Plant Structure, Growth & Development

1. Plant Structure, Growth & Development Chapter 35 ______________ Plant Responses Chapter 39

2. Chapter 35

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

4. 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

6. 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

8. 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

10. 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

11. Tendrils Spines Storage Reproductive Bracts

12. 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

14. 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

16. 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

18. 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

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

20. Apical meristems shoot root

21. Root structure & growth protecting the meristem

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

23. Chapter 39

24. 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

25. 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

26. 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

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

28. 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

29. Plant hormones • Auxin • Gibberellins • Abscisic acid • Ethylene • and more…

30. 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

31. 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

32. 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

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

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

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