STUDIES OF THE DEVELOPMENT OF ZOSTERA MARINA L.: I. THE EMBRYO AND SEED

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Abstract
Development of Zostera marina L. is described from proembryo to mature seed. The single cotyledon and the stem apex develop laterally from the nearly spherical young embryo. No radicle is produced; the hypocotyl develops into a swollen basal part (the original embryonic mass) and an elongated axial part formed by cell divisions at the base of the cotyledon and plumule. The cotyledon grows in part by cell divisions at its apex. Curvature develops in the axial hypocotyl mainly by unequal elongation of cells, whereas curvature of the cotyledon is mainly by unequal increase in cell numbers. A single procambial strand differentiates from the suspensor end of the basal part of the hypocotyl through the axial part into the cotyledon and plumule. The plumular stem apex develops a single-layered tunica over a several-layered corpus. It produces three leaf primordia, one with air-spaces organized, before dormancy sets in. Mid-vein procambium differentiates acropetally from the axial strand into each leaf primordium; lateral procambial strands differentiate acropetally and basipetally from near the base in each of the first two primordia. Two adventitious root apices differentiate external to the axial procambium at the base of the cotyledon. Seed coat development is described briefly. The nucellus remains a non-vascularized column of cells extending upward from the highly vascularized attachment region of the seed. The patterns of development reported support the interpretation that the starchy, shield-shaped main part of the embryo is the basal part of the hypocotyl and that the first leaflike structure is the cotyledon.
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