Plant Structure Growth Development Chapter 35 Plant Responses

Plant Structure, Growth & Development Chapter 35 _______ Plant Responses Chapter 39

Chapter 35

Hierarchical Organization in Plants Three basic plant organs: 1. Roots 2. Stems 3. Leaves

Roots Anchors vascular plant in soil Absorbs minerals and water Stores carbohydrates Root Parts: • Taproot – main vertical root of eudicots and gymnosperms, develops from embryonic root • Lateral roots – branch from taproot In monocots, embryonic root dies and small roots emerge from stem – fibrous root system

Stems Raise or separate leaves, exposing them to sunlight Also raise reproductive structures to facilitate pollen dispersal Alternating system of: • Nodes – point where leaves are attached • Internodes – stem segments between nodes Most of the growth of a young plant is in the apical (terminal) bud Leaves emerge at the axillary bud

Leaves Main photosynthetic organ in vascular plants Consist of: • Blade – generally flat • Petiole – stalk that join blade to stem at node Monocots – parallel veins Eudicots – branching veins Simple vs Compound leaves: • Simple- single, undivided blade • Compound – blade consists of multiple leaflets • Doubly compound – each leaflet is divided into smaller leaves

Evolutionary Adaptations of Leaves Tendrils – “lassoes” something to provide more support for the stem Spines – protection, do not photosynthesize Storage leaves – succulents Reproductive leaves – make plantlets that fall off and take root in soil Bracts – often mistaken for petals; surround a group of smaller flowers and attract pollinators

Tendrils Spines Storage Reproductive Bracts

Plant Tissues Dermal – plant’s outer, protective covering (epidermis and cuticle) Vascular – carries out long distance transfer of materials between roots and stems (xylem & phloem) Ground – neither dermal nor vascular; performs specialized functions such as storage, photosynthesis & support

Plant Cells Parenchyma – perform most of the metabolic functions of plant Collenchyma – flexible, support young parts of plant shoot Sclerenchyma – rigid support for mature plants, especially trees

Plant Cells, cont. Water conducting cells in Xylem (dead at functional maturity) • Tracheids • Vessel elements Sugar-conducting cells in Phloem (alive at functional maturity) • Sieve tube elements • Sieve plates • Companion cells

Growth in Plants Unlike animals, plant growth is not limited to an embryonic and juvenile period Indeterminate growth – at any time the plant as a whole has embryonic, developing and maturing organs Leaves, thorns and flowers undergo determinate growth

Growth in Plants, cont. Meristems - specific regions of growth • Contain stem cells: perpetually embryonic tissue • regenerate new cells Apical shoot & root meristem o Provide growth in length o primary growth Lateral meristem o Provide growth in girth o secondary growth

Apical meristems shoot root

Root structure & growth protecting the meristem

Woody stem How old is this tree? cork cambium vascular cambium late early 3 2 1 xylem phloem bark

Chapter 39

Response to Stimuli At the organismal level, plants and animals respond to environmental stimuli by different means Animals, being mobile, respond mainly by moving toward positive stimuli and away from negative stimuli Plants are stationary and generally respond to environmental cues by adjusting their individual patterns of growth and development • For this reason, plants of the same species can have much more variation in body form than animals of the same species

Tropisms Tropism – any growth response that results in plant organs curving toward or away from a stimulus • Phototropism – growth of a shoot toward (positive) or away (negative) from a light source • Photoperiodism – physiological response to a photoperiod (like flowering) • Gravitropism – growth of shoots and roots in response to the pull of gravity • Thigmotropism – directional growth in response to touch

Growth in Plants Etiolation – morphological adaptations for growing in darkness • Pale stems • Rapid stem elongation • Unexpanded leaves • Short, stubby roots • Example – young potato plant in the soil

De-etiolation (greening) • Stem elongation slows • Leaves expand • Roots elongate • Shoot produces chlorophyll • Receptor in cytoplasm called a phytochrome is responsible

Plant Hormones Hormone – signaling molecule that is produced in tiny amounts by one part of an organism’s body and transported to other parts, where it binds to a specific receptor and triggers responses in target cells and tissues (phew!!!!) Plant hormone = plant growth regulator One hormone can regulate a diverse array of cellular and developmental processes Multiple hormones can influence a single process

Plant hormones Auxin Gibberellins Abscisic acid Ethylene and more…

Auxin (IAA) Effects controls cell division & differentiation phototropism • growth towards light • asymmetrical distribution of auxin • cells on darker side elongate faster than cells on brighter side apical dominance

Gibberellins Family of hormones • over 100 different gibberellins identified Effects • stem elongation • fruit growth • seed germination plump grapes in grocery stores have been treated with gibberellin hormones while on the vine

Abscisic acid (ABA) Effects slows growth seed dormancy • high concentrations of abscisic acid • germination only after ABA is inactivated or leeched out • survival value: seed will germinate only under optimal conditions • light, temperature, moisture

Ethylene Hormone gas released by plant cells Effects • fruit ripening • leaf drop o like in Autumn o apoptosis

Fruit ripening Adaptation • hard, tart fruit protects developing seed from herbivores • ripe, sweet, soft fruit attracts animals to disperse seed Mechanism • triggers ripening process o breakdown of cell wall - softening o conversion of starch to sugar - sweetening • positive feedback system o ethylene triggers ripening o ripening stimulates more ethylene production

Apoptosis in plants Many events in plants involve apoptosis • response to hormones o ethylene o auxin • death of annual plant after flowering o senescence • differentiation of xylem vessels o loss of cytoplasm • shedding of autumn leaves What is the evolutionary advantage of loss of leaves in autumn?