Temperature and dates of male meiosis in trees

A review of methods is given, including conventional and non‐linear heat sum methods, statistical considerations and benefit of graphic methods. Actual dates in 1968‐71 of onset of meiotic stages of 11 tree species growing in natural conditions at Punkaharju and Rovaniemi, Finland, are presented. It is shown that no abrupt thermal threshold is necessary for the start of meiosis in the spring. The dormancy period has a great synchronizing effect on the progress of Larix meioses when different species or individuals of the same species are compared. It is suggested that the halt in the development of a plant at low temperatures, e.g., during meiosis, is caused by drying up of cell (PMC) protoplasts due to onset of freezing. Premises for finding the zero point of generative development after winter dormancy are evaluated. Disturbances in the flowering cycle of trees are discussed. Q 10 factors are shown to be of little avail as delineators of the meiotic development.