RAVEN 9/e

CHAPTER 36: PLANT FORM

WHERE DOES IT ALL FIT IN?

Chapter 36 builds up the general information on green plants provided in Chapter 30. A quick summary of Chapter 30 is essential for success at covering Chapter 36. In addition, students should be encouraged to recall the principles of eukaryotic cell structure, photosynthesis, and evolution associated with the particular features of plant cells.

SYNOPSIS

Plant meristem cells are analogous to stem cells in animals. They are small, unspecialized cells from which other plant cells differentiate. Cell division in apical meristems result in elongation of the root and shoot. They give rise to three types of embryonic tissue: protoderm that becomes epidermis, procambium that becomes primary vascular tissue, and ground meristem that becomes ground tissue parenchyma cells. Division in lateral meristems produces an increase in girth of shrubs and most trees. Woody plants have two cylinders of lateral meristems: cork cambium produces the cork cells of the outer bark, and vascular cambium cells give rise to secondary vascular tissue. Cork cambium also replaces the epidermis, and produces the bark on stems and roots in plants that experience secondary growth. The basic plan of a plant includes root and shoot, the latter being composed of stems, leaves, flowers, and fruit. Although the development of the basic plant form and structure is rigidly controlled, some aspects of leaf, stem, and root development are flexible. Plants undergo continuous development that is greatly affected by external events. Plants must be highly responsive to their environment as they cannot generally pick up their roots and grow somewhere else. The three basic plant tissue types include dermal tissue (epidermis), ground tissue (parenchyma cells), and vascular tissue (xylem and phloem).

Primary growth helps to establish the basic body-plan of plants, whereas secondary growth allows plants to increase in diameter. Secondary growth appears to have evolved independently in several groups of vascular plants, allowing the development of tree-like organisms with thick trunks. Early vascular plants lacked stems, leaves, and roots. These organs became useful with inhabitation of the land. The vascular tissues formed by the primary meristems functioned in conduction just as in modern plants. Sieve tube members conduct carbohydrates away from areas where they are manufactured or stored, typically roots and leaves, while vessel members and tracheids conduct water and minerals upward. These tissues are associated together in primary tissues. In secondary growth, phloem is located on the periphery, and xylem tissue develops more centrally in the stem. Conduction is continuous between root and shoot tissues.

Epidermal cells derived from dermal tissue cover all parts of the primary plant body. It may contain specialized cells. Guard cells are paired, chloroplast-containing cells located on either side of the stoma. They accommodate the passage of oxygen and carbon dioxide as well as the diffusion of water that results from transpiration. Trichomes are hair-like outgrowths that help regulate heat and water balance and can vary greatly in form. Root hairs are extensions of single cells on the outer surface of growing roots. They greatly increase a root’s surface area and are the entry points for water absorption.

Plant ground tissue exhibits several characteristic kinds of cells including parenchyma, collenchyma, and sclerenchyma. Only parenchyma and collenchyma are capable of further cell division. Sclerenchyma fibers and sclereids are lignified, structural elements that possess tough, secondary walls. Xylem conducts water upward from the roots and is composed of tracheids, dead cells that taper at the ends and overlap one another, and vessel members, dead, hollow, cylindrical cells arranged end-to-end. Water is conducted in a continuous stream throughout the plant body, passing through pits in tracheid cell walls, eventually diffusing into the atmosphere in transpiration. Vessel members, found primarily in angiosperms, conduct water and dissolved minerals more efficiently than do tracheids. Primary xylem is derived from procambium, and secondary xylem forms from the vascular cambium. Xylem also consists of parenchyma cells that are produced by special ray initials of the vascular cambium, and they function in lateral conduction and food storage, and fibers that are major components of modern paper. Phloem, the principal food-conduction tissue in vascular plants, is composed of sieve cells or sieve-tube members. Seedless vascular plants and gymnosperms have only sieve cells while most angiosperms have only sieve tube members that are more advanced and efficient than sieve cells. They function with living companion cells that have plasmodesmata connecting the cytoplasm of associated sieve tube members.

Roots have a simpler anatomy than do stems, and they do not produce projections analogous to leaves or flowers. Developing roots have four distinctive regions: root cap, zone of cell division, zone of elongation, and zone of maturation. Primary growth in the root occurs at the root tip. Daughter cells on the root tip side become root cap cells, whereas the others pass through the other zones, whose boundaries are not clear, before they complete differentiating. Root caps are unique to plant roots, and are composed of inner columella cells and outer, lateral root cap cells, which are continuously replenished by apical meristems. Root cap cells produce a mucilaginous material that eases root tips through the soil, and provides a medium for growth of beneficial nitrogen-fixing bacteria in some plants. Root caps respond to gravity, due to amyloplasts in columella cells although the exact nature of the gravitational response is not known. Mitosis in the zone of cell division occurs toward the edges of the inverted dome-shaped apical meristems. Daughter cells then divide into the three primary tissues: protoderm, procambium, and ground meristems. Activity in the zone of elongation causes lengthening of roots as new cells finally elongate. No further increase in cell size occurs above the zone of elongation, and the mature parts of the root remain stationary throughout the plant’s life. Cell differentiation takes place in the zone of maturation. Epidermal cells with very thin cuticles form on the root surface; many develop into root hairs. Parenchyma cells, produced by the ground meristems immediately interior of the epidermis, form the cortex whose inner boundary cells differentiate into the endodermis. Its primary walls contain suberin, produced in bands called Casparian strips. This waxy layer determines what minerals and nutrients enter the root xylem. The pericycle lies just inside the endodermis and is the perpetual source of lateral roots, and part of the vascular cambium in eudicots. Tissues interior to the endodermis constitute the stele. Primary xylem cells differentiate in the center of eudicot roots forming vascular bundles around parenchyma cells called pith. Primary phloem cells also differentiate near the xylem. Secondary xylem subsequently forms toward the inside of the root vascular cambium, and secondary phloem forms to the outside. Cork cambium forms the bark; however, cork cambium is not present in either monocots or herbaceous eudicot species. There are two primary root morphologies: tap roots or fibrous root systems. Both function in anchorage and absorption. Adventitious roots arise primarily along young stems of some plant species. They include aerial roots, pneumatophores, contractile roots, parasitic roots, food and water storage roots, and buttress roots, and they carry out an interesting variety of functions.

Stems form from initiatives in shoot apical meristems. Primordia tissues, located intermittently along stems, develop into other shoots, leaves, or flowers. The arrangement of leaves may optimize their exposure to the sun. Leaves are attached to stems at nodes, separated by regions called internodes. Leaf buds are terminal if attached to the tip of a twig, and axillary if formed along the length of it. Axillary buds form lateral branches. Woody stems may live for many years, and they develop distinctive markings that reflect prior events, such as bud scale scars the form when protective winter bud scales drop. Such markings are useful in wintertime identification of plants. Internally, apical meristems produce primary meristems that contribute to the length of stems and form the three primary meristems: protoderm, ground meristems, and procambium. In stems that exhibit primary growth, ground tissue divides into centrally located pith and an outer cortex. Also, vascular tissue is embedded in the ground tissue as scattered bundles in monocots, and as a cylindrical outer ring in eudicots. Vascular cambium produces xylem and phloem in the same manner as in roots. Cork is renewed constantly when the cork cambium produces cork to the outside and phelloderm cells to the inside. Cork inhibits water and gas exchange with the stem tissues except in unsuberized patches called lenticels. Bulbs, corms, rhizomes, runners and stolons, tubers, tendrils, and cladophylls are examples of stem modifications, and major modes of vegetative reproduction for some plants. They have all the features of stems: leaves at nodes, buds in leaf axils, and internodes. Commercial and private industry often uses segments of modified stems for species propagation.

Leaves are the primary photosynthetic organs of plants, and consequently, they are vital to life on earth. Initiated as uncommitted primordial by apical meristems, they are extensions of shoot apical meristems and stem development. Microphylls are small, early evolutionary forms of leaves primarily associated with the order Lycophyta. Megaphylls, characterized by numerous veins, are found on most plants. Flattened blades and slender petioles of most leaves demonstrate a shift from radial symmetry to dorsal-ventral symmetry. Stipules may be present. The veins are vascular tissues that run through the leaf in parallel patterns in monocots, and in netted patterns in eudicots. Leaf blades exhibit various morphologies, classified as simple or compound. They are arranged in alternately, as opposite pairs, or in whorls. Leaf surfaces consist of epidermal cells that lack chloroplasts and are covered by a waxy cuticle, and may have various glands and trichomes, but generally lack stomata. These are typically located on the lower epidermis. Mesophyll tissues, interspersed with veins of various sizes, occupy the interior of leaves. In eudicots, it is composed of tightly packed, chloroplast-laden palisade cells, and loosely arranged, spongy parenchyma cells. Spongy mesophyll is interspersed with intercellular spaces that connect to the stomata, permitting gas exchange with the environment. Differentiation of mesophyll does not occur in monocots. Modified leaves evolved in different environments, and include bracts, spines, reproductive leaves, window leaves, shade leaves, and insectivorous leaves.

LEARNING OUTCOMES

36.1 Organization of the Plant Body: An overview

  • Distinguish between the functions of roots and shoots.
  • Name the three types of tissues in a plant.
  • Compare primary growth and secondary growth.

36.2 Plant Tissues

  • Describe the functions of dermal, ground, and vascular tissues.
  • Name the three cell types found in ground tissue and their functions.
  • Distinguish between xylem and phloem.

36.3 Roots: Anchoring and Absorbing Structures

  • Describe the four regions of a typical root.
  • Explain the function of root hairs.
  • Describe functions of modified roots.

36.4 Stems: Support for Above-Ground Organs

  • List the potential products of an axillary bud.
  • Differentiate between cross sections of a monocot stem and a eudicot stem.
  • Describe three functions of modified stems.

36.5 Leaves: Photosynthetic Organs

  • Distinguish between a simple and a compound leaf, and between a pinnately and palmately compound leaf.
  • Compare the mesophyll of a monocot leaf with that of a eudicot leaf.

COMMON STUDENT MISCONCEPTIONS

There is ample evidence in the educational literature that student misconceptions of information will inhibit the learning of concepts related to the misinformation. The following concepts covered in Chapter 36 are commonly the subject of student misconceptions. This information on “bioliteracy” was collected from faculty and the science education literature.

  • Students do not fully understand the origins of endosymbionts in plants
  • Students believe that plants lack tissues and organs
  • Students do not equate pollination with sexual reproduction
  • Students think pollen are one and the same as sperm
  • Students are unaware that plants produce eggs
  • Students are unaware that plants undergo embryological development
  • Students are unaware of all of the functions of roots
  • Students are unaware of all of the functions of stems
  • Student believe that only leaves can carry out photosynthesis
  • Students believe that the reproductive structure of all plants are flowers
  • Students confuse spores with pollen
  • Students believe that all flowers are insect pollinated
  • Students believe that the pollen of any plant can be found in the wind

INSTRUCTIONAL STRATEGY PRESENTATION ASSISTANCE

This chapter epitomizes the complexity of plant form, resulting from plant growth. It is imperative that teachers think through and plan on how to present the information in exciting and understandable ways. Perhaps an explanation of cells to tissues to organs will be helpful. Or, consider initiating a discussion about the essential features needed for plant function: roots to anchor, stems to support, leaves for photosynthesis, tissues for conduction and storage and protection, etc. Have students understand the make-up of typical monocot and eudicot stems and roots, then “work backward” and examine the embryonic origins and subsequent differentiation of the respective cells and tissues. Once students understand the basic tissue types, they can work more expertly with this fascinating information. Hands-on sketching, however, interpretable to anyone except the artist, is a compelling way to encourage mastery, and it can be fun.

HIGHER LEVEL ASSESSMENT

Higher level assessment measures a student’s ability to use terms and concepts learned from the lecture and the textbook. A complete understanding of biology content provides students with the tools to synthesize new hypotheses and knowledge using the facts they have learned. The following table provides examples of assessing a student’s ability to apply, analyze, synthesize, and evaluate information from Chapter 36.

Application /
  • Have students describe how damage to apical meristems would affect plant growth.
  • Have students explain the evidence of organ structure in plants.
  • Ask students to explain how a plant lacking leaves and roots would be able to carry out photosynthesis.

Analysis /
  • Have students explain the advantages and disadvantages of an herbaceous stem over a woody stem.
  • Have students explain why a particular leaf has much fewer leaf hairs on its lower surface than on the upper surface.
  • Ask students to explain the benefits and risks up upright stem growth in plants.

Synthesis /
  • Ask students to explain the possible applications of the discovery of a mutation that increases the growth rate of vascular tissue formation.
  • Have students hypothesize why the agricultural value of a genetic engineering technique that can modify the role of ground tissue in a plant.
  • Ask the students to explain why trees in trees in construction areas exhibit a die-off of branches even it they are not damaged by people or machinery.

Evaluation /
  • Ask students evaluate any possible risks of using genetic engineering to increase the growth rate of root crops such as carrots.
  • Ask students to evaluate the consequences of people over-using herbicides that kill weeds by inhibiting meristem growth.
  • Ask students to evaluate the accuracy of using herbaceous eudicots as a model for all plants.

VISUAL RESOURCES

Three-dimensional models are very helpful, although they are generally better suited to small lectures or laboratory use. In large classes, overheads of 3-D drawings are most appropriate. Take time to explain the plant parts and processes so that students do not experience “information overload” regarding these valuable organisms in which many are not interested.

Secondary growth is difficult for beginning students. They especially don’t understand that when a cell divides one of the daughter cells moves inward and the other outward. One could illustrate this on an overhead using “cells” cut from colored pieces of acetate, with color differentiation being associated with cell type and cell position.

IN-CLASS CONCEPTUAL DEMONSTRATIONS

A. Cyber Plant.

Introduction