Plant, Cells, Tissue, Morphology & Growth (continued) [Chapters 35-37]

Lecture 2

What forms at the end of the first year in a young root?

The vascular cambium, which produces secondary xylem (wood) on the inside and secondary phloem (bark) on the outside

What are wood rays?

Secondary growth structures (made of parenchyma cells) that help move water around the stem, especially near wounds

• They can move water, sugar, nutrients side-to-side

  • As they run horizontally (radially) through stem or root

What is chlorenchyma?

Photosynthetic parenchyma tissue

What is the main function of parenchyma?

Starch storage, metabolism, healing, photosynthesis (Chlorenchyma)

• At a wound cite, parenchyma proliferate and create a callus (mass of undifferentiated cells)

  • Callus tissue later differentiates into new vascular tissue or epidermis as needed

What is the orientation of xylem and phloem in leaves?

Xylem is on top (closer to sunlight), phloem is on the bottom (efficient sugar export)

What is secondary xylem also known as?

Wood

What are the two lateral meristems involved in secondary growth?

Vascular cambium (internal transport) and cork cambium (external protection)

What does the vascular cambium do?

Creates secondary phloem outside (bark) and secondary xylem inside (wood)

What happens to old xylem and phloem during secondary growth?

Old phloem dies become bark, xylem accumulates and forms heartwood

• Newer active xylem that still conducts water becomes sapwood

What does the cork cambium produce?

Cork, which replaces the epidermis

What is lignin and its function?

A hardening substance in woody plants that makes older wood tougher (depositied into the sclerenchyma)

• Also found in xylem vessels and tracheids

What is heartwood?

The darker, older, harder center of the wood that no longer transports water (structural support / protection)

What are wood rays?

Tissues that radiate from the center of a woody stem to help water distribution (Lateral transport of water)

What happens in a young dicot root after one year?

Vascular cambium forms between xylem and phloem; cork cambium forms later (secondary growth)

What kind of root system do monocots have?

Fibrous root system

Do monocots form real wood?

No, they do not form true wood

How are monocot leaves structured?

Parallel venation, no branching, and flower parts in multiples of 3

What kind of root system do dicots have?

Tap root system with branching

How is venation different in dicots?

Netted venation with lots of branching

Where are stomata mostly located on dicot leaves?

On the bottom of the leaf

Why are most stomata on the bottom of a leaf?

To reduce water loss due to wind evaporation on top

What are palisade and spongy parenchyma?

Palisade absorbs sunlight (tightly packed); spongy creates air spaces and supports gas exchange

• Both types of chlorenchyma

What is mesophyll?

All the photosynthetic cells in the middle of a leaf

Where are vascular bundles located in the leaf?

Xylem is on top, phloem is on bottom of each bundle

What forms when a leaf’s vascular bundle enters the stem?

A leaf gap (which is filled with parenchyma cells)

What is the role of vacuoles in plant cells?

Regulate water, help store starch, and give acidic flavor

• They store organic acids which lower the pH inside plant cells

What are parenchyma cells like?

Thin-walled, live cells for metabolism and storage

• They are thin walled for efficient diffusion of water, gases, and nutrients

What are collenchyma cells like?

Live cells with unevenly thick walls for support, especially in young tissues and leaf corners (once mature there is no more)

What are sclerenchyma cells like?

Dead, very thick-walled cells that provide strength (e.g., in pear skin or fruit pits)

What is the function of tracheids in xylem?

Dead cells that conduct water via a long column of water molecules (has no open ends)

What is the function of vessel elements in xylem?

Larger, dead cells in angiosperms that allow more efficient water transport (they have perforation plates / large openings)

What do sieve tube elements in phloem do?

Barely-living cells (lack a nucleus and ribosomes) that transport sugar from source to sink

What keeps sieve tube elements alive?

Companion cells connected by plasmodesmata

• They help with loading and unloading sugars and STE metabolism

What are root hairs and their purpose?

Unicellular extensions of epidermal cells that increase surface area for absorption

• Trichoblasts make up root hairs

What do guard cells do?

Control the opening and closing of stomata for gas exchange and water retention

• Controlled by Abscisic Acid (ABA) and osmotic changes

What are cork cells?

Dead dermal cells that reduce water loss and protect the stem or root

• Contain liginin

What is cohesion in water movement?

Water molecules sticking to each other

What is adhesion in water movement?

Water sticking to surrounding surfaces like xylem walls

What drives the upward pull of water in xylem?

Evaporation of water from stomata (transpiration)

What is the transpiration stream?

The continuous flow of water from roots to leaves through xylem

What is the pressure flow hypothesis?

The model explaining movement of phloem sap from source to sink

• Sugar are transported in the phloem by osmotic pressure differences - flowing from source (high pressure) to sink (low pressure)

What is the structure of a dicot flower petals?

Flower parts (petals) in multiples of 4 and 5

What are wood rays comprised of?

Parenchyma Cells

What are circulating axillary meristems?

Shoot apical meristems located in axillary buds that can grow into branches or flowers

Why are palm trees considered petioles?

Palm trees are monocots and cannot make true wood, so their trunk is technically a modified, hardened petiole

Where is the epidermis located on a dicot leaf?

On both the upper and lower sides, called adaxial (upper) and abaxial (lower) surfaces

What is an internode?

The segment of a plant stem between two nodes

• A node is where leaves/buds/branches/flowers attach to the stem

Where are vessel elements found?

Only in angiosperms

How are vessel elements different from tracheids?

They have a larger diameter, allowing more water to be moved more efficiently

Are piths more prominent in dicots or monocots?

Dicots

What is the lateral meristem?

1. Vascular Cambium

2. Cork Cambium

Which structures within Monocot and Dicot have a central pith?

1. Monocot: Roots

2. Dicot: Stem