Separation of the outer bract enclosing the flower bud and spike axis results in the stimulation of amylase synthesis in the petals of gladiolus. Light is implicated in this process. Amylase production is localised in the petal epidermis (which is free of starch) and not generalised in the ground parenchyma (which contains abundant starch). Depletion of starch in the petal is strictly basipetal. It is proposed that the reducing sugars released from hydrolysis of starch in the ground parenchyma cause osmotic water intake, leading to petal expansion and flower opening.