It has long known that Autumn leaf colour appears when the green chlorophyll pigment is broken down. The yellow carotenoid pigments remain and red antocyanines are formed. This process is steered by genes, some of which make sure that the process starts at the right time while others make sure that chlorophyll and a number of other constituents of the leaf are degraded and others created. Scientists at the Royal Institute of Technology at Stockholm have distinguished some 35 genes in the leaves of an Aspen tree (Populus tremula) that are not expressed anywhere other than in huge quantities in autumn leaves. Trees cannot know what the temperature will be the following day or week, but they are able to sense the length of the nights. When the length of the night exceeds a certain critical value, the genes 'switch-on'. This critical value differs according to tree species; Birches (Betula spp.) from further South are programmed for the autumn to arrive much later. Scientists are now studying these genes to discover when and how these genes are switched on and off in order to identify those trees that have an optimal "calendar" for various climatic zones. Those where leaf colour does not turn too late and thereby loosing nutrients and suffering frost damage, or turn too early thereby stopping growth too early in the autumn. In the future it might also be possible to genetically modify this calendar extending the geographical distribution of diverse species. Karin Wilkman, Umea University E-mail: karin.wikman@adm.umu.se www.AlphaGalileo.org