Leaves: Definition, Function, Form, and Development

Photosynthesis, defense, support, protection, storage, dissipated heat, traps insects, and attract pollinators for propagation.

Function of leaves

Leaves photosynthesis

Intercepts light and facilitates gas exchange

Leaves

Must be thin and flat for maximum photosynthesis performance

Sclerophylly

Hardened leaves that reduce palatability and digestibility of leaves

Ligule

A membranous scale on the inner side of the leaf sheath that helps in angling the leaves towards the sun to catch more light.

Blade aka lamina

The flat, light-harvesting portion of the leaf

Margin

The edge of the leaf

Apex

This is the tip of the leaf - this is where water droplets accumulate.

Midrib

The central, thick, linear structure that runs along the lamina that contains vascular bundles, associated fundamental tissues and epidermis

Veins

Vascular tissue of the leaf and are located in the spongy layer of the mesophyll

Venation

Veins patterns

Petiole

The stalk that holds the blade into the light; prevents self-shading.

Stipule

Common in dicot, leaf blade

Leaf sheath

For monocots - leaf base wraps around the stem to form this.

Abaxial surface/side

a.k.a dorsal surface; lower side, veins protruding

Adaxial surface/side

a.k.a. ventral surface; upper side, smoother

Simple leaves

Flat, not divided - has a blade of just one part

Pulvis

Swollen part at the base of the petiole; allows leaf movement due to turgor pressure as response to stimuli

Compound leaves

Blade divided into several individual parts

Leaflet

Each small blade that is attached to a petiolule

Rachis

Extension of the petiole

Difference of phyllotaxy to compound leaves

Phyllotaxy is defined by the arrangement of leaves around the stem: compound leaves are attached to the rachis, not the stem itself.

Pinnately compound

Leaflets attaches individually along rachis

Odd

A.ka imparipinate

Even

A.ka paripinnate

One pinnate

Petiolule is attached to the rachis

Twice pinnate

Petiolule is attached to the rachilla

Trice pinnate

Moringa oleifera

Palmately compound

Leaflets attached at the same point

Peltate leaves

Umbrella-shaped leaves; petiole more or less centrally attached to the lamina on the abaxial side

Perfoliate leaves

stem pierces the leaf

Sessile leaves

Leaves that have no petiole; when leaves are small or very long and narrow, and self-shading is not a problem

Thigmonasty (Nastic movement)

Non-directional movement that does not matter where the stimulus comes from. e.g. makahiya, venus fly trap

Thigmotropism

Directional movement. The direction of stimulus matters; there is positive and negative thigmotropism. e.g. pea plant tendrils.

Prophylls

First leaves that form on the axillary bud.

Abscission zone

Separation region at the base of the leaves.

Frond

Leaves blade fern

Leaf dimorphism

In the same plant, there are two types of leaves in different morphological stage.

Auricle

Expanded region at the base of the lamina.

Cataphylls

Act as scale leaves; bud scales, function for protection

Cotyledon

Embryonic leaf in seed-bearing plants; first leaves to appear in germinating seed.

Forliage leaves

Photosynthetic leaves

Fiddlehead

Young part of the leaf.

Shoot apical meristem

Where leaves are only produced

Leaf primordium

At the base of the meristem that is a protrusion and where the leaf development starts.

Leaf primordia

Develops immature tissues (protoderm, procambium, and ground meristem) and comes from apical meristem

Process of primary morphogenesis

When the leaf primordia is initiated, it is going to undergo cell division, and the cells will grow, and the midrib will thicken. It will eventually form the petiole. There is also the marginal meristem/blastozone: cell division here leads to the formation of the lamina. Cells grow outward

Secondary morphogenesis

There is differentiation and maturation of the leaf, the lamina will continue to expand until it will reach its determined size. Trichomes may also develop here, stomata, etc.

Monocot leaf development

Apical meristems along the leaf primordia will grow along the leaf sheath forming a tube. The tip of the original leaf primordia is now at the side of the tube, forming the lamina - as the stem grows, the leaf is left behind.

Horizontal and vertical leaf

Stomatal distribution

Horizontal leaf

Stomata mostly in abaxial side of leaf: hypostomatic

Vertical leaf

Stomata in both adaxial and abaxial side: amphistomatic; approximately equal in both sides.

Epistomatic

Stomata in adaxial/upper side

Monocot leaf cross section

Arranged in a row

Dicot leaf cross section

Arranged randomly/scattered

Epidermis

Protective covering over the surface of the plant organs. Barrier against bacterial invasion, protection from abrasion, reflective to prevent overheating.

Single layer parenchyma cells

Makes up the epidermis

Epidermis

Where all interchange of material between the plant and the environment occurs

Cutin

Secreted by the epidermis to make wall impermeable to water

Cuticle

Formed after building up of cutin that prevents water

Guard cell and trichomes

Cell types in epidermis

Guard cells

Are gateways for CO2 to enter the impermeable epidermis and has stomatal pores.

Stoma

Formation of guard cells and stomatal pore

Trichomes

On the leaf epidermis that are hair-like. It provide shade on the leaf, prevent rapid air movement and slow water loss from stomata in lower surface.

Cuticular transpiration

Transpiration of gases or vapor directly through the external membranes

Stomatal transpiration

Dominant type of transpiration; most of water lost is lost via this.

Why is there transpiration? Why take all the water just to lose it?

Because plants still need water especially the leaves, and it keeps the plant upright (turgidity).

Do all plants have stomata?

No. Some aquatic plants don’t need stomata because gas exchange happens directly via diffusion.

Mesophyll tissue

Collection of tissues interior to the epidermis in which you can find sclerenchyma, collenchyma and parenchyma cells.

Chlorenchyma tissues

The palisade and spongy mesophyll cells

Palisade mesophyll cells

Main photosynthetic tissue of most plants. Upper portion. For light absorption that is more elongated and packed together, often only one layer thick, but in regions with intense penetrating sunlight, it can be 3-4 layers thick.

Spongy mesophyll cells

In the lower portion that has more spaces; loose aerenchyma ▪ Main function is for gas exchange.

Veins

Are vascular tissues located in the spongy mesophyll.

Light and moisture

Influences leaf development

Absence of light

Discourages the leaf/lamina to expand.

Light

Affects the chlorophyll development

Daylength

Affects morphology of leaf.

Intensity

How much light iyou are getting

Pulvim

Plural of pulvinus

Sun leaves

Exposed to light, smaller, thicker, due to less chlorophyll, and has more RuBisCo.

Shade leaves

Not very exposed to light, bigger but thinner, has more chlorophyll, and has less RuBisCo.

RuBisCo

Has most abundant on Earth.

Mesophytes general characteristics

Generally flat, thin, relatively large, and green in color.

Xerophytes general characteristics

Succulent leaves that help in water retention and conservation - Highly reduced leaves that resist water loss/spines

Hydrophytes general characteristics

Floating plants have thin large leaves with wide lamina - Submerged ones have thin and dissected leaves.

Mesophytes environment

Neither adapted to particularly dry nor particularly wet environments

Xerophytes environment

Survive in dry and arid conditions with low water availability.

Hydrophytes environment

Grow in water habitats or aquatic conditions - Some are submerged, while others float

Xerophytes cuticle

Thich waxy cuticle seals upper and lower epidermis

Hydrophytes

On the adaxial/upper side which repels water - Submerged hydrophytes may have thin or no cuticle

Mesophytes Stomatal distribution

Generally found at abaxial side.

Xerophytes Stomatal distribution

Tend to be at abaxial/lower side for less sun exposure

Hydrophytes Stomatal distribution

On adaxial/upper surface. Submerged hydrophytes have no stomata.

Xerophytes special features

Sunken stomata

Hydrophytes special features

Aerenchyma

Mesophytes supporting tissues

Sclerenchyma tissues help in supporting vascular tissues.

Xerophytes supporting tissues

Sclereids, specifically sclerenchyma tissue to keep the plant from wilting if there is not enough water for turgidity.

Hydrophytes supporting tissues

No special tissue needed for support since the water itself acts as support.

Tendrils

Grow indefinitely and contain cells capable of sensing contact with an object.

Tendrils function

Support and attachment.