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Leaves (External Structure of Foliage (Petiole: Stalk: prevents shading of…
Leaves
External Structure of Foliage
photosynthesis
light+CO2=carbohydrate aka glucose
prevent loss of water
must not allow entry of fungi, bacteria, or epifoliar algae
cannot be super tasty or nutritious bc animals will eat them
cannot be effect sails that the plant is blown over
cheap to build
one part of leaf to be flat and broad to absorb light for max CO2 absorption
flat light harvesting portion of a leaf is called the leaf blade
blades lower side is called the dorsal surface
upper side is called the ventral surface
Petiole: Stalk: prevents shading of other leaves
Flexibility allows for fresh air and cooling, also prevents bugs
If a leaf doesn't have a petiole it is a sessile leaf
plants that are sessile typically have no problem with photosynthesis and the reason their leaves are so packed together prevents evaporation
In many monocots
foliage leaves tend ti be very ling and tapered and self shading only occurs at the base
typically these plants lack a petiole instead the lead base wraps around the stem to form a sheathing leaf base
the blade can flex and flutter still without a petiole
A leaf blade can either be simple or compound
a simple leaf blade only has one part
compound leaf has multiple parts
some plants have pretorian leaves or several leaf blades called leaflets which are attached by a petioulule
Large compound leaves still only has small leaflets
order of compound leaves
rachis , first order leaflet, second order leaflet
compound leaves are useful in the fact that they produce turbulence as wind blows over them.
more turbulence produces more CO2 intake
turbulence takes away excess heat and can prevent fungus and bugs from spreading
leaflets never bear buds in the axils of their petioles and the tipoff the rachis never has a terminal bud
leaflets are always arranged in two rows never in a spiral
in cacti spines are short-shoot leaves whereas the long-shoot leaves are green fleshy and in most species microscopic
Veins: bundles of vascular tissue
reticulate venation: netted patterns of veins
Initiation and development of leaves
Basal Angiosperms and Eudicots
leaf primordium
base of meristem cells just interior to the protoderm grow outward forming a protrusion
grow into a narrow cone very rapidly becoming taller than the shoot apical meristem
as this leaf primordium grows it increases in thickness est. the bulk of the midrib
all cells in the wings are meristematic and their division and expansion enlarge and the lamina rapidly
a row of cells consists of a midrib and two small thin wings.
Monocots leaves are initiated by the expansion of some shoot apical meristem
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apical meristem cells adjacent to the promordium and giving it a hood-like shape
primordium becomes cylinder shaped
cylinder shaped primordium completely or almost completely encircles the shoot apical meristem
the shoot apex enlarges forms new stem tissue
in some monocots the lamina becomes broad and expanded like a eudicot lamina but grasses lilies and many others have linear strap-shaped leaves that grow continuously having no predetermined size
Morphology and Anatomy of Leaf Types
Succulents
desert habitat
succulent leaves are thick and fleshy
very few air spaces
photosynthesis occurs more deeply
bud scales
adaption for hibernation
usually on perennials
small and rarely compound so mechanical wind damage is not a risk for bud scales
Leaves of conifers
leaves are sclerophylls
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thick cuticle
and their epidermis and hypodermics cells have thick walls
unpalatable chemicals
needles of long leaf pine can be 40 cm long
mostly perennial
leaves can live for years
spines
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microscopic green leaves
spines are modified leaves of axillary buds
no mesophyll parenchyma or vascular tissue is present
after fibers mature they deposit lignin in their walls which make them resistant
tendrils
peas, cucumbers, and squash
can grow indefinitely
contain cells that are capable of sensing contact with an object
no leaf blade
Leaves with Kranz Anatomy
C4 photosynthesis
lack palisade parenchyma
ring mesophyll cells
helps with arid environment
Insect Traps
pitcher plants
venus fly traps
trigger hairs
watery digestive fluid
Internal Structure of Foliage Leaves
epidermis
flat thin foliage leaves
have large surface area where water can be lost
optimal for light absorption
leaf and stem epidermis is similar
on sunny days the leaf is usually warmer than the air around it
air is trapped on the underside of a leaf so water loss from stomata there is not so great: water molecules are trapped in quiet air and diffuse into the stomata
unilateral distribution of stomata has other beneficial consequences.
air borne spores of fungi are landing on leaves. rye leaves have 10,000 fungal spores per sq. centimeter but are on the upper surface where stomata are rare. thus fungi rarely penetrate the leaf
epidermis of leaves typically are very hairy
this is beneficial because it protects from sun, and insects, and mammals from eating them
also very waxy in some cases which protects against fungi
mesophyll
ground tissues interior to the leaf
upper surface of most leaves is called the palisade parenchyma
main photosynthetic layer of most plants
each cekk has most of its surface exposed to the intercellular spaces.
lower portion of the leaf is the stingy mesophyll
open loose aerenchyma that permits carbon dioxide to diffuse rapidly away from stomata into all parts of the leafs interior
palisade parenchyma near the upper surface persists max absorption of light and photosynthesis
Vascular Tissues
edict leaf usually has one large mid rub called a mid vein
lateral veins emerge from mid vein and then minor veins branch from that
minor veins are important in releasing water from xylem and loading sugar into phloem whereas the midrib and lateral veins are involved in mostly conduction
the midrib and lateral veins always contain both primary xylem and loading sugar into the lower side
many fibers are arranged into a bundle sheath
a sheath also makes it difficult for insects to chew into the vascular tissues
bundle sheath extension which help give ridgeity to the blade and believed to provide additional means by which water moves from the bundle out to the mesophyll
Petiole
tiny, but massive in plants like palms
one to five vascular bundles make up a leaf trace
in palms there can be bundles of hundreds
we don't understand the significance of bundle patterns
stipules are two small flaps of tissue at the base of a plant