This document discusses leaf structure and function. It begins by defining leaves and their basic anatomy. It then covers leaf classification, morphology, histology, and development. The key structures discussed include the epidermis, mesophyll, vascular bundles, petiole, and abscission zone. Gymnosperm and angiosperm leaves are compared in terms of their tissues and support structures. Leaf development starts from the shoot apical meristem and progresses through initiation, outgrowth, and maturation of tissues.

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Leaves
Principal appendage or lateral
organ of stem
Part of the shoot
Tissue systems: dermal vascular
dermal,
and fundamental
Determinate apical growth (vs.
stem—indeterminate)
LEAF
Biology 101
Structure-function relation Foliage leaf (rel. to photosynthesis)
PHOTOSYNTHESIS Lacks storage tissues
Large external surface Develops no periderm
Extensive air space system Primary tissues only
Abundance of chloroplasts i th
Ab d f hl l t in the
ground tissue
Close spatial relation bet.
Vascular and ground tissue
Classification of leaves Foliage leaves
Foliage Principal photosynthetic organs
Cataphylls
Hypsophylls
cotyledons
t l d
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Cataphylls Hypsophylls
Cata= down; phyllon= leaf Hypso= high
Leaves inserted at low levels of shoot Leaves inserted at high levels of the plant
Scales on bud and underground stem) Floral bracts (protection)
Protection or storage
Prophylls
Pro= before
First cataphylls on lateral branch
Monocots– 1 prophyll
Eudicots
E di t – 2 prophyllh ll
Cotyledon Phyllomes
First leaf of the plant General terms
Include foliage leaves, scales, bracts, floral
appendages
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FOLIAGE LEAF morphology
Blade/lamina– flattened
structure
Petiole
Leaf sheath
Simple and compound leaf
(leaflets)
Cladodes
Stipules
phyllode
Histology of MATURE leaf
Epidermis
ANGIOSPERM LEAF Epidermal cells
Guard cells with subsidiary cells
Trichomes
Ti h
Silica and cork cells
(Gramineae)
Bulliform cells
Fiber like cells
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Epidermis Wall structure of epidermis
Terrestrial Presence of cutin in the outer walls
Living tissue a. Thin – mesophytes and water plants
No well differentiated chloroplasts b. thick, lignified – xerophytes
c. Sili ifi d – grasses and allies
Silicified d lli
Aquatic
May show more abundant chloroplasts
Mesophyll
Mesos= in the middle
Living, lacunose parenchyma with chloroplasts
Mesophyte Dicots– palisade and spongy
Palisade-- development is affected by light
P li d d l t i ff t d b li ht
-- more chloroplasts
(sun vs. shade plants)
Vascular system
Vascular bundles or group of it = veins
Single vein– conifers, Equisetum
Dicot– largest vein occur in median position
(midvein) with
(midvein)—with rib
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Histologic composition
Monocots– usually equal in size or may vary (larger Collateral bundles– x is adaxial; p is abaxial
veins alternalte with smaller ones); median bundle Bicollateral – adaxial phloem occurs only in large
may be larger than others veins
Largest veins
(distribution of bundles)
- circular
- irregular
- crescent shape (if
single)
Veins (dicots)
Larger veins Smaller veins
May have primary entirelyprimary
and secondary tissues Tracheary elements
Vessels and sievetubes are tracheids; phloem
part may parenchyma
only at ultmate
endings
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Bundle sheaths
Part of ground tissue
Also called border parenchyma (dicots)
May contain chloroplasts
In monocots (Gramineae), two types exists:
1. Parenchymatous– with chloroplasts
2. thick-walled sheath/ mestom sheath—inner;
surrounded by parenchymatous sheath also
--- procambial origin
Supporting structures PETIOLE
Not so developed as in the stem Comparable to stem
Flat blades– vascular system Ground tissue =~ cortex of stem
Dicots -- less chloroplasts
– the bundle sheaths and extensions -- supporting structures: collen or scleren
ti t t ll l
-- collenchyma (large veins)
Vascular bundles
-- sclereids
-- collateral (Syringa)
Monocots– large amounts of sclerenchyma
-- fibers (assoc. with vascular bundles) -- bicollateral
-- concentric (most dicots)
Distribution of vascular tissues Petiole
Continuous or multi-stranded arc (open toward 1 Collateral bundle– x is adaxial; phloem is
adaxial) abaxial
Form a circle (with addtl. Bundles within circle) Bicollateral bundle—on both sides of the xylem
Numberous and arranged in several superposed arc If in arcs or circles– phloem oriented periphery
circles
scattered
*rachis and pedicels of leaflets—similar to petiole but
with less tissue
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Pinus leaf
Xeromorphic
GYMNOSPERM LEAF Low ratio of surface to volume
Epidermis heavily cuticularized/ thick-walled
Pinus leaf
Presence of hypodermis—thick wall; compact
yp ; p
(except with stoma)
Guard cells sunken (overtopped by subsidiary cells)
Vascular bundles surrounded by transfusion and
endodermis respectively
mesophyll not differentiated
Other features
Resin ducts
Vascular bundles– x adaxial side; p abaxial side
Xylem is endarch
Transfusion tissue
2 kinds of cells: a] living parenchyma cells with non-
lignified walls and b] thin-walled but lignified tracheids Development of the leaf
with bordered pits
Parenchyma cells- deeply staining
Tracheids (near xylem)
Albuminous cells (near phloem) – dense cytoplasm and
prominent nuclei
Universally present in gymnos
Function: water storage or auxiliary conducting system
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Origin from SAM
Periclinal division in the flank meristem
Lateral protrusion—occurs NEAR the surface
Leaf buttress formed
Leaf develops
Tunica and corpus Early growth and histogenesis
Participates in the formation of leaf primordium After initiation cell division, enlargement and
If single layer tunica– corpus maturation
If three-layered tunica-- tunica Stages of leaf development
a.
a Formation of foliar buttress
b. Formation of leaf axis
c. Formation of lamina
Adaxial meristem
Marginal meristem
a. marginal initials
b. b
b submarginal
i l
initials
As the leaf axis is elevated above
the buttress---procambium is
differentiated
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Vascularization Leaf abscission
Procambium of midvein differentiates first in the Separation of leaf from the stem without injury to
leaf axis the living tissues
As the lamina is formed, the procambium While giving protection to newly exposed surface
differentiates in the middle layers
y from dessication and infection
The development progresses BASIPETALLY Occurs in abscission zone
Abscission zone Histologic structure
Occurs within the Contains minimum of strengthening tissues
petiole or at its base Parenchymatous except in vascular tissues
Facilitating separation Vascular elements (tracheids) are short
a. Histologic structure of
g Weak
W k portion
ti
petiole
b. Presence of
separation layer
Separation layer Protection of the surface exposed
Cell walls are chemically modified a. Formation of scar or cicatrice
Cell walls increase in volume, swell, assume - deposition of suberin, lignin, or wound gum
gelatinous appearance b. Periderm formation beneath the scar
cells separate from each other or are easily
broken
Calcium pectate water soluble pectin
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