Anatomy and Function of Leaves: Structure, Function, and Adaptations - Prof. Robert E. Ree | Study notes Biology

BIOL 1120 REEDER

Anatomy and Physiology of the Leaf

I. Plant Structure

A. Leaf: specialized for photosynthesis; most variable plant organ

1. Mature leaf has three main parts:

a. Petiole: stalk that attaches the leaf to the stem at a location called the node

b. Stipules: some leaves have these leaf-like outgrowths at the petiole base

c. Blade: broad and flattened photosynthetic organ with veins of vascular tissue (midrib through the central

valley to the tip).

2. The leaf's outer surface (both upper and lower):

a. Epidermis: upper and lower

1) Secrets a layer of waxy, acellular cuticle (water proofing agent)

a) Cuticle thickness varies according to environmental differences

2) Contains microscopic openings called stomata surrounded by a pair of guard cells

a) Also found in stems and flower parts

b) Significant to the exchange of gases (oxygen for respiration and carbon dioxide for

photosynthesis) and prevents water loss and desiccation as well (transpiration: normal evaporative

loss)

c) stomata are especially numerous on the lower epidermis and varies between monocots and dicots

3) Except for the guard cells, the epidermal cells are not green and contain no chloroplasts: most cells are

living parenchyma whose cell wall facing the leaf outside is thicker than the cell wall facing inward

3. Sandwiched between the epidermal layers:

a. Two layers of mesophyll (photosynthetic tissue making up the bulk of the leaf)

1) Cells are modified parenchyma with numerous chloroplasts; exhibits turgor, serving as a hydrostatic

skeleton for the leaf as well

a) Palisade mesophyll: tightly packed, elongated cells usually located near the top surface of the leaf

perpendicular to the epidermis; may be one to several layers depending on the species; main

photosynthetic organ

b) Spongy mesophyll: loosely arranged cells with numerous air spaces that communicate with the

atmosphere via the stomata and provide for gas circulation

4. Running through the mesophyll: vascular tissue

a. Strands of vascular tissue called veins, or vascular bundles

b. Functions:

1) Skeletal support: veins are usually surrounded by one or more layers of parenchyma, sclerenchyma or

collenchyma comprising a bundle sheath; also parenchyma as a hydrostatic skeleton

2) Conducts materials to and from metabolically active tissues: xylem (usually located on the upper side

of the vein) and phloem (usually confined to the lower vein side).

5. Leaf Types:

a. Simple: having a single blade or a blade undivided; axillary bud is located in a leaf axil (angle between the

stem and petiole); poplar, oak, castor bean

b. Compound: mature leaf blade divided into several separate parts, or leaflets, connected only by the midrib

with no petiole; each leaflet appears to be a separate leaf; lateral or axillary buds never develop at the base

of leaflets

6. Leaf arrangement on a stem: used as a means of identification

a. Opposite: two leaves at a node opposite each other; lilac

1) Node: stem area where one of more leaves are attached

b. Alternate: one leaf per node with successive nodes at different angles: walnut

c. Whorled: several leaves (3 or more) arise from the same node located at spaced angles around the stem's

circumference; catalpa

7. Venation:

a. Parallel: primary veins run parallel to one another; monocots

b. Netted: veins branching so as to resemble a net; dicots

1) Palmately netted: several major veins radiate out from one point; virginia creeper

2) Pinnately netted: major veins branch off along the entire length of the main vein: rose

8. Differences in monocot and dicot leaves:

a. Dicots:

1) Broad, flattened blade and a petiole typically with netted venation

2) Mesophyll contains both palisade and spongy layers

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BIOL 1120 REEDER

Anatomy and Physiology of the Leaf

I. Plant Structure A. Leaf: specialized for photosynthesis; most variable plant organ

Mature leaf has three main parts: a. Petiole: stalk that attaches the leaf to the stem at a location called the node b. Stipules: some leaves have these leaf-like outgrowths at the petiole base c. Blade: broad and flattened photosynthetic organ with veins of vascular tissue (midrib through the central valley to the tip).
The leaf's outer surface (both upper and lower): a. Epidermis: upper and lower 1) Secrets a layer of waxy, acellular cuticle (water proofing agent) a) Cuticle thickness varies according to environmental differences 2) Contains microscopic openings called stomata surrounded by a pair of guard cells a) Also found in stems and flower parts b) Significant to the exchange of gases (oxygen for respiration and carbon dioxide for photosynthesis) and prevents water loss and desiccation as well (transpiration: normal evaporative loss) c) stomata are especially numerous on the lower epidermis and varies between monocots and dicots 3) Except for the guard cells, the epidermal cells are not green and contain no chloroplasts: most cells are living parenchyma whose cell wall facing the leaf outside is thicker than the cell wall facing inward
Sandwiched between the epidermal layers: a. Two layers of mesophyll (photosynthetic tissue making up the bulk of the leaf) 1) Cells are modified parenchyma with numerous chloroplasts; exhibits turgor, serving as a hydrostatic skeleton for the leaf as well a) Palisade mesophyll: tightly packed, elongated cells usually located near the top surface of the leaf perpendicular to the epidermis; may be one to several layers depending on the species; main photosynthetic organ b) Spongy mesophyll: loosely arranged cells with numerous air spaces that communicate with the atmosphere via the stomata and provide for gas circulation
Running through the mesophyll: vascular tissue a. Strands of vascular tissue called veins, or vascular bundles b. Functions: 1) Skeletal support: veins are usually surrounded by one or more layers of parenchyma, sclerenchyma or collenchyma comprising a bundle sheath; also parenchyma as a hydrostatic skeleton 2) Conducts materials to and from metabolically active tissues: xylem (usually located on the upper side of the vein) and phloem (usually confined to the lower vein side).
Leaf Types: a. Simple: having a single blade or a blade undivided; axillary bud is located in a leaf axil (angle between the stem and petiole); poplar, oak, castor bean b. Compound: mature leaf blade divided into several separate parts, or leaflets, connected only by the midrib with no petiole; each leaflet appears to be a separate leaf; lateral or axillary buds never develop at the base of leaflets
Leaf arrangement on a stem: used as a means of identification a. Opposite: two leaves at a node opposite each other; lilac 1) Node: stem area where one of more leaves are attached b. Alternate: one leaf per node with successive nodes at different angles: walnut c. Whorled: several leaves (3 or more) arise from the same node located at spaced angles around the stem's circumference; catalpa
Venation: a. Parallel: primary veins run parallel to one another; monocots b. Netted: veins branching so as to resemble a net; dicots 1) Palmately netted: several major veins radiate out from one point; virginia creeper 2) Pinnately netted: major veins branch off along the entire length of the main vein: rose
Differences in monocot and dicot leaves: a. Dicots: 1) Broad, flattened blade and a petiole typically with netted venation 2) Mesophyll contains both palisade and spongy layers

Guard cells shaped like kidney beans
stomata on lower leaf surface b. Monocots:
Often lack a petiole, are narrow, and wrap around the stem; parallel venation
Mesophyll may contain both palisade and spongy layers, or is not differentiated (corn and other grasses)
Guard cells shaped like Kidney beans, or dumb bells (grasses, reeds, and sedges)
stomata on both upper and lower leaf surface as well as the stem B. Leaf Functional Aspects Related to its Structure

Primary function is to collect radiant energy and convert it to the chemical energy stored in the bonds of organic molecules (glucose) via photosynthesis a. Leaf epidermis is somewhat transparent allowing light to penetrate into the leaf's photosynthetic mesophyll
Also, stomata involved in regulation of gas exchange (CO (^) 2,O (^) 2), water loss, and temperature
Water is obtained from the soil and transported in the xylem to the leaf mesophyll
The loosely arranged mesophyll allows for rapid CO 2 diffusion where it can dissolve in a water film before diffusing into the cells
The veins, comprised of both xylem and phloem, carries both water and minerals to the mesophyll as well as dissolved sugar resulting from photosynthesis to other plant parts
Connection between leaf structure and environment in which plant is adapted a. Water lilies have long petioles which allow the blade to float on the water's surface 1) Submerged parts have an internal system of air ducts b. Leaves of conifers are waxy needles, with most representative species being evergreen (keep their leaves throughout the year) 1) Leaves help in dry winter survival when soil temperature is low preventing roots from absorbing water: the leaf's thickness results in less surface area exposed to the air resulting in reduced water loss. 2) Other features include a thick waxy cuticle and sunken stomata which allow for gas exchange while minimizing water loss; because the needles are not lost, photosynthesis can resume immediately with spring; deciduous plants (those that shed their leaves) have a lag period before resuming growth. C. Stomatal Opening and Closing
Stomata open during the day (when CO 2 is required), and close at night (when photosynthesis stops); guard cells plus pore referred to as the stomata.
Two guard cells control the pore by changes in their shape a. When water moves into the guard cells, they become turgid resulting from their inner cell walls bending laterally (outward) at the center, producing a pore; the guard cell's ends also stretch easily (balloon-like) b. When water leaves the guard cells, they become flaccid as they lose turgor, closing the stomata
Triggering Mechanism: a. Light and darkness b. CO 2 concentration

Low concentration in the leaf induces stomatal opening, even in the dark

1. Light and CO 2 : as photosynthesis occurs during the day (light), CO 2 is reduced in the leaf, triggering stomatal opening c. Dehydration (water stress)

During prolonged drought, stomata remain closed (even during the day)
Abscisic acid (hormone) is released during water stress to close the stomata d. Internal biological clock that follows a 24 hour cycle: circadian rhythm
Plants placed in continual darkness persist in opening and closing their stomata the same time each day

Mechanism responsible: Potassium ion (K +^ ) mechanism a. When light’s presence and carbon dioxide’s availability for photosynthesis drops, an active transport of K + into guard cells from surrounding epidermal cells occurs b. K +^ accumulates in the guard cell's vacuoles c. The increased K +^ concentration lowers water's potential in the cells resulting in water's movement (osmosis) into the guard cells from surrounding epidermal cells d. The guard cells become turgid, change their shape, and the stoma opens e. More precisely:

protons (H +) are pumped out of guard cells (a) malic acid (malate) produced by photosynthetic activity (b) H +^ produced when malic acid ionizes
an electrochemical gradient forms on two sides of guard cell membrane (charge and concentration differ)

Modified leaves that form traps for insectivorous plants; adapted to attract, capture, and digest a. Plants growing in poor soil that is deficient in certain essential minerals, especially nitrogen b. Passive traps: leaves shaped to form a water reservoir containing digestive enzymes; odor or nectar attracts the prey; pitcher plant c. Active traps: leaves with hairs that can suddenly spring shut; Venus flytrap

Anatomy and Function of Leaves: Structure, Function, and Adaptations - Prof. Robert E. Ree, Study notes of Biology

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BIOL 1120 REEDER