primarystem213 - The Shoot System: Primary Stem Structure -...

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The Shoot System: Primary Stem Structure - 1 Shoot System The shoot system comprises the leaves and stems of plants. Leaves are located at nodes on the stem; the distance along the stem between nodes is known as an internode. Shoots develop from shoot meristem, which contains the apical shoot tip meristem from the epicotyl of the embryo, leaf primordia, and bud primordia, which are embryonic lateral shoot systems. Flowers, the reproductive organs of angiosperms, are modified shoots. Shoot meristems are located at the growing tips of stems, and in buds. Shoot Meristems The apical meristem is divided into two regions: tunica and corpus. The tunica meristem cells divided in a plane the produces additional surface meristem. The corpus meristem cells under the tunica divide in a plane that adds bulk to the shoot meristem. Generally there are two tunica layers and one corpus layer. As a shoot grows, buds are laid down by shoot meristem in the axils of leaf primordia. The repeating units of leaf and bud primordia are called phytomeres. These buds are dormant meristems that are activated at some later time in growth. Stem tissues are produced from the same three derivative meristems as root tissues are: Protoderm is responsible for Epidermis Ground Meristem differentiates into ground tissues Procambium produces the vascular tissues Coleus stem tip, l.s. Shoot tip, xs SEM of shoot tip
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The Shoot System: Primary Stem Structure - 2 The arrangement of derivative meristems is different in the stem from the root meristem zone. In the shoot meristem, procambium forms a cylinder of cells with ground meristem to the interior and to the exterior of the procambium cylinder. As expected, the protoderm is the outer layer of derivative meristem. Single central strands of procambium extend out into the leaf primordia from the shoot meristem. These are called leaf traces, and leave a vascular gap in the stem tissue at those points. The positioning of new leaf primordia, and hence branches, is regulated by the inhibition effect of existing primordia so that leaf and branching patterns generally spiral along the stem. Growth regulators in the shoot tip and leaf primordia are responsible for the inhibition (see later). Stem
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primarystem213 - The Shoot System: Primary Stem Structure -...

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