Topic 14.1 The Structure & Growth of Flowering Plants Biology 1001 November 9, 2005 II. Plant Growth and Organization of Primary PLANT Tissues An Overview of Plant Growth Plants exhibit indeterminate growth Growth continues throughout the plant’s life Most plants grow continuously A typical plant consists of embryonic, developing, and mature organs Leaf growth is determinate When do plants die? Annuals = (eg. Wildflowers and grains) complete a life cycle in a year Biennials = (eg. Beets and carrots) live two years, usually with an intervening cold period Perennials = (eg. Trees, shrubs and some grasses) live many years An Overview of Plant Growth Primary and Secondary Growth Primary growth produces the primary plant body – the whole herbaceous plant, or the youngest parts of a woody plant Secondary growth produces the secondary plant body – the “woody” parts of the stems and roots of a woody plant (leaves do not generally experience secondary growth) Involves the apical meristems located at the tips of roots and in the buds of shoots and lengthens the roots and shoots Involves the lateral meristems, the vascular cambium and the cork cambium, and adds girth to the plant Primary & secondary growth occur at the same time in different parts of the plant Figure 35.10!! Primary Growth of Roots Growth of the root occurs just behind the root cap, where new cells are produced in the apical meristem The root cap protects the root as it pushes through the soil; it also secretes a lubricating polysaccharide slime Growth occurs in three zones of cells at successive stages of primary growth The zone of cell division includes the apical meristem and its derivatives In the zone of elongation root cells elongate up to 10X their length In the zone of maturation cells complete their differentiation and become functionally mature Figure 35.12!! Organization of Primary Tissues in Young Roots Primary growth produces the epidermis, ground tissue & vascular tissue of the young root The vascular tissue (xylem & phloem), collectively called the stele, consists of a lobed core of xylem with phloem between the lobes in dicots In monocots the stele has a central core of parenchyma cells and alternating rings of xylem and phloem The ground tissue is called cortex The innermost layer of the cortex is the endodermis The outermost layer of the stele is the pericycle, from which lateral roots arise Figure 35.13!! Primary Growth of Shoots A shoot apical meristem is a dome-shaped mass of dividing cells at the tip of the terminal bud Leaves arise as leaf primordia from the sides of the apical meristem Axillary buds develop from clumps of meristematic cells left by the apical meristem at the bases of the leaf primordia Growth and differentiation occur as cells divide & elongate at older internodes below the shoot apex Figure 35.15!! Tissue Organization of Stems and Leaves Stems The epidermis covers the stem as part of the dermal tissue system The vascular tissue runs the length of the stem in vascular bundles, arranged in a ring in a dicot with xylem facing inward, phloem outward The ground tissue is arranged as pith & cortex (internal and external to the vascular tissue) in the dicot, and consists of mostly parenchyma cells, with some collenchyma and/or sclerenchyma Leaves The epidermis is interrupted by stomata for CO2 and H2O exchange; stomata are pores flanked by two guard cells, which regulate the opening and closing of each stoma Ground tissue in the leaf is called mesophyll (palisade & spongy) and consists of photosynthetic parenchyma cells Veins are the leaf’s vascular bundles; they are connected to the stem by leaf traces, and each is enclosed by a bundle sheath. They network throughout the leaf to bring the xylem & phloem into contact with all the ground tissue Primary Tissue Organization of Stems Figure 35.16a!! Tissue Organization of Leaves Figure 35.17!!