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Apical Dominance • Usually the growing terminal bud inhibits the development of the lateral buds, a phenomenon known as apical dominance - as the distance between the shoot tip and lateral buds, the influence of the apical meristem lessens and the lateral buds proceed with their development
Stem Function The two main functions of the stems are conduction and support • Conduction involves moving substances manufactured in the leaves through the phloem to other parts of the plant including developing leaves, stems, roots, developing flowers, seeds and fruits and the xylem carries water from the roots to the leaves, where water is transpired • Support involves holding the plant off the ground - supporting the principal photosynthetic organs of the plant (the leaves) as well as flowers, seeds and fruits
Stem Growth • The organization of the apical meristem of the stem is more complex than is the organization of the apical meristem of the root • The apical meristem adds cells to the plant body and forms leaf primordia and bud primordia that develop into lateral branches • The apical meristem of a stem lacks a protective cover like the root cap of roots
Stem Growth cont’d • Protoderm always originates from the outermost meristem cell layer • Procambium and part of the ground meristem (which will form the cortex and sometimes part of the pith) form from the peripheral meristem • The rest of the ground meristem (which forms some or all of the pith) is formed by the pith meristem
More Stem Growth • Usually the meristematic activity causing the elongation of the internodes is most intense at the base of the developing internodes - if elongation of the internodes occurs over a long period, the meristematic base of the internode may be called an intercalary meristem (a meristematic region between two highly differentiated regions) – intercalary meristems are very important in the growth of grasses and grass-like plants
Three types of primary structure 1. In some conifers and dicots, the narrow, elongated procambial cells (and consequently the primary vascular tissues that develop from them) appear as a more or less continuous hollow cylinder within the ground tissue - the outer region of the ground tissue is called the cortex and the inner region is the pith
Three types of primary structure 2. In other conifers and dicots, the primary vascular tissues develop as a cylinder of discrete strands separated from one another by ground tissue • The ground tissue separating the procambial strands (and later mature vascular bundles) is continuous with cortex and pith and is called the interfascicular parenchyma (between the bundles) • The interfascicular regions are often called pith rays
Three types of primary structure 3. In most monocots and some herbaceous dicots, the arrangement of procambial strands and vascular bundles is more complex - vascular tissues do not appear as a single ring, but instead develop as more than one ring or are scattered throughout the ground tissue - here ground tissue cannot be distinguished as pith and cortex - often called pith
Leaf Traces • At each node, one or more vascular bundles diverge from the cylinder of strands in the stem, cross the cortex and enter the leaf or leaves attached at that node • The extensions from the vascular system in the stem toward the leaves are called the leaf traces • The wide gaps or regions of ground tissue above the level where leaf traces diverge toward the leaves are called leaf gaps
Plant habitat and leaf structure • mesophytes - plants that require abundant soil moisture and a fairly humid environment - the most common plants - typically have fairly well developed epidermis, especially on upper surface of leaf, stomata on both sides of leaf • hydrophyte - plants that depend on a very abundant supply of water or which grow wholly or partially submerged in water - have thin epidermis, stomata only on upper surface • xerophyte - plants adapted to arid habitats - very thick epidermis, stomata open to stomatal crypts with protective hairs
Red Oak Leaves Shade Leaves Sun Leaves
