DEVELOPMENT, STRUCTURE AND FUNCTION OF SUBPALISADE CELLS IN WATER IMPERMEABLE SIDA SPINOSA SEEDS

Newly matured prickly sida ( Sida spinosa ) seeds were hard, but afterripening, heat or pressure permitted water entry solely via a predefined region of the chalazal area. In this region, the raising of a “blister” formed by separation of the palisade of the seed coat from underlying tissues preceeded measurable water uptake by prickly sida seeds. A single subpalisade layer, unique to the region beneath the blister, was involved in the sequence of events in seed water uptake. The lateral walls of subpalisade cells invariably broke near the palisade border and cell contents were extruded onto the surface. This report describes the cytological development of the subpalisade layer from 1‐21 days post anthesis. Cells beneath the potential “blister” near the chalazal slit developed into columnar subpalisades and cells beneath the subpalisades or beyond the margins of the potential blister, developed into oval, thick‐walled chalazal cap cells. By 6‐10 days, distinct features of the subpalisades included: 1) thin portions in lateral walls due to lack of secondary wall depositions at the palisade border; 2) progressive accumulation of fibrous material in numerous vacuoles; and 3) progressive coalescence of osmiophilic bodies and degeneration of cytoplasmic contents. At 21 days, the seeds were dehydrated, mature and hard, but the thin, lateral subpalisade walls were still intact and had not broken. The thin‐walled portions were predetermined weak sites that break, permit palisade separation, expose the area under the blister to available moisture and result in subsequent imbibition of water by the seed. The hydrophilic, fibrous material extruded from the ruptured subpalisade cells may attract water to the newly exposed surface and facilitate penetration of water into the nutritive and embryonic seed tissues.