THE MECHANISM OF ROOT CONTRACTION IN GYMNARRHENA MICRANATHA, A DESERT PLANT

The taproot of the ephemeric desert annual Gymnarrhena micranatha (Asteraceae) starts to contract soon after emergence and continues to do so at a constant rate over a period of 50 days or longer. This causes retraction of the main shoot apex from the soil surface to a depth of about 10 mm. Contraction takes place in cells of the endodermis, pericycle and primary phloem which expand radially while contracting longitudinally to about one‐half their initial length. This results in a wavy pattern in their radial walls. Partial hydrolysis of the cell wall is probably associated with secretion of multivesicular bodies from the cytoplasm. Contracted cell walls appear wavy where dissolution is incomplete or straight with undulated microfibrils where it is more complete.