The leaf is the main photosynthetic organ of most vascular plants

Leaves - Overview

Shootsystem

LeafBlade

Petiole

Leaves generally have a flattened blade and a stalk called the petiole, which joins the leaf to a node of the stem

Most Dicots have branch-like veins and palmate leaf shape

Monocots have parallel leaf veins and longer, slender blades

Leaves - Comparisons

Monocots and Dicots differ in the arrangement of veins, the vascular tissue of leaves

LEAF PARTS

1. Stoma (stomata) – small pores that act as a doorway for gases involved in photosynthesis (found mainly on the underside of leaves).

2. Guard Cells – Cells that regulate the opening of the stomata.

3. Lower and upper epidermis – outer layer of a plant that serves for protection, like skin.

LEAF PARTS CONTINUED…

4. Palisade layer – layer where majority of chloroplasts are found (site of photosynthesis).

5. Spongy layer – cells surrounded by air spaces; allow water, CO2 and O2 to diffuse during photosynthesis.

6. Waxy layer/cuticle – protects leaf from water loss and from feeding insects.

Leaves – Structure and Development

• Most dicots have 2 types of mesophyll– Palisade

mesophyllhigh

photosynthesis– Spongy

mesophyllair spaces for gas

& water exchange

• Monocot leaves have 1 type of mesophyll

CROSS SECTION OF A LEAF

Leaves – Structure and Development

• Leaves are several layers thick – each with different cell types

Epidermis Leaves• Leaf epidermis contains stomata - allow CO2

exchange • Stomata flanked by two guard cells, control open vs.

closed• The ground tissue in a leaf, called mesophyll, fills the

middleKey

to labels

Dermal

Ground

VascularCuticle Sclerenchyma

fibersStoma

Bundle-sheathcell

Xylem

Phloem

(a) Cutaway drawing of leaf tissuesGuardcells

Vein

Cuticle

Lowerepidermis

Spongymesophyll

Palisademesophyll

Upperepidermis

Guardcells

Stomatalpore

Surface view of a spiderwort(Tradescantia) leaf (LM)

Epidermalcell

(b)

50 µ

m1

00

µm

Vein Air spaces Guard cells

Cross section of a lilac(Syringa)) leaf (LM)

(c)

VASCULAR TISSUE

Xylem – cells that carry water and dissolved minerals from the roots to the leaves.

Phloem – cells that carry food (sugar) that is made in the leaves to all parts of the plant.

Use: Collectively the vascular tissue, xylem and phloem create a vein or vascular bundle to transport material in plants.

STOMA OPEN VS. CLOSED

Open – with light and when guard cells are filled with water.

Closed – without light and when guard cells are lacking water (dehydrated).

Why is it important to have stomata open?

Closed vs. Open

Question time????????