Plants The Essentials Classification of Plants Bryophytes Live

Plants The Essentials …

Classification of Plants Bryophytes – Live in moist environments and absorb water by diffusion – Lack lignin-fortified tissue required to support tall plants on land – Predominant life stage is the gametophyte generation – Ex: mosses

Classification of Plants Tracheophytes – Have transport vessels: xylem and phloem – Includes ancient seedless plants, like ferns, that reproduce by spores – Includes modern plants that reproduce by seeds Those with seeds are further subdivided into gymnosperms and angiosperms

Classification of Plants Gymnosperms – conifers or cone-bearing plants – Needle-shaped leaves with thick waxy cuticles – Ex: cedars, sequoias, redwoods, pines, yews, junipers Angiosperms – Flowering plants – Subdivided into monocots and dicots – Ex: roses, grasses, maples trees, fruit trees

Angiosperms: Monocots vs Dicots

Strategies That Allowed Plants to Move to Land Modern plants are believed to have descended from green algae (Chlorophyta ) Plants began life in the sea and moved to land Problems faced by moving to land included: – Supporting the plant body – Absorbing and conserving water

Strategies That Allowed Plants to Move to Land Modifications to live on land include: – Cell walls made of cellulose to support plant mass and shape – Roots and root hairs to absorb water & nutrients – Stomata to regulate water loss and allow gas exchange with the environment – Waxy cuticle to prevent water loss – Development of xylem & phloem

Plant Anatomy

Organs & Organ Systems in Plants Three basic organs evolved in plants: – Roots – Stems – Leaves They are organized into a – Root system and a – Shoot system have

Plant Structure & Function Roots Functions: – Anchoring the plant in the soil – Absorption minerals and water (1° root tip & hairs) – Storage of food.

Plant Structure & Function Roots Types of Root Systems: – Fibrous Roots consist of a mat of thin roots that spread out below the soil surface Monocots, including grasses, generally have fibrous root systems. This extends the plant’s exposure to soil water and minerals and anchors it tenaciously to the ground.

Plant Structure & Function Roots Types of Root Systems: – Tap Roots consist of a one large vertical root (the taproot) that produces many small lateral, or branch roots Many dicots have a taproot system Taproots not only anchor the plant in the soil but often store food that supports flowering and fruit production later.

As mentionned earlier, most absorption of water and minerals occurs near the root tips, where vast numbers of tiny root hairs increase the surface area enormously. – Root hairs are extensions of individual epidermal cells on the root surface.

Modified root systems include: Prop roots Aerial roots Adventitious roots

Plant Structure & Function Stems Organ consisting of an alternating system of nodes (point where leaves attach) and internodes, the segments between nodes Contains: – A terminal bud located near the shoot tip that causes elongation of a young shoot – Axillary buds, structures that have the potential to form a lateral shoot (branch)

LE 35 -11 Terminal bud Bud scale Axillary buds Leaf scar This year’s growth (one year old) Node Stem Internode One-year-old side branch formed from axillary bud near shoot apex Leaf scar Last year’s growth (two years old) Scars left by terminal bud scales of previous winters Growth of two years ago (three years old) Leaf scar

Modified stems include: Stolons, strawberry (top left); rhizomes, iris (top right); tubers, potato (bottom left); bulb, onion (bottom right)

Plant Structure & Function Leaves Function: – Main photosynthetic organ of most vascular plants – Some plants have leaves specialized for other functions such as support, protection, water storage, or reproduction

Simple versus compound leaves And as seen on the next slide, some leaves have been modified for other special functions besides photosynthesis ….

Spines Tendrils Bracts Storage Reproductive

Internal Structure of a Typical Plant Leaf Key to labels Guard cells Dermal Stomatal pore Ground Vascular Cuticle Sclerenchyma fibers Epidermal cells 50 µm Surface view of a spiderwort (Tradescantia) leaf (LM) Stoma Upper epidermis Palisade mesophyll Bundlesheath cell Spongy mesophyll Lower epidermis Guard cells Cuticle Vein Xylem Phloem Cutaway drawing of leaf tissues Guard cells Vein Air spaces Guard cells 100 µm Transverse section of a lilac (Syringa) leaf (LM)

Plant Growth Plants can be classified as: – Annuals Life cycle completed in one year – Marigolds, impatiens, vegetables – Biannuals Life cycle completed in two years – Foxglove, a plant from which digitalis is derived – Perrenials Life cycle continues for many years – Trees, shrubs, peonies, hosta,

Plant Growth Plants continue to grow as long as they live b/c plants have embryonic tissues called meristems that continually divide and generate new cells Pattern of growth is a function of the location of meristem – Apical (primary growth = length) – Lateral (secondary growth = width)

Figure 35. 12 Locations of major meristems: an overview of plant growth

Primary Growth Apical meristem is located at the tips of the roots and in the buds of shoots and allows for primary growth In the root you see the following zones of primary growth – Zone of cell division – Zone of elongation – Zone of maturation (differentiation)

Secondary Growth Lateral meristems allow for secondary growth which is an increase in girth Herbaceous plants have only primary growth Woody plants have secondary growth – Responsible for the thickening of roots & shoots

Secondary Growth Two different lateral meristems exist: – Vascular Cambium provides secondary xylem (wood) and secondary phloem – Cork Cambium Produces a tough covering called periderm that replaces epidermis Bark – All tissues outside the vascular cambium – Includes phloem derived from the vascular cambium, the cork cambium, and the tissues derived from the cork cambium

LE 35 -10 Primary growth in stems Shoot apical meristems (in buds) Epidermis Cortex Primary phloem Primary xylem Vascular cambium Lateral meristems Cork cambium Pith Secondary growth in stems Pith Periderm Cork cambium Cortex Primary phloem Primary xylem Root apical meristems Secondary xylem Secondary phloem Vascular cambium

LE 35 -18 a Primary and secondary growth in a two-year-old stem Epidermis Cortex Primary phloem Vascular cambium Primary xylem Pith Primary xylem Vascular cambium Primary phloem Cortex Phloem ray Xylem ray Epidermis th Grow Primary xylem Secondary xylem Vascular cambium Secondary phloem Primary phloem First cork cambium Periderm (mainly cork cambia and cork) Cork th Grow Primary phloem Secondary phloem Vascular cambium Secondary xylem Primary xylem Pith Secondary xylem (two years of production) Vascular cambium Secondary phloem Bark Most recent cork cambium Cork Layers of periderm

LE 35 -18 b Secondary phloem Vascular cambium Secondary xylem Cork cambium Late wood Early wood Periderm Cork Xylem ray Bark 0. 5 mm Transverse section of a three-yearold Tilia (linden) stem (LM)

Plant Tissues Three major types of tissues in plants – Dermal Tissues – Ground Tissues – Vascular Tissues

Figure 35. 7 The three tissue systems

Dermal Tissues Covers and protects the plant Includes endodermis, epidermis, and modified cells such as guard cells, root hairs, and the cells that produce the waxy cuticle

Ground Tissues The most abundant type of tissue in plants Found between dermal and vascular tissue Primary function is support, storage, and photosynthesis Three types – Parenchyma – Collenchyma – Sclerenchyma

Ground Tissues Parenchyma – “traditional” plant cells Some are photosynthetic and contain chloroplasts Others store sugars in plastids Called mesophyll cells in leaves – Primary cell walls are thin and flexible – No secondary cell walls

Ground Tissues Collenchyma – Thickened primary but no secondary cell wall – Support the growing stem Sclerenchyma – Thick primary & secondary cell walls fortified with lignin – Two forms are fibers (rope) and sclerids (seed coats)

Vascular Tissues Xylem – Carries fluids and dissolved minerals upward from roots into the shoots. – Composed of tracheids and vessel elements Elongated cells that are dead at functional maturity whose thickened cell walls form a nonliving conduit through which water can flow. Water moves through pits between tracheid cells

Vascular Tissues Phloem – Transports sugars made in mature leaves to the roots and to nonphotosynthetic parts of the shoot system. – Consists of sieve tube members whose end walls have sieve plates that facilitate flow of fluid from one cell to the next – Sieve tubes are living cells but are missing a nucleus and ribosomes and vacuoles – Companion cells are associated with the sieve tubes and nurture the sieve tube elements/connected to sieve tube via numerous plasmodesmata

Transport in Plants See Ch 36, 7 th ed. lectures….

Roots Epidermis-cortex-stele (vascular cylinder) Stele is surrounded by the pericycle

LE 35 -13 Epidermis Cortex Vascular cylinder Endodermis Pericycle Core of parenchyma cells Xylem 100 µm Phloem 100 µm Transverse section of a typical root. In the roots of typical gymnosperms and eudicots, as well as some monocots, the stele is a vascular cylinder consisting of a lobed core of xylem with phloem between the lobes. Endodermis Pericycle Transverse section of a root with parenchyma in the center. The stele of many monocot roots is a vascular cylinder with a core of parenchyma surrounded by a ring of alternating xylem and phloem. Key Dermal Ground Vascular Xylem Phloem 50 µm

LE 35 -14 100 µm Emerging lateral root Cortex Vascular cylinder Epidermis Lateral root

Control of Stomata

Transport of Water

Transport of Sugars

Plant Hormones – Substances produced by specialized cells in one part of an organism that influence physiology elsewhere in the organism – Typically needed in small amounts – Response depends on type of hormone, concentration of hormone, & the receptor on the target cell

Plant Hormones, cont’d Auxin or IAA (indoleacetic acid) – Promotes plant growth by facilitating elongation of developing cells – Produced at the tips of shoots and roots – Also active in leaves, fruits, and germinating seeds – Also influences plant’s response to light and gravity – Modified tryptophan amino acid

Plant Hormones, cont’d Gibberellins – Group of hormones that promote cell growth – Typically synthesized in young leaves, roots, and seeds but is often transported elsewhere – Also involved in promotion of fruit development and seed germination and inhibition of aging in leaves – Can lead to plant “bolting” if GA is too high

Dwarf pea plant treated with giberellin. Normal pea plants do not respond to giberellin since they already produce enough of this hormone naturally.

Seedless grapes treated with giberellin

Plant Hormones, cont’d Cytokinins – Group of hormones that stimulate cytokinesis (cell division) – Produced in roots and transported elsewhere – Influences organogenesis in plants Relative amounts of auxins and cytokinins determine whether roots or shoots will develop – Cytokinins stimulate growth of lateral buds – Can also delay senescence (aging) of leaves or fruit

Plant Hormones, cont’d Ethylene – Gas that promotes the ripening of fruit – Also involved in the stimulation of flower development – In combination with auxins, ethylene inhibits elongation of roots, stems, and leaves – Influences leaf abscission, aging & dropping of leaves

Plant Hormones, cont’d Abscissic Acid – Growth inhibitor – Maintains dormancy in seeds – Influence on abscission is controversial

Plant Responses to Stimuli Since plant roots are anchored, plants cannot move in response to environmental stimuli Instead, they change their growth pattern Tropism = growth pattern in response to an environmental stimulus

Plant Responses to Stimuli, cont’d Phototropism – Movement towards light – Influenced by auxin – Produced in the apical meristem – Moves downward by active transport into zone of elongation to stimulate elongation

Plant Responses to Stimuli, cont’d Phototropism & Auxin, cont’d – When all sides of the apical meristem are equally well lit, growth of stem is uniform and stem is straight – When stem is unequally lit, auxin concentrates on the shady side of the stem to stimulate elongation – Resulting differential growth pattern leads shadier side to grow more and causes the plant to turn towards the light

Plant Responses to Stimuli, cont’d Gravitropism – Response to gravity – Roots grow down, stems grow up – Mechanism not well understood – Relative [ ] of auxins & gibberellins are involved but their action depends on whether the target organ is the root or the stem

Plant Responses to Stimuli, cont’d Thigmotropism – Response to touch – Ex: Vines touching other objects respond by wrapping around the object

Photoperiodism Response of plants to change in photoperiod, the relative length of day and night Plants maintain a circadian rhythm, a clock that measures the length of day and night Mechanism is endogenous – Clock is internal and works even in the absence of light – Dawn & dusk reset the clock to maintain accuracy

Photoperiodism, cont’d Phytochromes exist as Pr or Pfr The two forms (Pr absorbs red, 660 nm / Pfr absorbs far-red, 730 nm) are photoreversible when exposed to light of appropriate wavelength Pr is the form synthesized in plant cells Pfr is the active form

Photoperiodism, cont’d Ratio of forms Pr to Pfr determines time of day Pr = Pfr during daylight hours Pr > Pfr during the night Night length resets the clock – Measurement of night length can be influenced by flashes of light during the night hours – Red light shortens the night length – Far-red light restores the night length

Photoperiodism, cont’d Plants initiate flowering in response to changes in photoperiod Long-day plants – Flower in spring & early summer when daylight is increasing Short-day plants – Flower in late summer and early fall when daylight is decreasing – Flower when night exceeds a critical length Day-neutral plants – Do not flower in response to daylight changes – Respond to other cues such as temperature

Plants Short-Day (long night) vs Long Day (short night)

Note: R signals day FR signals night

Plant Nutrition The uptake of nutrients occurs at both the roots and the leaves. – Roots, through mycorrhizae and root hairs, absorb water and minerals from the soil. – Carbon dioxide diffuses into leaves from the surrounding air through stomata.

Plant Nutrition Elements that plants need in larger amounts are macronutrients. – The nine macronutrients are carbon, hydrogen, oxygen, nitrogen, sulfur, phosphorous, potassium, calcium, and magnesium. – Nitrogen is often the mineral that has the greatest impact on plant growth

Plant Nutrition In order for plants to absorb nitrogen, it must be in the form of NH 4+ or NO 3 - (nitrate is favored) The main source of nitrogen for plants is the decomposition of humus by microbes Review the nitrogen cycle from ecology section: nitrogen fixing bacteria – ammonifying bacteria - nitrifying bacteria – denitrifying bacteria

Plant Nutrition Elements that plants need in very small amounts are micronutrients. – The eight micronutrients are iron, chlorine, copper, zinc, magnanese, molybdenum, boron, and nickel. – Most of these function as cofactors of enzymatic reactions. – For example, iron is a metallic component in cytochromes, proteins that function in the electron transfer chains of chloroplasts and mitochondria. – While the requirement for these micronutrients is so modest a deficiency of a micronutrient can weaken or kill a plant.

Plant Nutrition Plant nutritional adaptations often involve relationships with other organisms: Mutualism – Legumes have nodules composed of plant cells that contain nitrogen-fixing bacteria – In mycorrhizae, a fungus gains food in the form of sugar from a plant and gives the plant an increased water supply, minerals, substances to stimulate root growth, and antibiotics to fight bacterial infection

Plant Nutrition Plant nutritional adaptations often involve relationships with other organisms: – Mistletoe is a parasitic plant. Some parasitic plants are not photosynthetic and rely on other plants for nutrition – Epiphytes: grow on the surface of other plants rather than in soil; they are not parasitic – Carnivorous plants are photosynthetic but they get some nitrogen and other minerals by digesting small animals – Insert pictures of the plants mentionned….

Plant Reproduction Plants can reproduce asexually by several means to produce clones of themselves by vegetative propagation – Grafting, cuttings, bulbs, runners Sexual reproduction in plants generates genetic variation

Plant Life Cycle Life cycle = alternation of generations Includes both haploid (gametophyte) and diploid (sporophyte) multicellular stages. Diploid sporophyte produces haploid spores by meiosis Haploid gametophyte produces haploid gametes by mitosis

Plant Life Cycles Bryophytes – Bryophytes such as moss are primitive plants and the gametophyte generation dominates the life cycle. – The gametophyte undergoes photosynthesis – Archegonia and antheridia develop on the tips of the gametophyte and produce eggs and sperm respectively – The resulting zygotes develop into sporophytes which depend on the gametophyte for survival

Bryophyte Life Cycle

Plant Life Cycles Pteridophytes (Ferns) – Sporophyte produces spores from sporangia found in clusters called sori on underside of leaf frond – The spores germinate and form a gametophyte called a prothallus – Within the prothallus, archegonium will produce eggs and antheridium produce sperm – Upon fertilization, a zygote forms and develops into a sporophyte

Sexual Reproduction in Flowering Plants In angiosperms, the sporophyte is the dominant generation Spores produced by meiosis lead to the development of the male and female gametophytes, the pollen and the embryo sac of the flower. Flowers contain the gametophytes and are the reproductive organs of angiosperms

Figure 38. 2 Review of an idealized flower

Figure 38. 3 ax 1 Lily

Sexual Reproduction in Flowering Plants The parts of a flower: Sepals – which protect the floral bud before it opens Petals – attract insects and other pollinators to the plant with their color and fragrance Stamens – male reproductive organs Carpels – female reproductive organs Note that some plants have flowers with both male and female reproductive structures, others do not

Figure 38. 3 f Sagittaria: staminate flowers (left), carpellate flowers (right)

Figure 38. 1 Simplified overview of angiosperm life cycle

The Seed A seed consists of an embryo, a seed coat, and some kind of storage material (endosperm or cotyledons) Cotyledons form upon digestion of storage material in the endosperm – Dried peas see mostly two cotyledons & small embryo – Corn see mostly endosperm and a single cotyledon

The Seed, cont’d The Embryo – Top portion of embryo = epicotyl and becomes the shoot tip – Two young leaves (first true leaves) called plumules are attached to the epicotyl – Hypocotyl becomes the young shoot – Radicle develops into the root – Monocots also have a coleoptile surrounding & protecting the epicotyl/ The coleoptile may appear as a leaf but it is not the first true leaf for the plant

Germination & Development Seeds remain dormant until specific environmental cues are encountered First step is imbibition, the uptake of water

Flower formation involves a phase change from vegetative growth to reproductive growth It is triggered by a combination of environmental cues and internal signals Transition from vegetative growth to flowering is associated with the switching-on of floral meristem identity genes