Chapter 32 - Plant Structure And Life Span

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o Herbaceous

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are non-woody
 In temperate climates, the aerial parts of herbaceous plants die back to the ground at the end of the growing season
 Do not produce hard, lignified secondary tissues

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o Woody

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produce hard, lignified secondary tissues
 Aerial parts of woody plants persist

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• Annuals

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herbaceous plants that grow, reproduce, and die in 1 year or less

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• Biennials

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take two years to complete their life cycles before dying
o During their first season, they produce extra carbohydrates which they store and use during their second year where they typically form flowers and reproduce

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• Perennials

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herbaceous and woody plants that have the potential to live for more than 2 years
• All woody plants are perennials

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• Dormancy

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an organism reduces its metabolic state to a minimum level to survive unfavorable conditions

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• Deciduous

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plants that shed their leaves before winter and produce new stems with new leaves the following spring
• Other woody perennials shed their leaves over a long period, so some leaves are always present

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• Plants have different life history strategies

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o Environments that are particularly favorable have great competition for space. In these types of environments, perennials survive well but young plants do not.
o In relatively unfavorable environments, there is less competition for space and it is easier for young plants to establish a place for themselves. They are called opportunists; they grow and mature quickly during the brief periods when environmental conditions are most favorable

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• The plant body consists of cells and tissues

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o Tissue – group of cells that forms a structural and function unit
o Simple tissues – composed of only one kind of cell
o Complex tissues – composed of two or more kinds of cells
o Vascular plants:
 Ground tissue system – most of the plant body, has a variety of functions including photosynthesis, storage, and support
 Vascular tissue system – intricate conducting system that extends throughout the plant body, is responsible for conduction of water, dissolved minerals, and food
 Dermal tissue system – provides a covering for the plant body
o Roots, stems, leaves, flower parts, and fruits are organs because each is composed of all three tissue systems

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• The ground tissue system is composed of three simple tissues

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o Parenchyma
 Have thin primary walls
 Soft parts of a plant consist largely of parenchyma
 Usually the soft parts of fruits
 Perform photosynthesis, storage, and secretion
 Store starch grains, oil droplets, water, and salts
 Have the ability to differentiate into other kinds of cells
o Collenchyma
 Have unevenly thickened primary walls
 Extremely flexible structural tissue that provides much of the support in soft, non-woody plant organs
 Are alive at maturity
 Especially thick in the corners
 Is not found uniformly throughout the plant and often occurs as long strands near the stem surface and along leaf veins
o Sclerenchyma
 Have both primary walls and thick secondary walls
 Secondary cell walls become strong and hard because of extreme thickening
 Two types of sclerenchyma cells
 Sclereids – cells of variable shapes, are harder
 Fibers – long, tapered cells that often occur in patches or clumps, occur more in plants, and woody plants

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Primary and Secondary Cell Walls

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o Primary cell wall – stretches and expands as the cell increases in size
o Secondary cell wall – thick, strong cell wall that is deposited inside the primary cell wall

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Cells of the three simple tissues vary in their cell wall chemistry

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 Cellulose – polysaccharide composed of glucose units joined by beta 1,4 bonds
 Cellulose microfibril – strong, tiny strand
 Hemicellulose – group of polysaccharides that are more soluble than cellulose
 Pectin – cementing polysaccharide
 Lignin – complex strengthening polymer made up of monomers derived from certain amino acids

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The dermal tissue system consists of two complex tissues

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 Consists of
• Epidermis
• Periderm
 In herbaceous plants, the dermal tissue system is a single layer of cells [epidermis]
 Woody plants initially produce an epidermis but is split apart later
 Epidermis is the outermost layer of a herbaceous plant
• Epidermis – complex tissue composed primarily of relatively unspecialized living cells
• Have specialized guard cells and trichomes dispersed among these cells
• Generally contain no chloroplasts and are therefore transparent so light can penetrate the interior tissues of stems and leaves
 Cuticle – waxy layer that generally limits water loss from plant surfaces
 Stomata – minute pores in the epidermis that are surrounded by two guard cells
• Are generally open during the day when photosynthesis is occurring and usually close at night
 Trichomes – outgrowths or hairs
• Occur in many shapes and sizes
• Root hairs – simple, unbranched trichomes that increase the surface area of the root epidermis

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Periderm replaces epidermis in woody plants

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 Forms the outer bark of older stems and roots
 Composed mainly of cork cells and cork parenchyma cells
 Suberin – waterproof substance
 Cork parenchyma cells function mainly as storage

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• Meristems

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area where plant growth occurs; composed of cells whose primary function is to form new cells by mitotic division

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Growth

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• Indeterminate growth – the ability of roots and stems to grow throughout a plant’s life
• Determinate growth – they stop growing after reaching a certain size
• Primary growth – increase in stem and root length
• Secondary growth – increase in the girth of a plant
• Primary growth takes place at apical meristems

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Apical Meristem

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areas located at the tips of roots and shoots, including within the buds of stems
o Buds – dormant embryonic shoots that eventually develop into branches
o Root cap – protective layer of cells covers the root tip
o Shoot apex – a dome of minute, regularly arranged meristematic cells
o Leaf primordia – developing leaves
o Bud primordia – developing buds

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• Secondary growth takes place at lateral meristems

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o Lateral meristems – areas extending along the entire length of the stems and roots except at the tip
o Vascular cambium – layer of meristematic cells that forms a long, thin, continuous cylinder within the stem and root
 Located between the wood and bark of a woody plant
o Cork cambium – composed of a thin cylinder or irregular arrangement of meristematic cells
 Located in the outer bark
 Cells of the cork cambium divide and form cork cells toward the outside and one or more underlying layers of cork parenchyma
o Bark – the outermost covering over woody stems and roots; consists of all plant tissues located outside the vascular cambium

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o The vascular system consists of two complex tissues

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 The conducting cells in xylem are tracheids and vessel elements
 Xylem is composed of tracheids, vessel elements, parenchyma cells, and fibers
 Xylem parenchyma – performs storage functions
 Tracheids and vessel elements
 As they develop, both types of cells undergo apoptosis
 Mature tracheids and vessel elements are dead, and therefore, hollow
 Tracheids – long, tapering cells located in patches or clumps
• Water is conducted upward, from roots to shoots, passing from one tracheids to another through pits, thin areas in the tracheids cell walls where a secondary wall did not form
 Vessel elements
• Are hollow but have holes in their end walls
o These holes are known as perforations
• Are stacked one on top of the other and water is conducted from one vessel element into the next
• A stack of vessel elements are called a vessel
• Also have pits in their side walls that permit lateral transport of water from one vessel to another

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 Sieve tube elements are the conducting cells of phloem

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 Conducts food materials
 Composed of:
• Sieve tube elements – food materials are conducted in solution
• Are joined end to end to form long sieve tubes
• Sieve plates [end walls] have a series of holes through which cytoplasm extends from one sieve tube element into the next
 Companion cells
• Assists in the functioning of the sieve tube element
• Is a living cell complete with nucleus
• Nucleus is though to direct the activities of both the companion cell and sieve tube element
• Numerous plasmodesmata occur between a companion cell and its adjoining sieve tube element
 Fibers – usually extensive in the phloem of herbaceous plants and provide additional structural support of the plant body
 Phloem parenchyma cells