Article

Allochthonous ecosystems occur under extreme conditions which do not allow the growth of green plants. They are lacking in producers, in diaspore banks, and in herbivores. Allochthonous ecosystems are bes t known from extreme deserts. Two examples from the E Sahara (SW Egypt) are discussed. Here the completely vegetationless area is surrounded by a belt of accidental vegetation (in the sense of Kassas 1952) which is almost exclusively dependent on rainfall and duration of water resources. Since precipitations events do not take piace annually but in much longer periods (if any) an annual rhythm does not exist. The life form of a typical member of accidental vegetation is neither annual nor therophytic, and it only facultatively carri es out a perenniallife cycle. For this life form the new term "poikilorhythmic" is proposed.