primarystem213-page2 - for optimal access to pollinators...

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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 Functions The stem is the axis of the shoot system that provides mechanical support for and serves as the attachment site for leaves and reproductive shoots. Stems elevate leaves for photosynthesis and position reproductive shoots
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Unformatted text preview: for optimal access to pollinators and dispersal agents. • Stems conduct water and minerals from roots to the leaves and conduct solutes from leaves to storage and use sites. • Stems are responsible for the overall growth (height and girth) of the plant from the primary and secondary shoot meristems There is much variation in stem types and stem anatomy, although there are some common features, which we will discuss. Most secondary growth of plant shoot systems (increase in girth) occurs in stems. (Leaves are generally primary growth structures.) Our discussion of stems will include both internal anatomy and external features of primary and secondary growth in stems. Primary Growth of Stems There are three basic primary growth patterns in stems: • Early growth of most herbaceous eudicot stems and some woody eudicots • Early growth of some woody eudicots • Monocot stems Most woody eudicots Herbaceous eudicots Monocots...
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