NTA Level 5 • Semester 2 • PST05210

Morphology of the Leaf

Basic Pharmacognosy • Source Session/Topic 7

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Session 7: Morphology of the Leaf

Total Session Time: 120 minutes

Prerequisites

• None

Learning Tasks

By the end of this session students are expected to be able to:

• Define a leaf

• List external structures of a leaf

• List internal structures of a leaf

• Explain the anatomy of a leaf

• Explain the criteria used in classification of leaves

• Explain the pharmacognostic importance of leaves

Resources Needed:

• Flip charts, marker pens, and masking tape

• Black/white board and chalk/whiteboard markers

• LCD projector and Laptop

SESSION OVERVIEW

SESSION CONTENTS

STEP 1: Presentation of Session Title and Learning Tasks (5 minutes)

READ or ASK students to read the learning objectives and clarify

ASK students if they have any questions before continuing.

STEP 2: Introduction to Plant Leaves (15 minutes)

|Activity: Buzzing (5 minutes) |

| |

|ASK students to pair up and buzz on the following question for 2 |

|minutes |

| |

|What is a leaf? |

| |

|ALLOW few pairs to respond and let other pairs to add on points not|

|mentioned |

| |

|WRITE their response on the flip chart/board |

| |

|CLARIFY and SUMMARIZE by using the content below |

• A leaf is a plant organ that is found above the ground and is specialized

for photosynthesis

• The leaf is typically flat and thin

• Functions of the leaf

o Provides for transpiration and guttation

o Storage of food and water

• The leaf consists of a petiole, a lamina (or the leaf blade),

and stipules

o Petiole

▪ Attaches the leaf to the stem

▪ Some leaves are directly attached to the stem without a petiole

o The lamina

▪ Is flat and offers surface for trapping sunlight for photosynthesis

STEP 3: Internal Structures of a Plant Leaf (40 minutes)

|Activity: Brainstorming (5 minutes) |

| |

|Ask students to brainstorm on the following question: |

| |

|What are the internal structures of the plant leaf? |

| |

|ALLOW few students to respond? |

| |

|WRITE their responses on the flip chart/ board |

| |

|CLARIFY and SUMMARIZE by using the content below |

• The leaf is made up of three tissue systems (Dermal, Ground and Vascular)

• The dermal tissue system consists of lower and upper epidermis

• The ground tissue system is found between the epidermal layers of leaf

o Epidermis

▪ In dicot leaves, the epidermis is dorsiventral

▪ Generally, the epidermis is made up of a single layer of cells

that are closely packed and covered with a cuticle

▪ The epidermal tissue is made up of;

• Epidermal cells

• Guard cells

• Subsidiary cells

• Epidermal trichomes

o Epidermal cells

▪ Epidermal cells are the most numerous, largest, and

least specialized

▪ Epidermal cells are more elongated in the leaves of

monocots than in dicots

o Guard cells

▪ Consist of a pair of bean- shaped cells surrounding the

stoma

▪ They contain chloroplasts

▪ Other epidermal cells do not contain chloroplasts

o Subsidiary cells

▪ They are 2 to 4 cells that surround the stoma

▪ They do not contain chloroplasts

o The epidermal layer consists of stomata

▪ The stomata are part of a stoma complex which consists of

a pore surrounded on each side by guard cells and subsidiary

cells

▪ The stoma complex regulates the exchange of gases and

water vapour between the outside air and the interior of the

leaf

▪ There are more stomata over the abaxial that the adaxial

epidermis.

• Main function of the epidermis is to protect inner tissues

Figure 7.1: Transverse Section of a Leaf

[pic]

• Mesophyll

o Mesophyll is the entire tissue between the upper and lower epidermis is

Mesophyll is differentiated in two regions;

▪ Palisade parenchyma and

▪ Spongy parenchyma

• This differentiation gives two anatomical designations of the leaf

• Dorsiventral leaf

• Mesophyll is differentiated into palisade parenchyma (or palisade

mesophyll) on the adaxial (upper) side and spongy parenchyma (or

spongy mesophyll) on the abaxial (lower) side

• Common in dicot leaves

• Isobilateral leaf

• Leaf mesophyll is not differentiated this way (i.e., made of only

spongy or palisade parenchyma)

• Common in monocots

o Palisade Parenchyma (Palisade Mesophyll)

▪ Palisade parenchyma lies beneath the adaxial

epidermis

▪ Palisade mesophyll may be present below both surfaces or

occur only below the upper epidermis

▪ Consists of vertically elongated cylindrical cells in one

or more layers

▪ Cells are compactly arranged with the chloroplasts close to

the walls of the cell

▪ Main function is photosynthesis

o Spongy Mesophyll

▪ Lies below the palisade mesophyll, with irregularly shaped,

loosely arranged cells

▪ Contain fewer chloroplasts than palisade mesophyll

▪ Spongy cells facilitate exchange of gases with the help of

air spaces

▪ The pores or stomata of the epidermis open into sub

stomatal chambers, connecting to air spaces between the

spongy layer cells

o Vascular Tissues

▪ Veins are the vascular tissue of the leaf located in the

spongy layer of the mesophyll

▪ Based on pattern of venation, the vascular system of leaves

falls into two main classes:

• The reticulate (or net) venation typical of dicotyledons

• The parallel venation of the monocotyledons

▪ The veins are made up of xylem and phloem tissues

▪ The xylem typically lies over the phloem

▪ In dicotyledonous leaves the main veins are open and

usually collateral (less commonly bicollateral), closed in

monocot leaves

Figure 7.1: Transverse Section of a Leaf

[pic]

STEP 4: Pharmacognostic Importance of Epidermis (25 minutes)

• Leaves offer diagnostic characteristics which serve in quality control of

leaf drugs

o Shape, size and wall structure of the epidermal cells

▪ Straight –walled epidermal cells are found in medicinal plants such

as coca and Senna leaves

▪ Wavy-walled epidermal cells are in found in Datura stramonium,

Hyoscyamus niger and Atropa belladonna.

▪ Form, distribution and relation to epidermal cells of the stomata

▪ Stomata are classified on the basis of subsidiary cells into;

• Anomocytic Stomata

o When cells surrounding the stomata resemble other epidermal

cells

• Anisocytic stomata

o Stoma is surrounded by three or four subsidiary cells one of

which is markedly smaller than others

• Paracytic stomata

o Two subsidiary cells with their long axes parallel to the

pore

• Diacytic stomata

o Two subsidiary cells with their long axis at right angle to

the pore of the stomata

o Variations among anisocytic, paracytic and diacytic types

of stomata:

• Actinocytic type

o Subsidiary cells are arranged along the radii of a

circle. Form, distribution and abundance of epidermal

trichomes

• Trichomes offer physical and chemical protection from microbes, aphids

and insects.

o Types of trichomes are characteristic of a plant family or genus

▪ Clothing trichomes –cover surface of the leaf

▪ Glandular trichomes –may be unicellular or multicellular (they

store ergastic/drug substances)

• Multicellular trichomes may be;

o Uniseriate, biseriate, multiseriate or complicated branched structures

STEP 5: Classification of Leaves (25 minutes)

• External leaf characteristics (such as shape, margin, hairs, etc.) are

important for identifying plant species

• Leaves are determinant in growth i.e. they grow and achieve a specific

pattern and shape, then stop

• Other plant parts like stems or roots are non-determinant

• Leaves are classified on the basis of:

o Arrangement on the stem

o Alternate, opposite, whorled, rosulate,

o Divisions of the lamina

▪ Simple leaves, compound leaves (palmately compound leaves, pinnately

compound leaves, binately compound leaves)

o Leaf margin

▪ Entire, ciliate, crenate, dentate, denticulate, doubly toothed,

lobate, serrate, serrulate, lanceolate)

o Leaf Apex

▪ Acuminate, acute, cuspidate, ermaginate, obtuse, obcordate,

mucronate,

o Leaf base

▪ Acuminte, acute, cordate, cuneate,

o Surface of the leaf

▪ Farinose, glabrouse, verucose, viscid, pubescent, hirsute,

glandular)

STEP 6: Key Points (5 minutes)

• A leaf is a plant organ above the ground specialized for photosynthesis

• The leaf is made up of dermal, ground and vascular tissue systems

• The dermal tissue system consists of lower and upper epidermis

• The ground tissue system is found between the epidermal layers of leaf

STEP 7: Evaluation (5 minutes)

• What are the functions of the leaf?

• What are the external features of a leaf?

• What is the importance of the leaf in pharmacognosy?

References

Trease, G. E., Evans, W. C., & Evans, D. (2009). Trease and Evans

pharmacognosy. London: Saunders.

Joanne Barnes et al (2002), Herbal medicines 3rd Edition: Pharmaceutical

Press

Wallis, T. E. (2005). Textbook of pharmacognosy. New Delhi: CBS.

Robbers, J. E., Speedie, M. K., Tyler, V. E., & Tyler, V. E. (1996).

Pharmacognosy and pharmacobiotechnology. Baltimore: Williams &

Wilkins.

Heinrich, M., Kinghorn, A. D., Maizels, D., Gibbons, S., & Phillipson, J.

D. (2012). Fundamentals of pharmacognosy and phytotherapy. Edinburgh:

Churchill Livingstone/Elsevier.

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