The 'petals' of the strawflower are composed of phyllaries or involucral bracts; these are typical of the Asteraceae. In most members of the family, the phyllaries are green and range from few to many. Phyllaries can be seen on the underside of a dandelion and the back side of a sunflower. The involucral bracts are highly modified in strawflowers—enlarged, colored, papery, and dead at maturity.

Once the phyllaries of strawflowers form, the cells die. But the bracts still function to protect the pollen and stigmas of the disk florets. In the strawflower's case, the operation, called hygrochastic movement, is entirely a physical effect—a function of the differential between thick cell walls on the upper side of the phyllary and thin cell walls on the lower side. The thin walls allow the easy absorption of moisture; the dead thin-walled cells expand and contract with varying amounts of humidity or rain.

In the early morning, strawflowers are usually closed. The thin-walls dry, contract, and the inflorescences open as the relative humidity decreases with increasing warmth from sunlight. When rain threatens, the thin-walled cells moisten, expand, and the bracts close over the disk florets.

Hygrochastic movement happens fast enough to watch. Spray an open strawflower with water. Within seconds, the phyllaries begin curling upward until the inflorescence forms a tight ball.

(Compiled from: "Helichrysum", Hortus Third, Staff L.H. Bailey Hortorium, NY State College of Agriculture and Life Sciences, Cornell University, Macmillan, NY, 1976; "Xerochrysum subundulatum", Association of Societies for Growing Australian Plants, E.H. Graham Centre for Agricultural Innovation, Charles Sturt University and NSW Department of Primary Industries, 2004; and "Hygrochastic Movement in Floral Bracts of Ammobium, Acroclinium, Rhodanthe, and Helichrysum", J.C.Th. Uphof, American Journal of Botany, Vol II, No.3, 1924)