Essentials of the Living World Plant Form and Function Second Edition George B. Johnson Jonathan B. Losos Chapter 33 Plant Form and Function Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

33.1 Organization of a Vascular Plant A vascular plant is organized along a vertical axis the part below ground is called the root the root penetrates the soil and absorbs water and ions it also anchors the plant the part above ground is called the shoot 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

Figure 33.1 The body of a plant

33.1 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

33.1 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

33.1 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

33.2 Plant Tissue Types Most plants have three tissue types ground tissue in which the vascular tissue is embedded dermal tissue the outer protective covering of the plant vascular tissue conducts water and dissolved minerals up the plant and conducts the products of photosynthesis throughout

33.2 Plant Tissue Types There are three kinds of cells in plant ground tissue parenchyma cells they are alive at maturity they carry out the basic functions of living, including photosynthesis, cellular respiration, and food and water storage collenchyma cells they are also living at maturity they provide much of the support for plant organs in which secondary growth has not occurred sclerenchyma cells they usually do not contain living cytoplasm when mature they have tough cell walls called secondary cell walls

Ground Tissue Examples Figure 33.2 Parenchyma cells Figure 33.3 Collenchyma cells

33.2 Plant Tissue Types There are two types of sclerenchyma fibers which are long, slender cells that usually form strands sclereids which are variable in shape but often branched clusters of sclereids form the gritty texture one feels in the flesh of pears

Figure 33.4 Sclerenchyma cells in sclereids

33.2 Plant Tissue Types All parts of the outer layer of a primary plant body are covered by flattened epidermal cells, which are often covered by a waxy layer called the cuticle they protect the plant and provide an effective barrier against water loss

33.2 Plant Tissue Types There are several specialized epidermal cells that make up dermal tissue guard cells are paired cells that flank an opening called a stoma the guard cells regulate the passage of oxygen, carbon dioxide, and water vapor across the epidermis trichomes are outgrowths of the epidermis that occur on the shoot and give it a “fuzzy” appearance they play an insulating role and affect heat and water balance root hairs are extensions of the epidermis below ground and keep the root in intimate contact with soil particles root hairs increase the surface area of the root

Figure 33.5 Guard cells and trichomes

33.2 Plant Tissue Types There are two types of vascular tissues 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

33.2 Plant Tissue Types There are two principal conducting cells in the xylem, both of which are dead at maturity tracheids are elongated cells that overlap at their ends water flows from tracheid to tracheid through openings called pits vessel elements are elongated cells that line up end-to-end the end walls of vessel elements are almost completely open or be perforated to allow for the flow of water vessels conduct water much more efficiently than tracheids

Figure 33.6 Comparison of vessel elements and tracheids

33.2 Plant Tissue Types Food conduction in phloem is carried out through two kinds of elongated cells sieve cells have smaller perforations between cells sieve-tube members have some sieve areas with larger pores than do sieve cells these areas are called sieve plates sieve-tube members occur end to end, forming longitudinal series called sieve tubes specialized parenchyma cells, known as companion cells, occur in association with the sieve tubes

Figure 33.7 Sieve tubes

33.3 Roots Roots have a central column of xylem with radiating arms alternating within the radiating arms of xylem are strands of primary phloem surrounding the central column, and forming its boundary, is a cylinder of cells called the pericycle branch, or lateral, roots are formed from cells of the pericycle

33.3 Roots The outer layer of the root is the epidermis the mass of parenchyma in which the root’s vascular tissue is located is called the cortex its innermost layer lies just outside the pericycle and is called the endodermis the cells making up the endodermis are encircled by a thickened, waxy band called the Casparian strip this strip blocks the movement of water between the endodermal cells and instead forces the movement of water through the plasma membrane of the endodermal cells

Figure 33.9 A root cross section

33.3 Roots The apical meristem of a root is really three primary meristems protoderm becomes the epidermis procambium produces primary vascular tissues ground meristem differentiates into ground tissues, which is comprised of parenchyma tissue If apical meristem growth is outward, the cell division forms a thimblelike mass of unorganized cells called the root cap the root cap protects the root’s apical meristem as it grows through the soil

33.3 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

33.3 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 Figure 33.10 Lateral roots

33.4 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

33.4 Stems The places of 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

33.4 Stems Buds have their own immature leaves called stipules buds may either elongate or remain dormant their activity is controlled by a hormone that moves downward from the terminal bud of the shoot the hormone suppresses expansion of buds in the upper portions of the stem buds begin forming in the lower down portions of the stem where the amount of hormone is reduced

Figure 33.11 A woody twig

33.4 Stems 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

Figure 33.12 A comparison of dicot and monocot stems

33.4 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

33.4 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 cells

33.4 Stems The cork, the cork cambium, and the parenchyma cells collectively make up a layer called the periderm the periderm is the plant’s outside protective covering 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

Figure 33.13 Vascular cambium and secondary growth

33.4 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

Figure 33.14 Annual rings in a section of pine

33.5 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 once a leaf is fully expanded, its marginal meristems cease to grow

33.5 Leaves Additional leaf structures include a slender stalk called a petiole two leaflike organs, called stipules, may flank the base of the petiole where it joins the stem 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

Figure 33.16 Dicot and monocot leaves

33.5 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 vein palmately compound describes leaflets that are radiate out from a common point at the blade end of the petiole

Figure 33.17 A leaf in cross section

33.5 Leaves Leaves can be arranged in different patterns alternate leaves spiral around a shoot opposite occur on opposite sides of a shoot whorl circle the stem as a group

33.5 Leaves A typical leaf contains masses of parenchyma, called mesophyll, through which the vascular bundles, or veins, run a closely packed, columnlike layer or layers of parenchyma cells are found underneath the upper epidermis of a leaf this is called the palisade mesophyll it is packed with chloroplasts the rest of the leaf interior, except for the veins, consists of spongy mesophyll it has lots of interior spaces for gas exchange

Figure 33.17 A leaf in cross section

33.6 Water Movement Several factors are at work to move water up the height of a plant the initial movement of water into the roots of a plant involves 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

33.6 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

Figure 33.18 Capillary action

33.6 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

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

33.6 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

Figure 33.19 How transpiration works

33.6 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

33.6 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 Figure 33.20 How guard cells regulate the opening and closing of stomata

33.6 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 minerals also enter the root hairs because they 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

Figure 33.21 Root hairs

Figure 33.22 The flow of materials into, out of, and within a plant

33.7 Carbohydrate Transport Translocation is the process by which most of the carbohydrates manufactured in plants are moved through the phloem the movement is a passive process the mass flow of materials transported occurs because of water pressure generated by osmosis 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

Figure 33.23 How translocation works

33.8 Essential Plant Nutrients Minerals are involved in plant metabolism in many ways nitrogen (N) is an essential part of proteins and nucleic acids potassium (K) ions are used to regulate turgor pressure in guard cells calcium (Ca) is an essential part of cell walls magnesium (Mg) is a part of the chlorophyll molecule phosphorous (P) is a part of ATP and nucleic acids sulfur (S) is a key component of the amino acid, cysteine

33.8 Essential Plant Nutrients Other essential minerals for plant health include chlorine (Cl), iron (Fe), boron (B), manganese (Mn), zinc (Zn), copper (Cu), and molybdenum (Mb) Most plants acquire minerals from the soil, although some carnivorous plants are able to use other organisms directly as sources of nitrogen, just as animals do

Figure 33.24 A carnivorous plant

Inquiry & Analysis In the 23-cm section, is more 42K found in xylem or phloem? Above and below the 23-cm section, is more 42K found in xylem or phloem? Graph of Movement of 42K Through a Stem