15-1 copyright 2005 mcgraw-hill australia pty ltd ppts t/a biology: an australian focus 3e by knox,...

15-1Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Chapter 15: Structure of plants


Plant cell walls

• Cell walls define the size and shape of cells, contribute to metabolic processes and cell-cell communication

• Lying between adjacent cells is the middle lamella, which comprises pectins that bind the cells together


Fig. 15.3a: Cross-section of parenchyma cells from Coleus stem


Primary cell wall

• A plant cell wall is made up of a primary wall and a secondary wall

• The primary wall is the first layer formed by a developing cell

• This is composed of cellulose microfibrils, together with associated pectins, hemicelluloses and glycoproteins

• The primary wall is able to extend as a cell enlarges and matures

(cont.)


Primary cell wall (cont.)

• Primary cell walls vary in thickness, with thinner areas (primary pit fields), punctuated by numerous plasmodesmata, which are involved cell-cell communication


Fig. 15.4a: Parenchyma cell


Secondary wall

• As cells mature, many lay down secondary wall materials on the inner surface of the primary wall

• The secondary wall is usually formed in three distinct layers, each with cellulose microfibrils laid down in different orientations, and strengthened by the addition of lignin (a polyphenol)

• Some cells have many depressions, or primary pits, in the secondary wall


Plant tissue systems

The primary plant body is composed of cells produced

by the shoot and root apical meristems.

1. Dermal tissue– epidermis segregates internal plant tissues from the

external environment– epidermal cell structure varies, depending on whether it

occurs in the stem, root or leaf, although its main functions are protection and facilitating exchange of materials

– dermal tissues include the endodermis and exodermis

(cont.)


Plant tissue systems (cont.)

2. Ground tissue– These tissue systems, which include parenchyma,

collenchyma and sclerenchyma, make up the bulk of tissues in stems, roots and leaves


Fig. 15.5a: Parenchyma cells of potato tubers


Fig. 15.5b: Aerenchyma cells of the seagrass Amphibolis


Fig. 15.5c: Transfer cells in an embryo of the seagrass Amphibolis


Ground tissue

• Parenchyma: these tissues normally comprise large cells with thin primary walls and well-defined, pectin-rich middle

– chlorenchyma: photosynthetic, found in leaves and some stems

– storage parenchyma: contain nutrient reserves– aerenchyma: spongy cells with a large network of air

spaces, adaptation to low O2 environments

– transfer cells: ingrowths that increase cell surface area for rapid transfer of molecules

(cont.)


Ground tissue (cont.)

• Collenchyma: supportive tissue, providing strength to plant parts where bending and flexibility are required. Cells are elongated, with thickened cells walls that contain extra cellulose

• Sclerenchyma: supportive tissue that imparts rigidity and strength

– fibres: elongated cells with tapering end walls– sclerids: branched or even-shaped, and play roles in both

support and protection


Vascular tissue

• Vascular tissues are involved in transport of materials around the plant body

• Comprises xylem and phloem• Xylem transports water and dissolved minerals from

the roots to the leaves and comprises cell types, commonly vessels and tracheids

• Phloem transports the products of photosynthesis from the leaf to actively growing areas or to storage


Vessels

• Vessels, which are common in flowering plants, have open ends that allow an unimpeded flow of water from one cell to another


Fig. 15.8a: Xylem vessels in vascular tissues


Tracheids

• Tracheids, common to all vascular plants, have intact end walls, with water moving from cell to cell through bordered pits in the cell wall


Fig. 15.8b: Tracheids


Phloem

• Phloem comprises sieve tube cells associated with companion cells (parenchyma in nature)

• Sieve tube cells are elongated and possess thin primary cell walls composed largely of cellulose

• Companion cells control the activity of sieve tube cells by increasing the surface area for cell-to-cell interchange


Fig. 15.8c: Phloem


Stem structure

• Plant shoots comprise stems and leaves• The stem provides support, contains vascular

tissues and in some species is a storage organ


Fig 15.9: The structure of a flowering plant


Stem primary growth

• In dicotyledons (e.g. eucalypts and wattles) vascular bundles consisting of xylem and phloem, together with parenchyma cells, form a central cylinder, the stele

(cont.)


Fig. 15.10a: Ring of vascular bundles in Acacia


Stem primary growth (cont.)• Vascular tissue often forms a single ring, separating

the stem into outer cortex (ground tissue) from the inner pith

• Primary phloem forms outside the xylem on the same radius

• The earliest-forming xylem, (protoxylem) develops as thin strands towards the stem centre. Outside of this develops larger, thicker-walled xylem (metaxylem)


Fig. 15.10b: Vascular bundle in Acacia


Stem secondary growth

• Woody dicotyledons such as trees and shrubs can undertake secondary, or radial, growth

• Cambium, which produces sheets of cells laterally, may be vascular (wood) or cork (bark)


Fig. 15.12a, b and c: Secondary growth in the stem of silky oak (Grevillea robusta)

(a)

(b)

(c)


Vascular cambium

• This multilayered cylinder of meristem lies between the cortex and pith and is responsible for producing secondary vascular tissues such as secondary xylem and secondary phloem


Fig. 15.12d: Cross-section of secondary xylem and phloem


Cork cambium

• These meristematic cells generate the outer covering, or periderm, that replaces the epidermis in older stems

• The cambium produces cork (or phellem) on the outside of the stem and thin walled parenchyma, (the phelloderm) on the inside


Fig. 15.13b: Cross-section of bark

Copyright © Associate Professor Andrew Drinnan, University of Melbourne


Special functions of stems

• Not all plant stems are aerial; underground stems may be horizontal rhizomes, from which aerial branches or leaves arise for vegetative propagation or to serve as food storage, as in the potato tuber

• Desert plants– in arid and semi-arid regions, leaves tend to be absent or

reduced, with photosynthesis carried out by the stem– e.g. leaves of she-oaks (Allocasuarina sp.) are reduced

to small, scale-like structures in a whorl at nodes of stems or cladodes

(cont.)


Special functions of stems (cont.)

• Stems of marsh and aquatic plants– such environments are usually waterlogged, and

stems often contain extensive aerenchyma– in saline habitats, leaves are reduced and fleshy

stems photosynthesise and store water


Fig. 15.16: Stems of the saltmarsh plant Sarcocornia

Copyright © Associate Professor Andrew Drinnan, University of Melbourne


Root structure• Roots are the underground organs of vascular

plants, and grow from apical meristems at their tips• Roots have several functions, including nutrient

and water uptake from the soil, anchorage and support, synthesis of plant hormones and storage of nutritional reserves

• Root systems may be branched, a common trait of dicots, or consist entirely of adventitious roots, a trait among monocot species


Fig. 15.19a, b and c: Types of root systems


Root primary growth• A root consists of an outer epidermis, enclosing a

well-developed cortex and inner vascular cylinder

• The root cap, located at the tip of the root, protects the apical meristem

(cont.)


Fig. 15.20: Organisation of roots


Root primary growth (cont.)• The root cap comprises large parenchyma cells that

secrete mucigel, a polysaccharide that contains a mucilaginous matrix, and sloughed-off living cells

• The root surface area is supplemented by numerous fine root hairs, which are extensions of epidermal cells located just behind the growing tip

• The soil region around the root hair zone is the rhizosphere, within which occur interactions between the plant and its soil environment, and with free-living and symbiotic microorganisms

(cont.)


Root primary growth (cont.)• The root vascular tissues of dicotyledons comprise

exarch xylem, which forms from the outside to the inside of the root

• The vascular tissue is surrounded by the pericycle, a thin layer of cells from which lateral roots arise

• Outside the pericycle lies a single layer of cells, the endodermis, the radial walls of which endodermis are impregnated with suberin, a compound that is impermeable to water

• This Casparian strip forces water to pass through the symplastic pathway, enabling the plant to selectively control the movement of water and ions.


Special functions of roots• Roots of aquatic plants

– mangrove species often possess upright aerial roots, pneumatophores that function in gas exchange for aerobic respiration

• Roots and salinity– river red gums, which are mildly salt tolerant, have an

exodermis, a suberised cell layer that acts as a barrier to toxic chloride ions

• Storage roots– roots adapted for storage often have additional layers of

vascular cambia together with associated storage parenchyma cells (e.g. sweet potato and beetroot)


Leaf structure• Leaves are photosynthetic organs with limited life

spans• Evergreen leaves are shed continuously while

deciduous trees shed their leaves before the onset of a cold or dry period

• Simple leaves have a single lamina while compound leaves possess many leaflets

• Juvenile leaves can differ considerably from adult leaves in colour and/or form


Leaf structure and organisation• A leaf usually comprises upper and lower epidermis,

enclosing mesophyll tissue containing chloroplasts• The epidermis is covered by a thick cuticle and

studded with pores (stomata) which regulate water loss and gas exchange between plant and atmosphere

(cont.)


Fig. 15.33a and b: Leaf anatomy of Eucalyptus globulus

(a) (b)


Leaf structure and organisation (cont.)• Leaf hairs, secretory gland cells and crystal-

containing cells also characterise the epidermis• Mesophyll, which is the ground tissue of leaves,

comprises chloroplast-containing parenchyma cells that are the sites of photosynthesis

• Mesophyll is often differentiated into palisade mesophyll toward the upper surface of the leaf, with spongy mesophyll toward the lower

• Veins (vascular bundles) of flowering plants form an interconnected branching system, with xylem on the upper side and phloem on the lower


Modifications of leaf structure• Aspects of leaf morphology are directly influenced

by the environment– e.g. shade leaves are generally larger, but have less

palisade mesophyll than sun leaves

• Plants growing in particular habitats may exhibit distinctive leaf adaptations

– e.g. arid and semi-arid taxa often have small, scale-like leaves

• Some rainforest plants possess pore-like structures, hydathodes, that enable extrusion—guttation—of water under conditions of high atmospheric humidity

15-1 copyright 2005 mcgraw-hill australia pty ltd ppts t/a biology: an australian focus 3e by knox,...

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