Plant Form and function Chapter 33

Organization of a Vascular Plant Apical meristem Primary growth zone Lateral meristems Internode Vascular system Pith Lateral root Root Primary root Node Axilary bud petiole Blade Leaf Shoot Terminal bud Vein A vascular plant is organized along a vertical axis. The root penetrates the soil and absorbs water and ions and it anchors the plant. The shoot consists of the stem and leaves. The stem serves as a framework for positioning the leaves. The leaves are where most photosynthesis takes place.

Organization of a Vascular Plant Plants contain growth zones of unspecialized cells called meristems. Meristems are not only areas of actively dividing cells that result in plant growth, but also continuously replenish themselves. In this way, meristem cells function much like stem cells in animals.

Organization of a Vascular Plant Primary growth is initiated at the tips (of roots and shoots) by the apical meristems. The growth of these meristems results primarily in the extension of the plant body. Secondary growth involves the activity of the lateral meristems. The continued divisions of their cells results primarily in the thickening of the plant body.

Organization of a Vascular Plant There are two kinds of lateral meristems: Vascular cambium - gives rise to thick accumulations of secondary xylem and phloem. Cork cambium - forms the outer layers of bark on both roots and shoots.

Plant Tissue Types Most plants have three tissue types: Ground tissue - in which the vascular tissue is embedded.

Plant Tissue Types Dermal tissue - the outer protective covering of the plant. Flattened epidermal cells are the most abundant cells in the plant’s outer layer, or epidermis. The epidermis is often covered by a waxy layer called the cuticle. The epidermis and cuticle protect the plant and provide an effective barrier against water loss.

Root hairs Root hairs Trichomes (a) Stomata (c) (b) 186 µm Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Root hairs Trichomes 186 µm (a) Trichome Epidermal cells Stomatal opening Guard cells Root hairs Stomata 137 µm (c) (b) (top): © Andrew Syred/Science Photo Library/Photo Reseachers;(middle): © Dr. Jeremy Burgess/Science Photo Library/Photo Researchers; (bottom): © Dennis Drenner/Visuals Unlimited, Inc.

Courtesy of Wilfred Cote, SUNY College of Environmental Forestry Plant Tissue Types Vascular tissue - conducts water and dissolved minerals up the plant and conducts the products of photosynthesis throughout. There are two types of vascular tissue: Xylem is the plant’s principal water-conducting tissue. It forms a continuous system that runs throughout the plant body. Water (and dissolved minerals) pass from the roots to the shoots. When water reaches the leaves, most exits through the stomata. Phloem is the principal food-conducting tissue. Xylem (a) (b) (c) Tracheid Vessel element Tracheids Pores Pits Courtesy of Wilfred Cote, SUNY College of Environmental Forestry

Roots Roots have a central column of xylem with radiating arms. H2O Pericycle Primary xylem phloem Endodermis Casparian strip Sectioned endodermal cells Epidermis Cortex Roots have a central column of xylem with radiating arms. Alternating within the radiating arms of xylem are strands of primary phloem.

Roots The root elongates rapidly just behind its tip in the area known as the zone of elongation. Abundant root hairs, extensions of single epidermal cells, form above the elongation zone. This area is called the zone of differentiation. Epidermis Ground meristem Procambium Protoderm Apical Root cap (b) Monocot (a) Dicot Apical meristem Zone of elongation Cortex Phloem Xylem Roothair Pericycle Endodermis Primary phloem Primary xylem differentiation

Roots Roots branching is initiated as a result of cell divisions in the pericycle. The developing lateral roots grow out of the cortex toward the surface of the root. Developing lateral root Pericycle Cortex Epidermis Endodermis

Stems Stems often experience both primary and secondary growth. Stems are the source of an economically important product—wood. In the primary growth of a shoot, leaves first appear as leaf primordia. These are rudimentary leaves that cluster around the apical meristem. They unfold and grow as the stem elongates.

Stems The places on the stem where leaves form are called nodes. The portions of the stem between these leaf attachment points are called internodes. As the leaves expand to maturity, a bud develops in the angle between the leaf and the stem from which it arises. This area is called the axil. Terminal bud Axillarybud arising from theaxil Node Internode budscale scars Blade Petiole

Stems Epidermis (outerlayer) Collenchyma (layers below epidermis) Pith Vascular bundle Xylem Phloem Cortex Ground tissue Vascular bundles (a) (b) Within soft, young stems, the vascular tissue strands are arranged differently in dicots versus monocots. In dicots, vascular bundles (containing primary xylem and primary phloem) are arranged around the outside of the stem. In monocots, vascular bundles are scattered throughout the stem.

Stems In stems, secondary growth is initiated by the differentiation of the vascular cambium. This is a thin layer of actively dividing cells located between the bark and the main stem in woody plants, running between the xylem and the phloem. Cells that divide from the vascular cambium outwardly become secondary phloem. Cells that divide from the vascular cambium inwardly become secondary xylem.

Vascular cambium and secondary growth Primary xylem Secondary xylem Primary phloem Secondary phloem Annual growth layers Cork Periderm Cork cambium Vascular cambium Secondary phloem

Stems While the vascular cambium is being established, a second kind of lateral cambium develops in the stem’s outer layer. The cork cambium consists of plates of dividing cells that move deeper and deeper into the stem as they divide. Outwardly, this cambium divides to form densely packed cork cells. Inwardly, this cambium divides to produce a layer of parenchyma (ground) cells.

Stems The term bark refers to all of the tissues of a mature stem or root outside of the vascular cambium. Wood is accumulated secondary xylem.

Stems Because of the way it is accumulated, wood often displays rings. The vascular cambium divides more actively in the spring and the summer than in the fall and winter. The growth rate differences are reflected in alternating rings of growth of different thickness.

Leaves Leaves are usually the most prominent shoot organ and are structurally diverse. Growth occurs by means of marginal meristems. The marginal meristems grow outward and ultimately form the blade (the flattened portion) of the leaf.

Leaves Leaf blades come in a variety of forms: Simple leaves have a single, undivided blade. Compound leaves have a blade divided into leaflets. Pinnately compound describes leaflets that are arranged in pairs along a central axis. Palmately compound describes leaflets that radiate out from a common point at the blade end of the petiole.

Leaves Veins, comprised of xylem and phloem, run through the leaf. In most dicots, the veins have a net or reticulate venation. In most monocots, the veins are parallel.

Leaves Leaves can be arranged in different patterns: Alternate leaves spiral around a shoot. Opposite leaves occur on opposite sides of a shoot. Whorled leaves circle the stem as a group. Alternate (spiral): Ivy Opposite: Periwinkle Whorled: Sweet woodruff

Leaves A typical leaf contains masses of parenchyma, called mesophyll, through which the vascular bundles, or veins, run. Vein Air spaces Stoma Guard cell Upper epidermis Palisade mesophyll Spongy Cuticle Lower

Water Movement Vascular plants have conducting systems for transporting fluids and nutrients throughout the plant. Water and minerals enter a plant through the roots and are transported in the xylem. Carbohydrates synthesized by photosynthesis are transported throughout the plant in the phloem. H2O H2O and minerals Phloem Xylem Water and carbohydrates travel to all parts of the plant. and minerals Carbohydrates Water exits the plant through stomata in leaves. Stoma vapor Water enters the plant through the roots. Spongy mesophyll layer Water and minerals pass up through xylem.

Water Movement Several factors are at work to move water up the height of a plant. Osmosis: Water moves into the cells of the root because the fluid in the xylem contains more solutes than the surroundings. This osmotic force is called root pressure but, by itself, is not sufficient to “push” water up a plant’s stem.

Water Movement In addition to root pressure, capillary action adds “pull” to the movement of water up a plant stem. Capillary action results from the tiny electrical attractions of polar water molecules to surfaces that carry electrical charge. This attraction is called adhesion. But capillary action, by itself, is not strong enough to “pull” water up the plant stem.

Water Movement A final “pull” to the process of moving water up a plant shoot is provided by transpiration. Water evaporating from the top (leaf) of the tube pulls the column of water from the bottom (root). The column of water does not collapse because water molecules are attracted to each other. This process is called cohesion. The narrower the diameter of the tube, the more tensile strength, or resistance to separation, of the water column.

Water Movement The combination of gravity, tensile strength, and cohesion affects water movement. The whole process is explained by the cohesion-adhesion-tension theory.

Water Movement Transpiration is the process by which water leaves a plant. More than 90% of the water taken in by a plant is lost to the atmosphere, mostly through the leaves. Water first passes into the pockets of air in the spongy mesophyll and then evaporates through the stomata. High humidity and low temperatures increase transpiration rates. H2O 1 2 3 Dry air passes across the leaves and causes water vapor to evaporate out of the stomata. The loss of water from the leaves creates a type of “suction” that draws water up the stem through the xylem. New water enters the plant through the roots to replace the water moving up the stem. Dry air

Water Movement The only way that plants can control water loss on a short-term basis is to close their stomata. But plants need to balance closing their stomata with keeping them open for providing access to carbon dioxide. The stomata open and close because of changes in the water pressure of their guard cells.

Water Movement When the guard cells are plump and swollen with water, they are said to be turgid and the stoma is open. When the guard cells lose water, the stoma closes. H2O Thickened inner wall Stoma closed Stoma open Guard cell Chloroplasts Epidermal cell Nucleus (b) (a)

Water Movement Root hairs greatly increase the surface area of roots. Root hairs are turgid because they contain a higher concentration of dissolved solutes than the soil. Root hairs also contain a variety of ion transport channels that transport specific ions. This may involve active transport. The minerals are transported by the xylem while dissolved in water.

Carbohydrate Transport Translocation is the process by which most of the carbohydrates manufactured in plants are moved through the phloem. Carbohydrates are transported by mass flow, a passive process. Mass flow occurs because of water pressure - when carbohydrates are loaded into sieve tubes, water also enters due to osmosis; the water pressure forces the carbohydrates down the plant.

Carbohydrate Transport H2O 1 2 3 4 Sugar created in the leaves by photosynthesis (“source”) enters the phloem by active transport. When the sugar concentration in the phloem increases, water is drawn into phloem cells from the xylem by osmosis. The addition of water from the xylem causes pressure to build up inside the phloem and pushes the sugar down. Sugar from the phloem enters the root cells (“sink”) by active transport. Leaf cells Sugar Xylem Phloem Root cells An area where sucrose is made is called a source and an area where sucrose is delivered from the sieve tubes is called a sink. Sucrose moves from a source to a sink by a process described by the pressure-flow hypothesis.