BIO 131 Chapter 34: Plant Form and Function

34.1 - 34.4

Angiosperms

Angiosperms

• flowering plants

• create most of the food we eat and medicine we use

Indeterminate growth

• plants grow throughout their lifetime

• allows them to adapt to changes in their environment

Root system

• belowground portion of the plant body

• anchors the plant and conducts water and nutrients to the shoot

• can store products of photosynthesis

• takes in water and nutrients from the soil (root hairs)

Morphological diversity

• diversity between species

• different forms (root types and lengths, shoot size) minimizes competition for resources, allowing plants to thrive in a wide range of habitats

Phenotypic diversity

• diversity within species

• root/shoot form (size, shape) is varying within a species due to differing environmental conditions (temperature, wind exposure, amount of resources available)

• allows plants to adapt to their environments

Modified roots

• roots specialized for unusual functions

Adventitious roots

• modified roots that develop from the shoot system (rather than the underground root system)

• help anchor and stabilize the shoot system

• ex: anchor roots (e.g. poison ivy) and prop roots (e.g. banyan trees)

Pneumatophores

• upward-growing vertical modified roots

• specialized for gas exchange in plants where underground aeration/oxygen is limited, such as Mangroves

Storage roots

• modified roots for storage of nutrients

• carrots, sugar beets, etc

Shoot system

• aboveground portion of the plant body

• harvests light and carbon dioxide from the atmosphere to produce sugars

Shoot anatomy

• 1+ stems, nodes, internodes, leaves, axillary/lateral + apical buds (may develop into flowers or other reproductive structures if conditions are appropriate)

Modified stems (cacti)

• specialized stems function as water storage organs and contain the plant’s photosynthetic tissue

Modified stems (stolons)

• stems that grow horizontally along the surface of the soil

• produce adventitious roots and leaves at each node

• considered asexual reproduction

Modified stems (rhizomes)

• very similar to stolons (grow horizontally)

• produce new individuals at nodes underground

• tubers (potatoes, etc) are swollen underground rhizomes that function as carbohydrate storage, with each potato eye functioning as a node

• considered part of the shoot (not root) system despite being underground

Modified stems (thorns)

• stems modified with pointy growths for protection

Simple leaf

• petiole + blade

Compound leaves

• have blades divided into a series of leaflets

• pinnate (leaflets arise from many different points along the petiole) vs. palmate (leaflets arise from one point)

Morphological diversity (leaves)

• shape: simple vs. compound vs. doubly compound vs. needle-like

• arrangement: alternate vs. opposite vs. whorled vs. rosette

Phenotypic plasticity (leaves)

• do not grow indeterminately, but still exhibit plasticity based on growing conditions (such as sun vs. shade)

Modified leaves

• onion bulbs

• succulent leaves (water storage)

• tendrils found in climbing plants like grapevines

• colored leaves (poinsettias)

• traps (pitcher plants, venus flytraps, etc)

• cactus spines

Secondary cell wall

• contains cellulose and lignin, which is a compound that makes the cell wall very rigid

Plastids

• includes chloroplasts, which are the site of photosynthesis

• Also includes similar organelles specialized for storing pigments (chromoplasts), starch (amyloplasts), oils (elaioplasts), or proteins (proteinoplasts)

Vacuole

• contains an aqueous solution called cell sap

• store (+ sometimes digest) waste

• store water, nutrients, and sometimes pigments and poisons

Tissue

• group of cells that function as a unit

• grouped into tissue systems based on structural features and location within the plant

Simple tissues

• plant tissues that consist of one type of cell

Complex tissues

• plant tissues that consist of several types of cells

Dermal tissue system

• dermal (skin) tissue/epidermis

• outermost layer of cells; interface between organism and external environment

• function (shoot): protect the plant from water loss, pathogens, herbivores, etc; gas exchange

• function (root): root hairs; absorbs water and nutrients

• complex tissue

Dermal shoot tissue

• mostly flattened cells lacking cloroplasts

• fulfill protective role by secreting the cuticle

• stomata, trichomes

Cuticle

• waxy layer that forms a continuous sheet on the surface of leaves and stems

• hydrophobic —> reduces water loss by transpiration

• acts as physical barrier against pathogens

Stomata

• specialized structures (pores) in dermal tissue that bypass the blockage that cuticle causes

• allows gas exchange for photosynthetically active tissues

• surrounded by specialized guard cells which change shape to open or close the stomata in response to the amount of water available

Trichomes

• hairlike appendages made of specialized shoot cells

• wide variety of shapes, sizes, and densities

• functions include keeping leaf surfaces cool by reflecting sunlight, reducing water loss by blocking transpiration, providing barbs or storing toxic compounds to discourage herbivores, or trapping and digesting insects

Ground tissue system

• responsible for photosynthesis, most carbohydrate storage, and the synthesis + storage of products such as pigments, hormones, and toxins required for defense

• large structural role in the shoot system

• complex tissue composed of three different simple tissues: parenchyma, collenchyma, and sclerenchyma

Parenchyma

• “workhouse” cells

• characterized by relatively thin primary cell walls

• most common and versatile ground tissue cells—primary site of photosynthesis (in leaves), store starch granules in other organs

• alive and mostly totipotent

Totipotency

• ablility to give rise to any cell type

• allows a cutting to develop into a complete, mature plant

• important for healing wounds and asexual reproduction via rhizomes and stolons

Collenchyma

• characterized by unevenly thickened primary cell wall

• longer and thinner than parenchyma cells

• alive, mostly totipotent

• often found just under the epidermis of stems, especially outside vascular bundles

• not rigid—ability to stretch allows stems to bend without tearing or breaking

• provide structural support for shoots, especially those without woody secondary growth

• can support actively growing parts of the plant (primary cell wall is expandable)

Sclerenchyma

• cells characterized by the presence of thick, rigid secondary cell wall and relatively thin primary cell wall

• specialized for supporting stems and other structures after growth has ceased

• dead at maturity (no cytoplasm)

• fibers, sclereids

Fibers

• extremely elongated sclerenchyma cells

• one cell can be over half a meter long

• important in manufacture of paper, linen, cotton, and other fabrics

Sclereids

• relatively short sclerenchyma cells that often function in protection

• variable shapes

• compose the tough coats of seeds and nut shells

• responsible for the grainy texture of pears

Vascular tissue system

• functions: support and long-distance transport of water, dissolved nutrients, and products of photosynthesis in vascular plants

• consists of two complex tissues, xylem and phloem

Xylem

• function: conducts water and dissolved nutrients from root to shoot

• structure: parenchyma fibers + 2 types of water-conducting cells formed adjacent to each other (tracheids and vessel elements)

Tracheids

• found in the xylem of all vascular plants

• long, slender cells with tapered ends

• sides and ends have pits (gaps in secondary cell wall) which allow water to move up vertically and laterally

Vessel elements

• found in angiosperms and a few other groups of vascular plants

• shorter and wider than tracheids

• have both pits and perforations (openings in the end walls lacking both primary and secondary cell walls)

• stacked to form open pipes called vessels

• conduct water more efficiently than tracheids (less resistance to flow)

• cells are dead at maturity, with secondary cell wall essentially acting as a pipeline

Phloem

• function: conducts sugar, amino acids, hormones, and other substances in two directions (roots to shoots, shoots to roots)

• structure: 2 specialized types of cells (sieve-tube elements and companion cells) connected by plasmodesmata, both alive at maturity and lacking a secondary cell wall

  • often includes fibers that aid in structural support

Sieve-tube elements

• function: transport of sugars and other nutrients

• structure: long, thin cells with perforated ends called sieve plates; lack most organelles

Companion cells

• NOT conducting cells

• function: provide materials to maintain the cytoplasm and plasma membrane of sieve-tube elements

Meristems

• populations of undifferentiated cells that retain ability to undergo mitosis

• persist throughout the plant’s lifetime (allows for indeterminate growth)

• some differentiate, some remain meristematic

Apical meristems

• meristems located at the tip of each root and shoot

• responsible for primary growth

Primary growth

• growth that results from the division of apical meristem cells and their differentiation

• common in all plants

• goal: extend the root and shoot system, which increases the plant’s ability to absorb/acquire resources

• primary component of plants that lack woody tissues

• compose the primary plant body

Primary meristems

• regions that are partially differentiated, but retain the character of meristematic cells because they keep dividing

• derived from apical meristems

• 3 types: protoderm, ground meristem, procambium

Protoderm

• primary meristem that gives rise to the dermal tissue system

Ground meristem

• primary meristem that gives rise to the ground tissue system

Procambium

• primary meristem that gives rise to the vascular tissue system

Organization of primary root system

• root cap

• 3 distinct populations of cells (zones) behind the root cap

Root cap

• group of cells that protects the root apical meristem

• regularly loses cells replaced by the meristem

• importing in sensing gravity and determining the direction of growth (auxin granules, gravitropism)

• secretes slimy, polysaccharide-rich substance that helps lubricate the root tip (reduce friction and protect root apical meristem)

Zone of cellular division

• contains the apical meristem, where cells actively divide, and the procambium, where additional cell division occurs

Zone of cellular elongation

• composed of cells recently derived from the primary meristems

• cells increase in length, primarily by taking up water

• region most responsible for the growth of roots throughout the soil

• expansion provides force that pushes the root cap and apical meristem throughout the soil

Zone of cellular maturation

• older cells complete their differentiation into dermal, vascular, and ground tissues

• most important segment in terms of water and nutrient absorption

• epidermal root hairs

• where lateral roots begin to grow (from within the inner vascular tissue)

Organization of primary shoot system

• vascular bundles

• eudicot vs. monocot arrangements

Vascular bundles

• groups of vascular tissues (xylem and phloem)

• phloem usually faces outward, xylem inward

Eudicot structure

• vascular bundles are arranged in a ring near the stem’s perimeter (arrangement is associated with ability to produce woody secondary growth)

• pith: ground tissue toward the center of the stem

• cortex: ground tissue between the epidermis and the pith; surrounds vascular bundles

Monocot structure

• vascular bundles are scattered throughout the ground tissue

Secondary growth

• occurs in trees and other woody plants

• increases the width of roots and shoots

• function: increase the amount of conducting tissue available and provide structural support required for extensive growth, which allows the plant to compete for light

Cambium (lateral meristem)

• special type of meristem

• forms a cylinder that runs the length of a root/trunk/branch made up of one layer of meristematic cells

• meristematic cells divide in a way that increases the width of roots/trunks/branches

• 2 distinct types—vascular and cork

Vascular cambium

• cylinder of meristematic cells between the secondary xylem and secondary phloem

• associated with wood

• produces new cells toward both the interior (secondary xylem) and exterior (secondary phloem), but generally more toward the interior

Cork cambium

• cylinder of meristematic cells located near the outer perimeter of trunks/roots/branches

• associated with bark

• produces new cells only toward the exterior

Initiation of secondary growth

• vascular: single layer of cells between the xylem and phloem within the vascular bundles becomes meristematic

• cork: ring of cells just below the epidermis becomes meristematic

Vascular cambium growth

• produces sclerenchyma cells/fibers for additional strength + parenchyma cells in laterally radiating rows (rays) across the xylem/phloem which transport water and nutrients across a trunk

• mature woody plants are dominated by secondary growth (primary xylem/phloem degrades as diameter increases)

Bark

• cork cambium + cork cells (dead at maturity) + secondary phloem

• replaces primary epidermal tissue as the trunk matures, eventually taking over its roles

• helps prevent water loss (cork cells produce layer of wax and other materials that make it impermeable to water and gases)

Lenticels

• small, spongy openings in bark that let gas exchange occur

Heartwood

• the darker-colored inner region of the trunk

• darkened by deposition of resins, gums, and other protective compounds in xylem that no longer transport water

Sapwood

• lighter-colored functional outer xylem

Tree rings

• early wood (spring/early rainy season growth): large, thin-walled, relatively light

• late wood (dryer/cooler season growth): small, thick-walled, relatively dark

• rings of alternating early/late wood result in annual growth rings, and width of rings can indicate growing conditions each year