A Review of the Genetics of Betula

The present knowledge on birch genetics is reviewed as regards the fields of cytology, crossability within and between species, flowering, plant growth rhythm, tree growth and quality, and hybridization. Incompatibility occurs but can be partly overcome by crossing at low temperature. Species hybridization was more successful when the female parent was of low ploidy. Crosses within one subsection are not more successful than crosses between subsections. Earliness of male and female flowering can be improved by selection. Flowering is probably polygenically inherited. Clinal variation in north-south direction occurs for growth rhythm characters, the northern provenances show the earliest growth cessation. Long-distance transfers northwards or southwards result in reduced growth. The ratio between the family and the population variance components varies with type of character studied and the geographic range of the material studied. Long-distance crosses within a species or between species do not seem to lead to any hybrid vigour. Definite conclusions must await more comprehensive crossing programs.