plant structure bi

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)