shaped from haustoria of savanna mistletoes, are common in the curio markets of South Africa. They are carved from mistletoes that can be more than 50 years old; excessive use may not be sustainable.

A largely African aspect of mistletoe ecology is their occurrence on succulents, where some ramify within the tissues of their hosts. They reach their most advanced and reduced stage in V. minimum of the eastern Cape, a really weird plant. Its shoot (less than 3 millimetres long, see below) is far smaller than the berry it produces; in fact, the whole plant looks like its host plant's fruit.

Non-endophytic mistletoes are almost globally restricted to arid regions of Africa and their physiology is intriguing. We do not know whether they use the stored water or the xylem stream of their host plant. Mistletoes open their stomata and fix carbon during the day; succulents, on the other hand, open their stomata at night — an uneasy relationship. Further physiological pieces of this fascinating jigsaw are needed.

Host relations also need research. Polhill and Wiens suggest, for example, that host specialization is more frequent when potential host diversity is low. Some species are even restricted to parasitizing other mistletoes (for example, the epiparasitism of V. goetzii and V. loranthicola). Others are single dots on the African map, such as V. grandicaule, the largest Viscum, which only grows on fig trees on the island of Pagalu, off Gabon.

African mistletoes are similar to those found elsewhere in the world as well as having many unique characteristics. With the publication of this book, work on African mistletoes can now safely enter an “after-description phase”.