PLANT ORGANS: LEAVES

Leaf

Is an organ of a vascular plant, and the food making factories of plants. Comes in many different shape and sizes.

FUNCTIONS:

• Are the solar energy and CO2 collectors of plants. In some plants, they have become adapted for specialized functions. Are the principal structure,

•Produced on stems, where photosynthesis takes place.

Blade

The broad, flat portion of a leaf.

Petiole

The stalk that attaches the blade to the stem.

Stipules

Leaflike outgrowths usually present in pairs at the base of the petiole.

Vine

Have both xylem and phloem elements which are continuous with similar tissues of the stem through those of the petiole.

Midrib

Runs centrally through the leaf- blade from its base to the apex; this produces thinner lateral veins which in their turn give rise to still thinner veins or veinlets.

Phyllotaxy

Is the mode of arrangement of leaves on the stem.

• Pinnately compound

• Palmately compound

2 Compund leaves

Venation

Arrangement of veins in a leaf.

• Parallel venation

• Netted venation

2 Kinds of Venation

Netted veins

•Palmately netted in which several major veins radiate out from one point.

•Pinnately netted in which major veins branch off along the entire length of the midvein (the main or central vein of a leaf).

Leaf

Is a complex organ composed of several tissues organized to optimize photosynthesis.

Epidermis

Cuticle

Stomata

Guard cells

Subsidiary cells

Mesophyll

Veins or vascular bundles

Bundle sheaths

8 Anatomy of a Leaf

Epidermis

A layer of cells one cell thick that provides protection for the inner tissues. These cells are clear to allow light to reach the photosynthetic tissues.

Cuticle

The outermost layer of both the upper and lower surfaces of the leaf. It is clear and waxy to prevent against water loss.

Cutin

Consists primarily of a waxy substance.

Stomata

Openings in the surface of the leaf and stems for gas exchange. The lower surface of a leaf usually has more. Water vapor also passes out through these holes.

Guard cells

Two of these special cells surround each stomate and regulate the opening and closing of the stomate.

Subsidiary cells

Provide a reservoir of water and ions that move into and out of the guard cells as they change shape during stomatal opening and closing.

Mesophyll

Sandwiched between the upper epidermis and the lower epidermis. the cells of this, which are parenchyma cells packed with chloroplasts, are loosely arranged with many air spaces between them that facilitate gas exchange.

• Palisade mesophyll

• Spongy mesophyll

2 Sublayers of mesophyll

Veins or vascular bundles

Contain the vascular tissue that is continuous with that in the stem. Xylem carries water and minerals upward. Phloem carries dissolved food throughout the plant.

Bundle sheaths

Composed of parenchyma or sclerenchyma cells.

Bundle sheaths extensions

Extend through the mesophyll from the upper epidermis to the lower epidermis.

Guattation

Occurs when transpiration is negligible and available soil moisture is high.

Hydathodes

Opening at the tip of leaf vein.

Guard cells

Act as hydraulic valves.

Leaf abscission

Normal falling of leaves or other plant parts such as fruits or flowers.

Abscission zone

Area where leaf petiole detaches from stem.

• Reason:

• Plant metabolism slows down

• Plants water requirement

Bud scale

Spines

Bracts

Tendrils

Bulb

5 Modified Leaves

Bud scale

Modified leaves that protect the delicate meristematic tissue of the bud from injury and drying out.

Spines

Modified leaves that are hard and pointed, are found on many desert plants, such as cacti.

Bracts

A modified leaf associated with a flower or inflorescence but not part of the flower itself.

Tendril

A leaf or stem that is modified for holding on or attaching to objects.

Bulb

Rounded, fleshy, underground bud that consists of a short stem with fleshy leaves.

Carnivorous plants

Are plants that capture insects.The leaves are adapted to attract, capture, and digest their animal prey.