Leaves

Human Uses of Leaves

food; tea from small evergreen shrubs and herbal teas; seasonings from mint family like basil, thyme, etc

perfumes; dyes; fibers; medicines; waxes

Eudicot Leaf Structures

blade: broad, flattened part of the leaf

petiole: the stalk/stem-like structure that attaches the leaf to the node

axillary buds:

sessile leaves: leaves with no petiole; are directly attached to node

primary veins diverge from one another in various ways

(recall that eudicots are plants that have two embryonic leaves)

Monocot Leaf Structures

leaves usually narrow and long; leaves attach to stem with sheath

primary veins/venation are/is parallel to one another

Conifer Leaf Structure

needle-like leaves

Leaf Characteristics

a: palmate leaf of buckeye

b:opposite

c: pinnately compound

d: alternate, simple but lobed leaves

e: parallel-veined leaf of grass

f: palmately veined of maple

g: whorled leaves

h: pinnately veined, lobed leaf of oak

i: linear leaves of a yew

j: globe-shaped succulent leaves

k: fan-shaped leaf of ginko showing dichotomous venation

Leaf Margins/Edges

smooth: very few indentations or serration

toothed/serrated: jagged back and forth texture, appears sharp

lobed: leaf tissue wraps closer to veins of leaf

Quercus Lobata (Valley Oak)

native to central valley

Leaf Venation

monocot: veins are parallel to one another

dicot: netted veins that branch

pinnate: one central vein with other major veins branching off from it (similar to feather)

palmate: several major veins branching off from a common point (like a palm with fingers radiating out)

Simple vs Compound

simple: only one leaf blade

compound: two or more leaflets in a blade

• pinnate: leaflets arranged along a central axis

• palmate: leaflets arise from a common/central point at the end of petiole

(leaflets do not have axillary buds)

Leaf Positioning on Stems

alternate: one leaf per node; usually offset by a certain number of degrees

opposite: two leaves per node; rotated usually about 90 degree with respect to leaves above and below

whorled: three or more leaves per node

(the positioning helps the plants to maximize exposure to sunlight)

Epidermis Structure & Function

outer cell layer that covers upper and lower surfaces

mostly non-photosynthetic; translucence allows light to pass to underlying tissues

provide support for leaf and prevents water loss by secreting waxy waterproof  material called cutin to form cuticle 

Stomata

a pore/opening in the epidermis of leaves that is flanked by two guard cells that regulate its opening and closing and thus regulate gas exchange and transpiration

Guard Cells

a pair of specialized cells surrounding a stoma

when water is low, guard cells are limp and close

when water is high, guard cells become turgid and open the stoma

Trichomes

specialized “hairs” on leaves that absorb water and minerals in epiphytes and deter predators

Mesophyll Tissue

a parenchyma (chlorenchyma) tissue between the upper epidermis and the lower epidermis

Mesophyll Tissue Anatomy

palisade parenchyma: columnar cells usually in the upper side of leaf; most photosynthesis occurs here

spongy mesophyll: irregularly shaped cells, usually in lower side of leaf, have open areas between cells that facilitate gas exchange 

hydrophytes: large air spaces for gas exchange and flotation (called aerenchyma)

*mesophyll layer in monocots all looks the same; it is not differentiated into palisade or spongy

Bundle Sheath Cells

parenchyma cells that surround xylem and phloem

book def: the parenchyma and/or sclerenchyma cells surrounding a vascular bundle

Kranz Anatomy

leaves have two forms of chloroplasts: large chloroplasts with few to no grana and small chlorophylls in mesophyll

Transpiration

quantities of water evaporate from leaves through stomata

moist cells are exposed to absorb CO2 and release O2, but some O2 moisture evaporates from mesophyll

book def: loss of water in vapor form; most transpiration takes place through stomata

Positive Effects of Transpiration

cools plant as water evaporates

forces water up from roots

produces water vapor and rain for other plants in area

Cohesion Tension

happen in xylem

as water evaporates from mesophyll of leaves water from other cells and from xylem from roots, water gets pulled upward

Stomatal Movements

in sunlight K+ ions are pumped into guard cells; this causes water to move into guard cells by osmosis and become turgid; turgid cells bend because they are attached to each other at both ends

remember that water follows solute

Tendrils

long threadlike structures, develop from leaves

Bud Scales

modified waxy leaves that protect young buds, contain growth inhibitor, prevent growth until spring

Bracts

colorful, modified leaves, attract pollinators

ex: poinsettia

Other Functions of Leaves

water storage: thickened leaves to store water; cuplike structures to catch rainwater

food storage: onion, cabbage leaves

defense: sharp, non-photosynthetic spines on cacti

capture prey: carnivorous plants; trichome hair in venus fly trap trigger plant to shut

Spines, Thorns, & Prickles

spines: modified leaves

thorns: modified stems

prickles: epidermal extensions of the stem

Sundew Plant

parasitic

grows in nitrogen poor soil

hair-like trichomes w bio-adhesive mucilage containing nanofibers and digestive enzymes (chitinases and proteases)

(charles darwin’s favorite plant)

Senescence

series of irreversible changes and breakdown of cell components that leads to cell death

in annuals: occurs in plant as a whole

in perennials: occurs only in parts of plants- flowers senesce after pollination and some xylem cells die; leaves senesce before dry and cold seasons

Leaves Changing in Fall

chlorophyll is broken down, leaving xanthophylls, carotenoids, and anthocyanins

avoids leaves getting weighed down with snow etc

(example of planned senescence)

Leaf Abscission

complex process that leads to dropping of leaves

abscission zone: separation layer at base of petiole; thin walled cells will form

cork layer: suberized cells that help to seal wound along abscission layer; seals completely when leaf falls

ethylene hormone: promotes abscission