Seeds ofZostera marinaL. are shed in Prince Edward Island waters during mid to late summer and germinate the following spring. Occasionally seeds germinate in nature before winter; they can be germinated during autumn and winter in the laboratory. Elongation of cells of the cotyledon and doubled-back axial hypocotyl pushes the embryo out of the seed coat. Cell increase plays no part in this or subsequent elongation of cotyledon and hypocotyl. Leaves emerge through the slit-like opening at the top of the cotyledonary sheath. Hairs develop from cells of the convex surface of the basal swollen hypocotyl; two adventitious roots grow from the cotyledonary node, develop root hairs, and anchor the seedling. Stelar tissues of the hypocotyl and cotyledon differentiate at germination. No characteristically thickened tracheary elements differentiate in hypocotyl or cotyledon. In the hypocotyl, phloem differentiates around the central xylem parenchyma and abuts externally on endodermis which develops Casparian strips. Xylem parenchyma differentiates adaxially against the phloem in the single cotyledonary strand. Differentiation of procambium and vascular tissues in the leaf primordia follows as these expand. Vascularization of the stem nodes is complicated by differentiation of strands to the roots and leaves. All internodes except the first have one pair of cortical bundles. The pair of lateral veins of the first true leaf differentiate in connection with the stele at that node; the lateral bundles from each later-formed leaf pass through one internode as cortical bundles and unite with the stele in the node below.
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