Phloem differentiation: induced stimulation by gibberellic Acid.

Gibberellic acid solutions supplied through micropipettes to explants from dormant branches of white pine (Pinus strobus L.) stimulate changes in the cambial zone. Immature sieve cells expand radially and exhibit cytological changes usually associated with spring maturation. Diflerentiation of sieve cells continues in response to treatment and is recognized by the birefringence of secondary walls when examined with the polarizing microscope.