Plant Structure and Growth Topic 9.1
To review • Study the chart on pg 239 for your different types of plants • The seeded vascular plants include the angiosperms and gymnosperms • Do you remember the difference between angiosperms and gymnosperms??? • In this unit we will be referring to angiosperms unless otherwise noted.
3 tissue systems in plants • Dermal • Vascular • Ground
Dermal tissue • Outer protective covering • Single cell layer • Root hairs are extensions of epidermal cells • Most dermal tissue is covered by waxy cuticle which prevents water loss
Vascular tissue • Includes xylem and phloem
xylem • xylem is dead at maturity, only secondary cell wall remains • Consist of 2 cell types, either vessel elements (wide/short) or tracheids (long, thin, tapered). • carries water and minerals • Pits between cells allow water to flow through
phloem • Carries sucrose through cells called sieve-tube members. • Alive at maturity but lack nuclei, ribosomes and vacuoles. • Companion cells lie adjacent and provide for sieve tube members via plasmodesmata
Ground tissue (3rd type!) • Photosynthesis, support, storage • “filler” tissue • In dicots, center of stem is GT referred to as pith, outer stem GT is cortex.
Ground tissue made of 3 cell types • Parenchyma – unspecialized cells. Lack secondary walls, have large central vacuole. Important in psyn and food storage. All plant cells begin as unspecialized parenchyma cells. • Collenchyma – lack secondary walls but have thick primary walls. Form strands which support plant parts. • Sclerenchyma – thick secondary walls w/ lignin (strength). Include fibers and sclerids
All 3 types of tissue originate from meristematic tissue. • Meristematic tissue retains the ability to divide. • If a meristematic cell divides one cell begins differentiation (the derivative) and the other cell remains meristematic (the initials) • Where would you expect to find meristematic cells in a plant?
Roots • Obtain water and minerals • Anchor plant • May store food • Have a protective epidermis • Cortex conducts water from soil to interior vascular tissue. May also store material • Endodermis surrounds vascular tissue • Vascular tissue
Shoot system • Stem – alternating nodes (point of leaf attachment) w/ internodes in between. • Axillary bud – at angle between leaf and stem • Terminal bud – developing leaves and compacted nodes and internodes at tip of plant • Apical dominance – when terminal bud inhibits growth of axillary bud.
Leaves • Blade – flat, photosynthetic • Petiole – stalk of leaf monocots – leaves lack petiole, veins are parallel dicots – leaves have netted venation, petiole.
Leaf Anatomy • Covering of wax over epidermis. Stomata, tiny pores surrounded by guard cells permit gas exchange • Mesophyll – parenchyma and ground tissue containing chloroplasts. 2 layers, spongy (air spaces) and palisade (lots of chloroplasts) • A branch of the vascular bundle continues into petiole and divides in leaf blade, providing support and transport
Monocot vs Dicot • See page 242 to review
Roots - 2 main types • Taproot – one main vertical root with branch roots from main root • Fibrous root – no main root. Roots are thinner and spread throughout soil. Good for preventing erosion
Root modifications • Prop root – adventitious root growing from lower part of stem as a brace (corn) • Storage root – parenchyma cells store carbohydrates and water (beet, carrot) • Pneumatophore (air root) – extend above soil or water surface. Help with oxygen uptake (mangrove, cypress knee) • Buttress root – at bottom of tree for stability (fig tree)
Storage Prop Buttress Pneumatophore
Stem Modifications • Bulb – vertical underground stem with enlarged base for food storage (onion) • Tuber – horizontal underground stem for starch storage (potato) • Rhizome – horizontal stem just below surface for asexual reproduction (ginger) • Stolon – horizontal above ground stem for asexual reproduction (strawberry)
Bulb Tuber Stolon Rhizome
Leaf Modifications • Tendril – coil around objects for support (peas) • Reproductive leaves – tiny plants form on leaf margins. Fall to ground and take root • Bracts – AKA floral leaves. Surround flowers to attract pollinators. • Spines – reduce water loss
Bract Tendril Spine Reproductive Leaf
Meristem – 2 types • Apical meristem – found at tips of root and shoot. Produces primary tissue (non-woody) and primary growth (growth in length) • Lateral meristem – produces secondary (woody) growth. 2 types: • Vascular cambium produces secondary xylem and phloem (see next slide) • Cork cambium produces cork cells of outer bark
Secondary Growth • Wood is the accumulation of secondary xylem with lignified walls. • Growth rings occur due to seasonal cycles of growth (dormant, fast growth, slow growth) • In secondary growth epidermis splits and is replaced by tissue made by cork cambium which produces cork cells with suberin – impregnated walls. This layer is called periderm. • Lenticels are splits in periderm through which gas exchange occurs.
Bark = phloem + periderm • In old trees, heartwood is old resin filled wood while sapwood is actively conducting
Auxins • Plants have hormones too! • Auxins (a class of hormones) are found in embryos (seeds), apical meristems and buds. They increase the flexibility of cell walls so shoots can bend toward light • The auxin indoleacetic acid (IAA) collects on the side of the stem AWAY from the light causing that side to elongate and the plant to bend TOWARD light
Tropisms – growth or movement toward or away from a stimulus • Phototropism – light • Thigmotropism – touch • Geotropism – gravity • Chemotropism – chemicals • Ex) plant shoots exhibit positive phototropism and roots exhibit negative phototropism