Vegetative organs - roots.pptx

Vegetative organs - roots • Accounts •4 for ~50% of plant biomass. Basic functions (with many modifications) Anchorage • Absorption • Conduction of materials through vascular tissue • Storage of carbohydrates, sugars, etc. • Root Hairs • The functional water-absorbing part of the root. Water...

Vegetative organs - roots • Accounts •4 for ~50% of plant biomass. Basic functions (with many modifications) Anchorage • Absorption • Conduction of materials through vascular tissue • Storage of carbohydrates, sugars, etc. • Root Hairs • The functional water-absorbing part of the root. Water is ONLY absorbed by the root hairs • Root hairs are just long extensions of the cell wall/membrane of the root dermal cells. • • Responsible for generating root pressure • One of the mechanisms by which water is pumped to the rest of the plant. • Actively pump soil ions across cell membrane • Osmotic potential changes, water follows. Root hairs Longitudinal structure of roots • Divided into zones • Beginning • from root tip: Root cap (c in the diagram) • • • Originate as living cells in the Zone of Cell Division (see below), but lignify and die as they specialize toward the tip of the root. Provides protection from friction as the root grows downward through abrasive soil. Responsible for gravitropism (the downward growth of roots) • Root cap has cells known as statocytes, containing modified leucoplasts called statoliths. These respond to specific hormonal signals and regulate the expansion of cells on one side or another of the roots to ensure that they keep growing downward. • More on this mechanism later in the course. Longitudinal structure of roots • Zone • This • of Cell Division (a in the diagram) is a meristem Cells are actively dividing here, and specializing into other cells as they grow away from it (vascular tissue, root cap, parenchyma, dermal tissue, etc.) a Longitudinal structure of roots • Zone of Elongation (b in the diagram) No cell division here • Only CELL GROWTH • Cells get larger • The growth of these cells is the mechanism that pushes the root tip through the soil. • b a Longitudinal structure of roots • Zone of Maturation (d in the diagram) • Defined • by the presence of root hairs. When you see the first root hair, you are in the Zone of Maturation d • DOES NOT keep going all the way up the root. Root hairs peter out after a couple of inches. • Above that is the final “zone” of the root. b a Branch roots Pericycle • Root systems are highly branched with large numbers of multicellular roots (each with root hairs, zones of cell division, elongation, maturation, etc.) • These branch roots originate above the zone of maturation. • Branch roots originate deep in the center of the root (at a structure called the pericycle) and grow outward penetrating the tissues of the existing root until they emerge. Roots – dicots vs. monocots Transverse root structure • Monocots and dicots have differences in structure, but do the same things • Roots are structured like a series of concentric circles in cross section (transverse section) • Outermost is the epidermis – Outer protective covering • Next inward is the cortex – mostly storage parenchyma Transverse root structure • Endodermis located inside cortex •A valve system that allows water inside the vascular cylinder (everything from the endodermis to the center of the root) but NOT back out. • Water travels through the root in 2 ways Via apoplast – Basically soaking through the cell walls without ever entering the cells themselves – passive transport. • Via symplast – Active transport into the living cells, across the cytoplasm, and out the other side. • • Accomplished by enzymes – aquaporins • Costs energy and can be regulated. Water movement via apoplast and symplast • Apoplast • Symplast – red arrow – blue arrow • This movement proceeds as diagrammed UNTIL water hits the endodermis. • Endodermis wall is lined transversely and radially with casparian strips. • • Waxy seals that prevent apoplastic movement. Water movement into vascular cylinder is SYMPLASTIC only. • And can thus be regulated. Transverse root structure • Pericycle is inside the endodermis. • Meristematic layer responsible for development of branch roots. Originate here, then penetrate through the endodermis, cortex, and epidermis. • Ensures continuity of the vascular tissue. • Transverse root structure • Dicot • Also (eudicot) vascular cylinder called a stele. Usually defined by “arms” of xylem arranged in a cross shape. • Phloem is the tissue between each xylem arm. • xylem phloem Transverse root structure • Monocot vascular cylinder • Usually defined by a ring of xylem (surrounded by phloem) surrounding a central pith. • Pith is a central support structure that can be made of many things • Hard, lignified tissue or open spaces or chambered spaces or wispy, cottonlike fluff, etc. structural differences, but the vascular tissue acts exactly the same way as in eudicots. xylem • Minor phloem Variations on a theme - Taproots • One of the major types of roots Specialized for anchorage, nutrient storage. • Consist of a large central storage root with many branch roots. • Many herbaceous vegetable plants (carrots, etc.) and most woody plants have taproots. • Fibrous roots • Large • surface area, no main taproot – stabilize soil effectively. Most herbaceous plants (and all grasses - Family Poaceae) Adventitious roots • Roots arise out of any non-root tissue such as stems, leaves, etc. Prop roots of corn Modifications • Storage • Prop (taproots) roots – corn, tall grasses Cypress “knees” • Pneumatophores – roots that allow passage of oxygen into plant tissues in anoxic environments (such as wetlands). Mangroves, baldcypress “knees”, etc. • Clinging roots – English ivy, etc. • Aerial roots – absorb water from the air through a modified epidermis called “velamen” • Haustorial roots – some plants parasitize other plants – penetrate the host roots with modified haustorial roots. Orchids with velamen-covered aerial roots Haustorial roots of mistletoe (Phoradendron sp. – Family Santalaceae)