Plant Physiology – Plant Tissues

• Flowering plants consist of two major regions: the root system and the shoot system

• How do plants grow? plants grow throughout their lives – have two

major categories of cells: meristem cells – embryonic, undifferentiated

cells capable of cell division apical meristem – (tip) located at the ends of

roots and shoots – results in primary growth lateral meristem – (side) also called cambium

– growth in width – results in secondary growth

differentiated cells – specialized in structure and function

• Major structures of plants (roots, stems, leaves) consist of three tissue systems:

1.Dermal tissue – covers outer surfaces of the plant body–epidermal tissue (epidermis) –

outermost cell layer–periderm – replaces epidermal

tissue on the roots and stems of woody plants as they age

2. Ground tissue – all nondermal and nonvascular tissues

– parenchyma – most abundant, carry out most of metabolic activities (photosynthesis), also function in storage of sugars or starches, and secretion of hormones

– collenchyma – elongated, many-sided cells, source of support (especially in young or herbaceous plants)

– sclerenchyma – cells with thick, hardened secondary cell walls reinforced with lignin – also used to support and strengthen plant (found in xylem, phloem, nut shells, outer covering of peach pits)

3. Vascular tissue

A. Xylem – conducts water and minerals from roots to shoots – two types:• Tracheids – thin cells with slanted ends that

overlap – overlapping walls contain pits – allow water and

minerals to pass from one tracheid to next• Vessel elements – meet end to end, larger in

diameter than tracheids – ends either flat or overlapping– form wide-diameter “pipelines” called vessels

from roots to leaves• xylem develops thick walls to help support plant• when functional, xylem cells die leaving hollow

tube of cell wall

B. Phloem – conducts sugars, amino acids, and hormones– sieve tubes – constructed of a single strand of cells

called sieve-tube elements• adjacent cells meet at the sieve plates which have

holes – interiors of adjacent sieve-tube elements are connected through these holes forming continuous conducting system

– each sieve-tube element is nourished by a smaller adjacent companion cell – connected to sieve-tube element by cytoplasm-filled channels called plasmodesmata• companion cells regulate movements of sugars into

and out of sieve tubes

Plant Physiology/Anatomy – Roots • Root function: anchorage, absorption, and storage• Root systems

– taproot systems – consist of a primary root which becomes longer and stouter with time and many smaller roots that grow from primary root

– fibrous root systems – many slender roots of equal size

• Primary growth of roots – longitudinal section

• root cap – protective cover at tip covering apical meristem – protects against abrasive soil

• meristematic region – region of cell division

• elongation region – cells begin to elongate

• cell differentiation – cells mature and differentiate into various types of tissue (i.e. xylem, phloem, etc)

• root hairs – microscopic extensions of epidermis – increase surface area for water absorption

Root Cross Section

• epidermis – outer protective layer – layer from which root hairs grow

• cortex – loosely packed parenchyma cells used for storage (starch)

• vascular cylinder (stele) consists of:1. endodermis – regulates movement of

materials into center of root (xylem)• each cell has special bandlike region – Casparian

strip – casparian strips contain suberin (fatty, waterproof material)

2. pericycle – gives rise to lateral (secondary roots)3. xylem – forms “x” in center4. phloem – located in patches between xylem

“arms”5. vascular cambium – “sandwiched” between

xylem and phloem – gives rise to secondary vascular tissues

Plant Physiology/Anatomy – Stems• Main functions: support and conduction• All stems have:

1. buds – undeveloped, embryonic shoots• terminal bud – located at tip of stem – covered

with bud scales when dormant– leaves bud scars on stem between growing

seasons• axillary buds (lateral buds) – form stems bearing

leaves or flowers

2. nodes – area on a stem where each leaf is attached• internodes – region between successive nodes

Dicot stem cross section1. epidermis – outer

covering for protection• in woody stems,

contains cork cambium which produces periderm

• eventually replaces epidermis with cork cells that are waterproof and form bark

2.cortex – photosynthesis (herbaceous stems), storage, and support

3.Vascular bundles – arranged in a ring• xylem and phloem

separated by vascular cambium – results in secondary growth

4. Pith - center of herbaceous

stems used for storage

• woody stems – secondary xylem is made to interior, secondary phloem made to exterior of vascular cambium• younger, functional secondary

xylem is called sapwood (closest to bark)

• older wood in center of stem is heartwood – no longer functioning – plugged with pigments, tannins, gums, resins, and other materials – dense, provides support

Monocot stem cross section• vascular bundles are scattered• no vascular cambium – no secondary

growth

Dicot Stem Monocot Stem

Water (and mineral) transport in stems• First, water moves by osmosis from the soil

through the root into the xylem in the vascular cylinder • cells in root have more dissolved materials in

them compared to the soil

• Water follows two routes into the center of the root:

1. Symplastic route – through the cells

2. Apoplastic route – around the cell walls

Cohesion-Tension Theory • also called Transpiration–Cohesion • water is pulled up the xylem powered by the

evaporation of water from the leaves –transpiration plants lose water (as water vapor) through

microscopic pores on the underside of leaves – stomata

tension created by this process pulls water upward from root xylem into stem xylem (like sucking on a straw) – as water is pulled up, additional water from the soil is drawn into the roots

• only possible with an unbroken column of water in xylem throughout plant

– water forms unbroken column because water molecules are cohesive (tend to cling to each other because of hydrogen bonding) AND water molecules tend to cling to the walls of xylem (adhesion)

• Water potential gradient plays an important role in this theory

Water Potential• free energy of water, a measure of a cell’s ability

to absorb water by osmosis • water potential of pure water is 0 – when solutes

are dissolved in water, its free energy decreases (negative number) – water moves from a region of higher (less negative) water potential to a region of lower (more negative) water potential

• a water potential gradient exists in a plant from the least negative (the soil) up through the plant to the most negative (the atmosphere – has much less water in it than the soil) – this literally pulls water from the soil up through the plant

Root pressure • water that moves into a plant’s roots from the soil is

pushed up through xylem to the top - less important mechanism for water transport

• occurs because nutrient mineral ions are actively pumped into xylem, decreasing water potential

• water moves into root from soil because of difference in water potential btw soil and root cells

• may result in guttation – liquid water is forced out through special openings in leaves

Translocation of sugar in phloem• Sugar produced by leaves is converted into

sucrose and then loaded into phloem and translocated in solution to rest of plant

• Moves both upward and downward• Pressure-flow hypothesis – dissolved sugar

moves in phloem by means of a pressure gradient (difference in pressure) which results when sugar is translocated from a source (area of excess sugar supply, usually a leaf) to a sink (area of storage in the form of starch)

• sugar is moved from leaf into sieve tube members by active transport (increases concentration)

• water moves in after the sugar because of osmosis – this increases hydrostatic pressure

• at the destination (sink) sugar is unloaded from sieve tube members – reduces concentration and water tends to also flow out into surrounding tissues reducing hydrostatic pressure

Plant Physiology/Anatomy – Leaves • Leaves are the primary site of photosynthesis

• Leaf cross section covered by upper and lower epidermis – epidermis

covered by waxy cuticle to reduce water loss lower epidermis has tiny pores that allow gas

exchange – stomata

• each stoma is flanked by guard cells – changes in shape open and close stomata

• usually open during the day when CO2 is needed and closed at night when photosynthesis is not occurring

• light and concentration of CO2 trigger opening and closing – works by triggering movement of H+ and K+

• K+ is actively pumped into guard cell vacuoles increasing solute concentration - causes water to enter (increase turgor pressure) and open stoma

• In the afternoon/evening the reverse occurs, closing stomata (decreased turgor pressure) – abscisic acid assists in this process by causing K+ to rapidly diffuse out of guard cells – made by roots during time of water deficiency

• mesophyll – “middle leaf” – made up of parenchyma tissue packed with chloroplasts - main site of photosynthesis palisade layer – upper layer of column-shaped cells –

main site of photosynthesis spongy layer – layer just below palisade cells, irregularly

shaped with lots of air spaces between to facilitate gas exchange – also photosynthesize but primary function is to allow for diffusion of gases

Veins (vascular bundles) – contains xylem and phloem, extend through mesophyll

Flowering Plants – Angiosperms • Largest, most successful group – flowers function in

sexual reproduction

• flowers are reproductive shoots

• receptacle – tip of stalk on which some or all flower parts are borne

• sepals (calyx) – cover and protect bud

• petals (corolla)– vary in shape, color, and fragrance – attract pollinators

• stamens – male reproductive organs filament – thin

stalk on which anther sits

anther – saclike structure in which pollen grains form

•pistil – female part of flowers

stigma – sticky tip to catch pollen

style – necklike structure through which pollen tube grows down to ovary

ovary – juglike structure that contains one or more ovules (develops into seeds)

each ovule contains an embryo sac that forms an egg and two polar nuclei

Flower Fertilization• starts with pollination – transfer of pollen from

anther to stigma pollen grain contains a tube cell and two sperm

cells tube cell forms a pollen tube and digests its way

down to the ovary – sperm cells follow

• one sperm cells fertilizes egg (becomes embryo) the other fertilizes the two polar nuclei forming the endosperm (food source for embryo in seed)

•process is called double fertilization•ovule develops into seed•surrounding ovary develops into fruit•seed – embryonic plant, endosperm (food source), wrapped in a seed coat

Comparison of Monocots and Dicots

Control of Flowering in Angiosperms

• Photoperiodism – plant’s response to light involving the relative lengths of day and night

• Important factor in control of flowering

Plant Type Flowering and light Examples

Long – day Bloom when days are longest and nights shortest (midsummer)

Radishes, spinach, and lettuce

Short – day Bloom in spring, late summer, and autumn when days are shorter

Poinsettias, chrysanthemums, and asters

Day – neutral Flower without regard to day length

Roses, dandelions, tomatoes

• Flowering is actually controlled by length of the night• Short-day plants will only flower if exposed to a long

continuous period of darkness (must have a critical period of darkness or more)

• Long-day plants will only flower is exposed to a shorter continuous period of darkness (must have a critical period of darkness or less)

Plant Hormones

• Auxin– Found in the embryos of seeds, meristems of apical

buds and young leaves, and young developing shoots– Responsible for phototropism – plant growth in

response to light (plant stems exhibit positive phototropism and roots demonstrate negative phototropism)

– Auxin causes positive phototropism of shoots and seedlings

– Auxin causes elongation of cells on the side of the shoot necessary to cause growth towards the light

– Auxin accumulates on the side of the stem away from the light source

Phototropism