Leaves

concepts

numerous types of leaves and functions

shoot system

leaves have to be cost effective for energy uptake

support

storage

protection

nitrogen procurement

shows division of labor and integration

present in all leaf/ stem combos

do not grow new cells every year like wooded plants

almost all contain primary wall, rarely secondary wall

External structure of foliage leaves

parts of the leaf related to photosynthesis

petiole/stalk

other than photosynthesis, leaves have important functions

must not allow entry of fungi, bacteria, or epifoliar algae

must not be nutritious and delicious to animals ⚠

must not lose excessive amounts of water 🚰

must not be effective sails so wind could blow plant over

must be cheap to make and replace 💰

leaf blade/lamina

dorsal surface

large, flat, light harvesting side of leaf ☀

smooth, also called ventral surface/ adaxial

not must photosynthesis/light occurs here 🕶

has large veins protruding on this side

called abaxial side as well

leaf shape

holds leaf into the light

helps leaf receive adequate sunlight against leaves above them

long thin shape allows the leaf to flutter in wind

small or very small leaves are called sessile leaves

great when wind is blowing, not efficient when there is no wind ✅ ❌

fluttering helps prevent insects from landing on leaf

monocots such as grasses, lilies, and agaves lack petiole

they do not require petiole as self-shading is not a problem

are not called petiolate ⭐

called petiolate

this is due to adequate amounts of sunlight reaching majority of plant

form sheathing leaf base that wraps around stem

lack of sunlight in these plants occur at base

simple leaf(one part)

compound leaf(multiple parts)

small blades (leaflets)

point of attachment for leaflet (petiolule)

rachis (extension of petiole)

Two types of compound leaves

palmately compound (attached at same point) 👥

pinnately compund (attached individually along rachis) 👤

advantages

more disruption when wind blows= more CO2 and removal of wastes

less surface for insects and bacteria to be on

small leaves can flex more without tearing

easily recognized as leaves but can be mistaken as simple leaf with stem

always arranged in 2 rows, never spiral, whorled, or decussate

no real agreement on how different leaf shapes may be adaptive

plants can have multiple types of leaves

can be based on location and age of plant

some produce two types of leaves simultaneously

some can be different sizes on the same stem

can be disadvantageous compared to compound leafs

at base of leaf (petiole) is abscission zone perpendicular to petiole

involved on cutting of leaf when usefulness is up

swelled cells will cause leaf scar

internal structure of foliage leaves

Mesophyll

vascular tissue

epidermis

petiole

Initiation and development of leaves

Morphology and Anatomy of other leaf types

arranged in a netted pattern called reticular venation

in monocots with long, strap-shaped leaves, veins run side by side with few interconnections

called parallel pattern

epidermis must be somewhat waterproof, simultaneously translucent and allow CO2 entry

leaf and stem epidermises basically similar, contain

water lost through epidermis is called transpiration

ordinary epidermal cells

guard cells

Large % of flat, tubular ( shaped like paving stones)

trichomes (either glandular or nonglandular)

dorsivental nature of leaves causes upper and lower epidermis to exist in significantly lower micro climates ⭐

number of stomata is greater in lower epidermis than upper epidermis, aids in water retention

some plants do not have stomata on upper epidermis

helps prevent disease caused insects landing on leaf 🚫

Leaves contain an upper and lower epidermis

lower epidermis contains crypts that created pocket of nonmoving air

trichomes are abundant in lower epidermis

helps reabsorb water molecules that leave stomata ♻

makes the leaf epidermis hairy, affect leaf biology in many ways (especially notable for lower epidermis)

prevent rapid air movement and water loss ❤

make walking and consumption difficult for insects ⛔

provide excellent foothold for appropriately sized insects

can be poisonous, preventing consumption ☠

contain coating of cutin and wax on outer wall

helps retain water, difficult for fungi digestion ⚠

in fog, water droplets form here instead for stomatal pores which would inhibit CO2 &O2 movement

palisade parenchyma/palisade mesophyll contains most of leaf's photosynthetic tissue

lower portion of the leaf contains spongy mesophyll

ground tissue interior to epidermis is called mesophyll

has a large surface with cells slightly separated

allows for CO2 dissolution

can be on both surfaces of the leaf

rapid CO2 diffusion occurs here

appears in the center of leaf or is lacking

horizontal leaves absorb sunlight from overhead 🌞

vertical leaves can absorb sunlight from both sides equally 🌅

eudicot leaf usually has one large midrib/midvein

occur between palisade parenchyma and spongy mesophyll

these emerge into lateral veins and minor veins

minor veins important for releasing water in xylem and sugar loading in phloem

lateral vein and midrib contain both

primary xylem on upper side of leaf

primary phloem on lower side of leaf

both veins conduct and support leaf and may contain arranged fibers in a sheath called a bundle sheath

sheath makes it difficult for insects to chew vascular tissue

nonconducting cells such as mucilage, tannin, or starch present here

minor veins important for exchange of material

minor veins must have large surface in contact with palisade and spongy mesophyll

do not contain fibers or nonconducting cells

larger veins can contain mass of fibers below, above, or both called bundle sheath extension

this gives blade rigidity and may help transport H2O 🚰

considered part of the leaf and transition between stem and lamina

petiole mesophyll is like cortex

can be tiny or large depending on the plant

compact and not especially arenchymatous

contain considerable amount of collenchyma when supporting heavy lamina

leaf traces are 1-5 bundles of vascular tissue connecting to the petiole

click to edit

can be arranged in many shapes such as rings or plates

can even form a midvein with smaller bundles entering lamina as lateral veins

they can be distinct or fuse together in numerous bundles

significance of bundle patterns is unknown

could be proper distribution of sugars

patterns are not always disadvantageous

can contain two small flaps of tissue called stipules

can protect apical meristem

can contribute to photosynthesis

basal Angiosperms and eudicots

Monocots

leaves are only produced through activity of a shoot apical meristem

at base of meristem, cells interior to protoderm grow outward

form protrusion called a leaf primordium

extands as narrow cone as surpasses apical meristem

consists of leaf protoderm and leaf ground meristem.

all are meristematic

contain dense cytoplasm and small vacuoles

strand of cells is center differentiates into provascular tissue and primary xylem& phloem

increases thickness higher it grows, establishes midrib

row of cells extend outward on either side of primordium

initiate lamina

new leaf has midrib and two small wings

wings are meristematic

enlarge lamina quickly

stomata, trichomes, and vascular bundles differentiate

petiole becomes distinct from midrib

for perennial plants, leaves are initiated in summer/autumn

no mitosis or cell division occurs, only maturation

leaves leaf very vulnerable during this period

dormant in developmental stage on axillary buds until next growing season 🍂

develop leaves before seed of plant is dry and dormant

leaves are initiated by expansion of shoot apical meristem cells to form leaf primordium

adjacent apical meristem cells grow upwards with primordium

become part of primordium

give primordium a hood-like shape ☂

more apical meristems join until primordium is a cylinder or surrounds shoot apical meristem

grows forward and becomes a sheathing leaf base

original conical leaf primordium gives rise to lamina

outer surface of tube is abaxial epidermis and inner surface is adaxial epidermis

as shoot apex enlarges, new stem tissue is formed and new leaf primordium is initiated

new primordium will develop into tube in previous leaf's sheathing base ♻

in some moncots, lamina becomes broad and expanded like eudicot 🍃

however they have strap-shaped leaves and no predetermined size

lamina grows by meristem located at base where it is attached to top of sheathing leaf base

remains active mitotically, producing new cells to extend leaf

lamina can be regenerated despite many types of destruction to leaf

this also means protoxyleam and protophloem are constantly being stretched and disrupted 💥

new vessel elements and sieve tube member differentiate rapidly & conducting is never interrupted ⚡

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above basal meristem is where stomata and other features are differentiated

also includes initiation of primary xylem and phloem

higher above basal meristem tissues mature and differentiate similar to stems but upside down 🙃

Bud scales image

Spines image

Leaves of conifers image

Tendrils image

Scerophyllous Foliage leaves image

Insect traps image

succulent leaves image

leaves with Kranz Anatomy

thick and fleshy leaves, which reduces surface to volume ratio for H20 conservation 🚰

can be spherical (optimal surface to volume shape), or cylindrical ⏺

characteristic of families Crassulaceae, Portulacaceae, and Aizoaceae

reduced surface area has consequence of reduced capacity for CO2 uptake

inside the leaf

lack of air spaces makes mesophyll more transparent, allowing light to penetrate farther in leaf

photosynthesis occurs more deeply than in foliage leaves

Mesophyll has very few air spaces, reducing internal evaporation and H2O loss through stomata

Lithops and Frithia are so translucent they act as optical fibers

leaves located almost completely underground

exposed leaf allow sufficient light in and conducts energy underground

must produce more sugar by photosynthesis than are used in their own construction and metabolism

include barberry, holly, Agave and Yucca

evolved perennial, existing on plant more than two years

this liimits the amount of sclerenchyma in foliage leaves

makes sclerenchymatous leaves possible

hardness makes then more resistant to animals and the elements ✅

scelerenchyma is often present a layer below the epidermis and in bundle sheaths ⭐

cuticle is very thick and waxy

almost all species' leaves are

epidermis and hypodermis cells have thick walls

contain unpalatable chemicals ⚠

have thick cuticle

always simple, never compound and have only a few forms

scerophylls

needles of varying sizes occur in all pines and spruces

small, flat, scale-like leaves form shield around stems of junipers and cypresses

in conifers of the southern hemisphere, leaves are large and broad held away from stem

leaves are perennial and remain on stem for many years

are evergreens

vascular bundles can produce new phloem each year but not xylem

only three conifers shed their leaves each autumn Larix, Taxodium, Metaseuoia and are all deciduous

one of the most common modification of leaves is evolutionary conversion into bud scales

protect dormant shoot apical meristem in perennial plants from low temperatures and drying action of wind in winter

primary role is protection not photosynthesis

are small and rarely compound, not at risk from mechanical wind damage

petiole short or absent, must remain close to stem and folded over

frequently produce thin layer of corky bark, on exposed portions. provides greater protection 🍾

have tougher and thicker wax than regular leaves 😠

cacti have two types of leaves

green cactus body has microscopic green leaves

clusters of spines are on their axillary buds

are modified leaves of axillary buds

used for protection 🕶

no blade and mutated to have no lamina ❌

needle shaped

no mesophyll or parenchyma present

after spines mature, they deposit legnin in their walls making them hard and resistant to decay

then dry and die out, not disadvantageous as cactus has other photosynthetic tissues

can grow indefinitely and capable of sensing contact with objects ⏳

when leave touches something, stops growing in contact spot and grows opposite way

includes peas and squash

will then coil around object and use it for support

no lamina forms, would be detrimental

foliage leaves can sense light direction and reorient themselves while tendrils sense solid object and grow around them 〰

lack palisade parenchyma and spongy mesophyll

have prominent bundle sheaths composed of large chlorophyllous cells

have special metabolism called C4 photosynthesis

surrounding each sheath is a ring of mesophyll cells that appear to radiate form vascular bundle

possess a mechanism of CO2 transportation that requires Kranz anatomy and adapts C4 plants to arid environments

obtain nitrogen from insects amino acids

classified into two types

usually evolved in habitats that lack nitrates and ammonia 🔒

leaves appear highly modified but are similar to foliage leaves

insect traps have occurred in many families

active traps (move during capture)

passive traps (incapable of movement)

thin, parenchymatous, and capable of photosynthesis

numerous stomata and vascular bundles

mesophyll containing arenchyma and chlorenchyma

lamina is tubular rather than flat, secretes watery digestive fluid ☠

epidermis in digestive region is absorptive rather than impermeable

throat of pitcher contain numerous trichomes that point toward liquid, easy for insects to walk on, lead to their death ❌

largely metabolic rather than structural

trichomes and lamina must be able to sense and respond to insect

entire leaf blade curls around insect so many trichomes come into contact dropping digestive liquid on insect ☠

Venus fly trap contains motor cells on midrib that are extremely turgid and swollen

once one trichome is stimulated by an insect, all adjacent trichomes bend towards victim ⚠

share features with foliage but upper surface covered in glandular trichomes that secrete sticky digestive liquid 🚩

creates hair trigger when stimulated

stimulated when motor cells lose water quickly and trap closes two halves of lamina upward

after digestion is complete, midrib motor neurons fill up with H2O, swell and open again