Seed Plant Structure & Function All Materials © Cmassengale

Seed Plant Overview:

• The plant body consists of two basic parts--- the shoot system and the root system
• Shoot system is above ground and includes organs such as leaves, buds, stems, flowers, and fruits
• The functions of the shoot system include photosynthesis, reproduction, storage, transport, and hormone production
• The root system is below ground and includes roots as well as modified stem structures such as tubers and rhizomes
• The functions of the root system include anchorage, absorption, storage, transport, and production of certain hormones

• Seed plants contain 2 types of vascular tissue (xylem & phloem) to help transport water, minerals, & food throughout the root & shoot systems

• Plant cells have several specialized structures including a central vacuole for storage, plastids for storage of pigments, and a thick cell wall of cellulose

• Plant cells are all box-shaped in appearance

• Plant cells are arranged into tissues and tissue systems

• A simple tissue is composed of only one type of cell; a complex tissue is composed of more than one cell type.

• Plants are composed of specialized cells and tissues

• In plants, the formation of new cells, tissues and organs is restricted almost entirely to regions known as meristems

Meristems:

• There are 3 main types of meristematic tissue in vascular seed plants   ---- apical, intercalary, & lateral meristems

• Apical Meristem occur at the tips of roots and shoots and are responsible for the length-wise extension of the plant body known as primary growth

• Cells in apical meristems are produced by mitosis & then differentiate into specialized cells & tissues

• The apical meristem gives rise to 3 primary meristems --- Protoderm (gives rise to outer covering of plant), Procambium (gives rise to vascular tissue), and Ground meristem (gives rise to the 3 types of plant cells)

Root Tip with Meristems

• Some monocots, such as grasses & bamboo, have intercalary meristems located above the bases of leaves and stems  allowing them to regrow quickly after being cut down

• Woody plants have meristem between xylem & phloem to produce wood

• Lateral meristems produce outward growth in plants or secondary growth

• Lateral meristems are called cambium (cambia, plural)

• Vascular cambium makes wood & vascular tissues and cork cambium makes cork & bark

• Plants without lateral meristems have only primary, not secondary growth and are called herbaceous plants

Specialized Plant Cells:

• Plants have 3 basic types of cells --- parenchyma, collenchyma, & sclerenchyma

• Parenchyma cells are the most abundant & least specialized

• Parenchyma are loosely-packed, cube shaped or elongate cells with a large central vacuole & thin cell walls

Parenchyma Cells

• Parenchyma cells can specialize for various functions (storage, photosynthesis, etc.)

• Parenchyma form the bulk of non-woody plants such as the fleshy part of an apple

• Collenchyma cells are irregular in shape with thicker cell walls & support the growing parts of plants

Collenchyma cells

• Collenchyma cell walls are also flexible to support new growth regions of the plant (example: tough strings on a celery stalk)
• Sclerenchyma cells support non-growing parts of plants because they have thick, rigid, non-stretchable cell walls

Sclerenchyma Cells

• Sclerenchyma cells often die at maturity leaving empty, box-like structures 
• Two types of sclerenchyma cells are fibers & sclereids  
• Fibers are cells up to 50cm long that usually occur in strands such as linen & flax

Sclerenchyma Fibers

• Sclereids have thicker cell walls & come in many shapes

• Sclereids may be single cells or groups of cells and give pears their gritty texture & give hardness to peach pits & walnut shells

Other Plant Tissues & Systems:

• Tissues are groups of cells with similar structures & functions

• Plants have 3 tissue systems --- ground, dermal, and vascular tissues

• Plant tissues make up the main organs of a plant --- root, stem, leaf, & flower

• Ground tissue makes up most of the plant's body, dermal tissue covers the outside of the plant, & vascular tissue conducts water & nutrients

Dermal Tissue:

• Covers the plant body and consists of epidermis in young plants & non-woody plants that  is replaced later by periderm in woody plant 
• Epidermis is made of parenchyma cells in a single layer
• Epidermis on stem and leaves prevents water loss by transpiration & produces a waxy material called cuticle

Epidermis Of a Leaf

• Epidermal cells on roots form extensions called root hairs to absorb H₂0 & nutrients

• Openings in the epidermis on the underside of a leaf where gases are exchanged are called stomata (stoma, singular)

• Sausage-shaped guard cells are found on each side of the stoma to help open and close the pore to prevent water loss

• Dead cork cells replace epidermis in woody stems & roots

Ground Tissue:

• Ground tissue constitutes the majority of the plant body and contains parenchyma, collenchyma, and sclerenchyma cells
• Ground tissue of the leaf (called mesophyll) uses the energy in sunlight to synthesize sugars in a process known as photosynthesis

Spongy Mesophyll of Leaf

• Ground tissue of the stem (called pith and cortex) develops support cells to hold the young plant upright
• Ground tissue of the root (also called cortex) often stores energy- rich carbohydrates

Vascular Tissue:

• Vascular tissues transport water and dissolved substances inside the plant and helps support the stem
• The 2 types of vascular tissue are xylem & phloem
• Xylem carries water and dissolved ions from the roots to stems and leaves
• Phloem carries dissolved sugars from the leaves to all other parts of the plant
• Xylem has 2 kinds of conducting cells --- tracheids & vessel elements 

  
Xylem Cells

• Tracheids are long, narrow sclerenchyma cells with walls and pits for water to move between them  
• Vessel elements are short, wide sclerenchyma cells without end walls stacked on top of each other
• Angiosperms (flowering plants) have tracheids & vessel elements, while gymnosperms (cone bearers) only have tracheids
• Phloem moves sap (dissolved sugars & minerals) from source (where they are made) to sink (where they will be used)

Phloem Cells

• Phloem is made of cells called sieve tube members and companion cells

• sieve tube members are stacked to form tubes called sieve tubes with porous sieve plates between the cells for movement of sugars

• Companion cells are along each sieve tube member & help in loading sugar into the sieve tube

Root System:

• Roots grow underground (subterranean part of the plant)
• Roots have 3 main functions --- (1)anchor plants (2) absorb and conduct water & minerals (3) store food

Root & Shoot System of a Plant

• The first root to emerge from the most seeds is called the primary root or taproot & can grow deep to reach water 

• Taproots may store food (carrot & beet)

Taproot

• Monocots have highly branched, roots called fibrous roots that grow near the surface & spread out to collect water
• Fibrous roots such as in grasses often help prevent erosion

Fibrous Roots 

• Adventitious roots grow from a stem or leaf above ground in some plants and serve to prop up or support the plant (corn)

Adventitious Roots

• Aerial roots obtain water & minerals from the air & enable the plant to climb (orchids & ivy)

Aerial Roots of an Orchid

Structure of the Root:

• Root cap covers the apical meristem (growth tissue) at the tip of the root & produces  a slimy substance so roots can more easily grow through the ground

• Apical meristem replaces cells of the root cap as they are damaged

Root Structure

• Epidermis covers the outside of the root & has extensions called root hairs that absorb water & minerals and increase the surface area of the root

Root Hairs

• The core of the root is called the vascular cylinder & contains xylem & phloem

• A band of ground tissue called cortex surrounds the vascular cylinder

• A single cell layer called endodermis separates the cortex & vascular tissue

• Endodermal cells are coated with a waxy layer called the Casparian strip so water is channeled into the vascular tissue

• The Pericycle is the outermost layer central vascular tissue & forms lateral roots

• Vascular tissue in dicots is at the center of the root

• In monocots, vascular tissue forms a ring around pith at the root center

• Secondary growth begins when the vascular cambium forms between Xylem and Phloem

• Vascular Cambium produces secondary xylem toward the inside of the root and secondary phloem toward the outside

Root Functions:

• Anchor plants
• Absorb water & nutrients (13 essential)
• May store carbohydrates (carrots, potatoes, turnips)
• May store water for dry periods
• Plant Cells use minerals, such as nitrogen and potassium in LARGE amounts; called macronutrients 
• Minerals needed in SMALL amounts are called micronutrients
• Mineral deficiencies or excess minerals can kill plants

Stem Structure & Function:

• Adapted to support leaves
• Transport water & minerals 
• Transport sugars (usually sucrose) from Source (where they're made) to Sink (where they're stored)
• Movement of sugars is called translocation
• Store food and/or water
• Tubers (potatoes) underground food storage stems
• Stems grow from the tip or apical meristem
• Stems increase in circumference by lateral meristems
• Leaves are attached to stems at nodes & have lateral buds that can develop into new stems or branches
• Internode is space between nodes on a stem

• The tip of each stem usually has a Terminal Bud enclosed by specialized leaves called Bud Scales

• Vascular Tissue is arranged in bundles with xylem toward the inside & phloem toward the outside

• Vascular bundles are scattered throughout monocot stems 

• Vascular bundles are arranged in rings in dicot stems 

• Secondary grow (woody growth) occurs in dicots, but less often in monocots
• Plants with only primary growth (non-woody) are called herbaceous

• Darker wood in the center of a tree trunk is called Heartwood and is composed of old, non-functional xylem
• New, functional xylem makes up lighter wood in the tree trunk and is called Sapwood
• Sapwood gets wider, but heartwood remains the same size
• Bark is the protective covering of Woody Plants & consists of Cork, Cork Cambium, and Phloem

• Springwood forms in spring when there is lots of water & xylem cells are wide with thin walls
• Summerwood forms when there is less water & smaller xylem cells with thick walls form
• In a Stem Cross Section, the abrupt change between Small Summerwood Cells and the following year's Large Springwood Cells produces an ANNUAL RING

Translocation of Sugars:

• Phloem cells move sugars through a plant
• Sugars made in photosynthetic cells are PUMPED into Sieve Tubes by ACTIVE TRANSPORT at the Source
• Turgor Pressure Increases as Water enters the Sieve Tube by Osmosis
• TURGOR moves the SAP toward the SINKS
• Known as the PRESSURE-FLOW HYPOTHESIS

Transport of Water:

• Transport of Water and mineral Nutrients occurs in the Xylem
• Water movement in plants is driven by Transpiration (evaporation of water from leaves & stems)
• As water evaporates, more water is pulled into the roots
• Transpiration produces a NEGATIVE pressure in the xylem pulling water UPWARD
• Water molecules are cohesive (attracted to each other) which also pulls water upward
• Water is also adhesive sticking to the walls of the xylem
• Known as Cohesion Theory of water movement

Leaf Structure & Function:

• Flat surfaces helps leaves capture sunlight for photosynthesis
• Convert carbon dioxide and water into simple sugars (glucose)
• Some leaves store food (onion) and water
• Protect (cactus spines)
• Used  Dyes, Fibers, Fuels, Drugs, Wax, Soap, Spices and Food
• Attached by a stem-like petiole to the plant
• Simple leaves have one blade, while compound leaves have several leaflets
• Covered with a single layer of cells called epidermis (upper & lower)
• A waxy cuticle prevents water loss
• Openings called stomata on the underside of leaves for gas exchange (CO₂ & O₂)
• Two guard cells on either side of the stomata open & close the openings

• When guard cells LOSE water, the stoma CLOSE, while the stoma OPEN when guard cells gain water & swell

• Stomata are CLOSED during the HOTTEST parts of the day to prevent water loss from leaves

• Below the epidermis are 2 types of chlorophyll containing MESOPHYLL cells ---palisade & spongy

• PALISADE mesophyll cells are closely packed columnar cells (most photosynthesis occurs here)

• SPONGY mesophyll cells are loosely packed with air spaces containing CO₂ & O₂

• VASCULAR BUNDLES (xylem & Phloem) in the spongy mesophyll appear as VEINS on the surface of the leaf

• VENATION is the arrangement of veins in a leaf
• Monocots leaves (such as Grasses or Corn Plants) have Parallel Venation
• Dicots leaves form a Branched network called Net Venation

MONOCOT (left)        DICOT (right)

• In carnivorous plants (Venus Fly Trap), the leaves trap insects for food so the plant can get enough nitrogen (grow in N₂ poor soil)