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Angiosperms: Phylum Anthophyta, the flowering plants

1.  Overview of seed plant evolution 2.  Traits of flowering plants 3.  The angiosperm life cycle 4.  Seed (fruit) dispersal (continued) 5.  Seed morphology and germination 6.  Hormones (Auxin) briefly (we’ll come

back to this)

Fruit and Seed dispersal •  Fruits protect seeds during development and

sometimes aid in their dispersal •  Fleshy fruits or seeds are adapted to animal

dispersal •  Dry fruits can be adapted to air or water dispersal,

animal dispersal, or to release the seeds at maturity •  Seeds themselves often have their own dispersal-

adapted morphology, and adaptations for survival and germination

Fruit adaptations that enhance seed dispersal: Red berries are animal dispersed, while dandelion fruits are wind-dispersed (right). Some fruits, such as these burrs, hitch a ride

on the fur of animals

The seeds of many plants have elaiosomes—fleshy attachments which attract ants. Ants carry the seeds back to their nests, eat the elaiosome, and often discard the seed. (One example is our native wild ginger, Asarum caudatum)

Don’t forget: many plants also reproduce asexually. Two examples: the maternity plant (Kalanchoe, left), aspen (Populus) groves (right)

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Seed adaptations for survival and germination

•  Many seeds exhibit dormancy, a temporary condition of low metabolism and no growth or development. Some seeds can survive like this for decades or more. What are the potential benefits of dormancy?

•  Dormancy in some seeds is simply broken by favorable environmental conditions, but others only germinate after specific cues

•  What would you expect the cues to be for seeds living in deserts, fire-prone habitats (such as California chaparral), or at high latitudes? How about for seeds borne in berries eaten by mammals?

Figure 38. 8 Review: Three types of seed structure

The four steps of seed germination: 1. imbibition of water, 2. enzyme digestion of stored food, 3. embryo begins growth and radicle is pushed through the seed coat, and 4. shoot tip grows toward soil surface.

Germination of a barley seed is shown below.

Figure 38.10 Two ways that young shoots break through the soil surface.

Figure 38.7 The development of a dicot plant embryo Hormones

One bad apple…

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A word about Auxin:

Auxin is a term used for any chemical substance that promotes the elongation of coleoptiles (though they have multiple functions in monocots and eudicots)

The natural Auxin occuring in most plants is IAA - indoleacetic acid

Plants exhibit phototropism and gravitropism

•  Phototropism: in response to light (shoots)

•  Gravitropism: in response to gravity (pressure really) – roots

•  Thigmotropism: in response to mechanical pressure/touch (vines)

How do plants grow toward the light? (phototropism)

Auxin causes cell elongation along the dark side of the shoot, causing the shoot to bend towards light

It also works in conjunction with other hormones….for example in fruit ripening

A word about Auxin:

Auxin is a term used for any chemical substance that promotes the elongation of coleoptiles (though they have multiple functions in monocots and eudicots)

The natural Auxin occuring in most plants is IAA - indoleacetic acid

Thigmotropism: pressure/mechanical

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What auxin does in plants

•  Promotes shoot elongation •  Suppresses shoot branching (apical

dominance) •  Inhibits root elongation •  Promotes root branching •  Promotes growth of fruit

Structure and Growth

Plant ‘systems’ •  The plant body has a hierarchy of organs,

tissues, and cells, like multicellular animals –  Have organs composed of different tissues, which are

in turn composed of cells •  The basic morphology of vascular plants

–  Reflects their evolutionary history as terrestrial organisms that draw nutrients from two very different environments: below-ground and above-ground

Figure 35.2 Morphology of a flowering plant: an overview

•  Three basic organs evolved: roots, stems, and leaves

•  They are organized into a root system and a shoot system

Roots •  A root

–  Is an organ that anchors the vascular plant – Absorbs minerals and water – Often stores organic nutrients

•  In most plants – The absorption of water and

minerals occurs near the root tips, where vast numbers of tiny root hairs increase the surface area of the root

Roots cont.

•  Gymnosperms and eudicots: taproots with lateral roots

•  Seedless vascular and monocots: fibrous root system: spread out

•  Many plants have modified roots: adventitious roots arise above ground from stems and even leaves

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Many plants have modified roots

(a) Prop roots (b) Storage roots (c) “Strangling” aerial roots

(d) Buttress roots (e) Pneumatophores

Stems (shoot system)

•  Nodes: point of leaf attachment •  Internodes: segments between nodes •  Axillary buds can form new shoots or

branches •  Terminal buds can lead to apical

dominance (grow up!)

Figure 35.4 Modified shoots: Stolons, strawberry (top left); rhizomes, iris (top right); tubers, potato (bottom left); bulb, onion (bottom right)

Leaves

•  Main photosynthetic organ - but in many, the stems can dominate too.

•  Blade and petiole (monocots don’t have petioles - base of the leaf forms a sheath around leaf.

•  Leaf types:

Leaf types

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Figure 35.5 Simple versus compound leaves

Leaf types

Figure 35.6 Modified leaves: Tendrils, pea plant (top left); spines, cacti (top right); succulent (bottom left); brightly-colored leaves, poinsettia (bottom right) Figure 35.19 Leaf anatomy

Note: “Cuticle” = waxy or fatty layer on outer wall of epidermal cells

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The plant body – organs: root and shoots •  Root system and shoot system are designed to

draw ‘nutrients’ from two very different habitats •  Many are modified to accommodate specific

conditions or strategies •  Leaf types can be confusing (like fruits), but

you need to know them •  Leaf anatomy includes many different kinds of

cells: living to dead. Pallisade parenchyma and spongy parenchyma can help determine orientation of leaf and stomata.

•  Some leaf morphologies are adapted for specific conditions: oleanders and water conservation

The plant cell—tissue systems, tissues, and cells

A review of the tissue systems and cells – this is really here for you to read and review…

Pay attention to the ‘new context’ – we’ll look at monocots vrs. eudicots

Figure 35.7 The three tissue systems

Figure 35.7 The three tissue systems Tissue Systems

Tissues/cells

Ground Parenchyma

Collenchyma

Sclerenchyma

Vascular Xylem

Phloem

Dermal Epidermis

Figure 35.18 Primary tissues, and their functions, in young stems

The three tissue systems

Epidermal tissue: protection, control of water loss, and a variety of other functions

Parenchyma tissue: photosynthesis, storage, and/or secretion

Collenchyma tissue: support of young, growing parts

Xylem tissue: water & mineral transport; Phloem tissue: “food” (sugar) transport

Sclerenchyma tissue: support of mature plant parts

(a.k.a. “pith ray”)

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Figure 35.16 Organization of primary tissues in young stems. Note difference from root: vascular tissue is arranged in bundles, with ground tissue in center. Also note difference in arrangement of bundles between dicot and monocot.

Figure 35.15 Organization of tissue systems and tissues in young roots

“Epidermis:” Dermal system, epidermal tissue

“Cortex:” Ground system, parenchyma tissue

“Stele:” Vascular system, xylem & phloem tissues

Figure 35.13 Organization of primary tissues in young roots. Note the difference between the monocot and the dicot in the arrangement of the xylem & phloem in the stele. Tissue Systems Tissues

(& cell types)

Ground Parenchyma (parenchyma cells, transfer cells)

Collenchyma (collenchyma cells)

Sclerenchyma (fibers & sclereids)

Vascular Xylem (tracheids or vessel members, also some parenchyma cells, fibers, & sclereids )

Phloem (sieve cells or sieve-tube members, also specialized parenchyma cells called companion or albuminous cells, some fibers & sclereids)

Dermal Epidermis (ground cells, guard cells, trichomes, and others, also some fibers & sclereids)

Three tissue systems of plants

•  Dermal tissue - outer protective covering – Epidermis/periderm analogous to skin – Cuticle - waxy coating to preserve water

•  Vascular tissue - transport system – Xylem: carries water and nutrients from roots to

leaves. Support and food storage too. – Phloem: transport organic nutrients (sugar),

amino acids, lipids, hormones etc. •  Ground tissue - “everything else”.

– Pith (internal to vascular), Cortex – Function in storage, photosynthesis, & support

Figure 35.19 Leaf anatomy

Note: “Cuticle” = waxy or fatty layer on outer wall of epidermal cells

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•  Primary Tissues are arranged in a ‘ring’ in eudicot stems, and are scattered in monocot stems

•  The stele, within the root, contains xylem that is more centralized in eudicots and more scattered in monocots

•  You remember all those cell types

Plant Growth 1.  Meristems and overview of plant growth

2.  Apical meristems and primary growth

3.  Lateral meristems and secondary growth

Figure 35.10 Locations of major meristems Remember: A major adaptation of land plants is the meristem—a region of perpetual cell division that allows the plant to grow rapidly

There are two types of meristems: Apical and lateral.

Apical meristems lengthen the plant (“primary growth”). Roots and shoots

Lateral meristems thicken the plant with bark and wood (“secondary growth”).

Figure 35.11 Illustration of primary and secondary growth: Morphology of a winter twig

Plant Growth

1.  Meristems and overview of plant growth

2.  Apical meristems and primary growth

3.  Lateral meristems and secondary growth

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Figure 35.12 Primary growth of a root. Notice that the apical meristem produces three primary meristems, which produce the three primary tissue systems (dermal, ground, and vascular).

Figure 35.13 Organization of primary tissues in young roots. Note the difference between the monocot and the dicot in the arrangement of the xylem & phloem in the stele.

Figure 35.14 The formation of lateral roots. Lateral roots arise from the pericycle—the outermost cell layer of the stele. Figure 35.12 Primary growth of a root. Notice that the apical meristem

produces three primary meristems, which produce the three primary tissue systems (dermal, ground, and vascular).

Figure 35.15 The terminal bud and primary growth of a shoot. Just like in the root, the apical meristem produces three primary meristems, which develop into the three tissue systems.

Figure 35.16 Organization of primary tissues in young stems. Note difference from root: vascular tissue is arranged in bundles, with ground tissue in center. Also note difference in arrangement of bundles between dicot and monocot.

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Plant Growth

1.  Meristems and overview of plant growth

2.  Apical meristems and primary growth

3.  Lateral meristems and secondary growth

Figure 35.10 Locations of major meristems Remember: Secondary growth happens at the 2 lateral meristems

The interior lateral meristem, called the vascular cambium, produces secondary xylem (to the interior) and phloem (to the exterior).

The exterior lateral meristem, called the cork cambium, produces cork cells to the exterior.

Figure 35.18 Secondary growth of a stem (Layer 1) Vascular cambium:

Fusiform initials make cells elongated vertically, form secondary xylem (to inside) and secondary phloem (to the outside)

Ray initials make cells elongated horizontally-transfer water and nutrients, store starch

Figure 35.18 Secondary growth of a stem (Layer 2)

Cork cambium: meristem for tough thick covering - replaces epidermis. Forms from cortex to fill in gaps as epidermis is stretched and broken by lateral expansion of stem

Rays maintain connectivity between secondary xylem and secondary phloem, allow exchange of nutrients and water, store starch and organic nutrients

Figure 35.18 Secondary growth of a stem (Layer 3) Anatomy of a three-year-old stem. How can you tell the stem is 3 years old?

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Secondary growth of a stem. How many years old is it? Notes:

“Wood” = secondary xylem, or everything on the interior side of the vascular cambium

“Bark” = everything exterior of the vascular cambium, including the secondary phloem and the periderm (cork cambium + cork)

Also remember: Secondary growth happens in roots too.

Figure 35.20 Anatomy of a tree trunk

Oldest xyelm nonfuctional

Oldest phloem

sloughed off

A summary of primary and secondary growth in a woody stem

A summary of primary

and secondary growth in a woody stem

Figure 35.10 Locations of major meristems Remember: A major adaptation of land plants is the meristem—a region of perpetual cell division that allows the plant to grow rapidly

There are two types of meristems: Apical and lateral.

Apical meristems lengthen the plant (“primary growth”).

Lateral meristems thicken the plant with bark and wood (“secondary growth”). There are two of these

Oldest xyelm nonfuctional

Oldest phloem

sloughed off

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Oldest xyelm nonfuctional

Oldest phloem

sloughed off

Record of events from xylem rings

•  Old growth trees can provide disease and fire records

Aztecs and plagues…