One of the world's oldest and rarest trees, a desert species of which just 231 are left, is able to use a "surrogate mother" from another species in its drive to survive, French scientists report.The Saharan cypress, specimens of which have lived up to 4,700 years, was once plentiful in the Sahara, but has now dwindled to near extinction, with just a small group of ageing trees left in the desert's Ajjer plateau.