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3. HD Behnke.In:DA Baker, JA Milburn, eds. Transport of Photoassimilates. Harlow, Essex: Loughman Scientific and Technical, 1989, pp79-137.
4. K Esau. Anatomy of Seed Plants. New York: Wiley, 1977, pp321-322.
5. Ecophysiology of phloem loading in source leaves