The study of the population dynamics of annual and biennial plants reveals that many species of these groups do not behave as their classification suggests. Some species that are listed as monocarpic e.g. Senecio jacobaea may be polycarpic depending on environmental conditions. Furthermore, strict biennial behaviour appears to be limited to a few species (e.g. Gentianella germanica, Linum catharticum, Melilotus alba and Pedicularis paluslris). Delayed flowering is common amongst many species (table 3); it may occur if plants fail to attain a certain minimal or critical size before the winter (the period of vernalisation). The fraction of rosettes that reaches the critical size and flowers may vary between years. An example is given for Cynoglossum officinale in the coastal dune area of Meyendel (the Netherlands; fig. 2): much rainfall during the growing season is followed by a high fraction of flowering in the next year. Only under favourable conditions biennials may flower in the second year. Following Bakker c.s. (1966) a classification of annuals and biennials is proposed in which frequency of reproduction per individual, size requirement for reproduction and vernalisation are keywords. In table 3 a list of species for which such information could be found is provided.