1. Ager, A., A.C. Rönnberg-Wästljung, J. Thorsén and G. Sirén, 1986. Genetic improvement of willows for energy forestry in Sweden. Swed. Univ. Agr. Sci., Dept. of Ecology and Environmental Research. The Energy Forestry Project, Report No. 43.
2. Ahuja, M.R., 1987. In vitro propagation of poplar and aspen. In: J.M. Bonga and D.J. Durzan (Eds.), Cell and Tissue Culture in Forestry, Vol. 3, pp. 207–223. Martinus Nijhoff, Dordrecht.
3. Ammirato, P.V., 1983. Embryogenesis. In: D.A. Evans, W.R. Sharp, P.V. Ammirato and Y. Yamada (Ed.), Handbook of Plant Cell Culture, Vol. 1, pp. 82–123. MacMillan Publishing Co., New York/London.
4. Ammirato, P.V., 1987. Organizational events during somatic embryogenesis. In: C.E. Green,D.A. Somers, W.P. Hacket and D.D. Biesboer (Eds.), Plant Tissue and Cell Culture, pp. 57–81. Alan R. Liss, New York.
5. Arnold, S. von and I. Hakman, 1988. Regulation of somatic embryo development in Picea abies by abscisic acid (ABA). J. Plant Physiol. 132: 164–169.