Topic 14.1 The Structure & Growth of Flowering Plants Biology 1001 November 14, 2005

III. PLANT HORMONES  Hormones are chemical signals that coordinate the parts of an organism A hormone is a molecule produced in one part of the body and transported to another where it bind to a receptor and triggers a response in the target cell or tissue In plants, hormones control growth & development by affecting the division, elongation, & differentiation of cells Plant hormones are small molecules required in very small amounts and they often have multiple effects on the plant Some hormones respond to environmental stimuli Hormones produce their effect by altering gene expression, affecting the activity of enzymes, or changing the properties of membranes

~Note multiple effects and interactions of plant hormones~

AUXIN  The term auxin is used for any chemical substance that promotes the elongation of coleoptiles  Auxins have multiple functions in flowering plants  The auxin that occurs in plants is indoleacetic acid  IAA moves in one direction only in a plant – from shoot tip to base – called polar transport Rate is 10mm/h – faster than diffusion Involves transport proteins concentrated at the basal end of each cell Apical meristems are the major sites of auxin synthesis  A primary role of auxins, at low concentrations, is to stimulate cell elongation in young developing shoots

Cytokinins, Auxin & the Control of Apical Dominance  Cytokinins stimulate cytokinesis (cell division) and influence the pathway of cell differentiation Cytokinins are produced in actively growing tissues Cytokinins achieve their effects in the presence of auxin  Apical dominance is the ability of the terminal bud to suppress growth of the axillary buds  Cytokinins & auxin interact in apical dominance Auxin transported down the shoot from the from terminal bud inhibits growth of axillary buds Cytokinin entering the shoot from the roots counters the effect of auxin by signalling axillary buds to grow Apical dominance determines the shape of the plant

IV. Secondary growth of roots & stems Secondary growth is the increase in girth of the stems & roots of woody plants due to the lateral meristems  The vascular cambium adds secondary xylem (wood) and secondary phloem (part of bark)  The cork cambium replaces the epidermis with a thick protective covering called periderm (cork cambium and cork cells)  Secondary growth occurs in the older parts of the plant at the same time that primary growth continues to elongate the roots and stem

Secondary Growth of a Stem Figure 35.18!!

What Happens During Secondary Growth? The vascular cambium is a cylindrical meristem one cell thick located between the primary xylem & the primary phloem  Forms from parenchyma cells that regain their ability to divide  Vascular cambium cells called fusiform initials produce secondary phloem to the exterior & secondary xylem to the interior  Ray initials produce xylem & phloem rays, radial files of parenchyma cells  The diameter of the vascular cambium increases with secondary growth As the girth of the stem increases, the tissues exterior to the vascular cambium (secondary phloem etc.) rupture and the cork cambium develops from parenchyma cells in the cortex  The cork cambium produces cork cells which replace the epidermis  Cork cells deposit a waxy substance, suberin, in their walls and them die  The cork cambium and cork cells together form the protective periderm  All tissues exterior to the vascular cambium are collectively called bark

The Vascular Cambium and Secondary Growth The cells of the vascular cambium are called initials  Initials divide either transversely to produce new cambium initials or radially to form xylem or phloem cells In each year of growth the vascular cambium lays down a new layer each of secondary xylem and phloem  The secondary xylem layers accumulate and form wood  The secondary xylem cells produced in the spring are larger than the ones produced in the autumn – this causes the annual growth rings of a tree trunk Figure 35.19!! Figure 35.18b!!

Features of an Older Woody Plant Secondary phloem does not accumulate because it continually gets sloughed off as the girth of the tree increases – there is just the most recent layer As old layers of periderm slough off a new cork cambium forms to the inside and produces a new layer of periderm In roots the older woody parts function to anchor the plant and transport water and solutes to the shoot The secondary xylem of the trunk becomes differentiated into heartwood and sapwood – the younger sapwood functions to transport the xylem sap while the heartwood provides support Anatomy of a tree trunk Figure 35.20!!