Seed coat imposed dormancy: Histochemistry of the region controlling onset of water entry into Sida spinosa seeds

Fresh Sida spinosa L. seeds do not imbibe water and germinate because the seed coat is impermeable to water. Seeds imbibe water when a portion of the coat in the chal‐azal area separates from an underlying layer of subpalisade cells. Thin‐walled subpalisade cells are limited to the area beneath the chalazal area. Palisade cells in the coat region that become permeable are tall, only lightly lignified, contain abundant hemicelluiose and have large cell lumena. Palisade cells in regions of the coat that remain impermeable to water are short, have heavily lignified thick walls, small cell lumena and are tightly packed. We propose that Sida spinosa seed coat dormancy is released when moisture eventually reaches the hygroscopic hemicellulose deposits in cell lumena in the lightly lignified palisade cells of the chalazal region. The subsequent expansion of the palisade cells causes the thin‐walled subpalisade cells to break, resulting in separation of palisade from subpalisade cells and free passage of water through the exposed surface to the embryo, culminating in germination.