MORPHOLOGY OF MONOCOTYLEDONOUS AND DICOTYLEDONOUS PLANTS

Objectives

This blog post provides readers with the following objectives. The reader will be able to:

o     Describe the external features of monocotyledonous and dicotyledonous plants.

o     Outline the functions of roots, stems and leaves of plants.

o     Identify modified roots, stems and leaves of plants and their functions.

External Features of Monocotyledonous and Dicotyledonous Plants

Plants are the living, multicellular, eukaryotic organisms, which belongs to the kingdom Plantae. Plants produce their own food by photosynthesis and provide shelter, place and food for animals. There two classes in the angiosperms or flowering plants; monocotyledonous and dicotyledonous plants. They are classified base on the number of seed-leaves (cotyledons) contained in the seed.

A typical flowering plant consists of two main systems: root system and shoot system. 

The Root System

The root system is the organ of a plant which lies below the surface of the soil. It comprises the main root, together with all the lateral branches. There are two types of root system.

1. Tap root system (Primary root): a root system which develops from the radicle of the embryo. E.g., carrot, radish, etc. Smaller secondary roots known as the lateral roots are produced on the primary root. The secondary roots in turn produce tertiary roots.

2. Adventitious root system: this root develops from any parts of the plant other than the radicle. It may develop from a node or inter node of the stem, or even from the leaf. Types of adventitious root system. 

a. Fibrous roots: in monocotyledonous plants, the primary root is short lived and is replaced by a large number of roots of similar size. These roots originate from the base of the stem and constitute the fibrous root system e.g., rice, maize, etc.

b. Foliar roots: these roots develop directly from the leaf, from the petiole, or the vein e.g., rubber plant and Bryophyllum.

Functions of Roots 

1. It anchors the plant into the soil

2. It absorbs water and minerals from the soil

3. Roots may also serve as sites of food storage and hormone synthesis.

4. Roots conduct Water and minerals to the aerial parts of the plant

5. Some root crops are edible structures. E.g., sweet potato, carrot, and angelica

6. Roots are also used in vegetative propagation.

7. Roots also help in environmental protection by preventing soil erosion.

Modification of Roots

Roots may be modified to different shapes in order to perform special functions besides their normal functions.

Modification of Root for Storage of Food

Storage roots are swollen with nutrients and water to prepare for unfavorable conditions. E.g., carrot, cassava, sweet potato

Modification of Root for Mechanical Support

1. Prop roots: Prop roots grow from the stem to the ground. They give extra support to the plants. E.g., Ficus, maize. 

2. Stilt roots:  they develop near the base of the stem and grow downwards to provide mechanical support. E.g., red mangrove. 

3. Climbing roots: these roots twine around and clasp the support and help in climbing, e.g., Betel (Piper betel) etc.

4. Clasping roots: Clasping roots grow from the nodes of the soft stem to cling on to other plants. It provides mechanical support to the plant. E.g., pepper, ivy.

5. Root buttresses: large trees have great plank-like roots that radiate from the base of the stem. These consist of partly root and partly stem e.g., Bombax, silk cotton, flamboyant.

  

Modification of Roots for Gaseous Exchange

Pneumatophores: These roots grow vertically up and come out of the water or marshy soil. The roots have a large number of breathing pores for exchange of gases. E.g., white mangrove

The Shoot System

The shoot system consists of stems, leaves, and flowers. The stems, roots and leaves form the vegetative organs of plant while the flowers reproductive organs.

Stem

Stem develops from the plumule of the embryo of a germinating seed. It bears nodes and internodes. It also bears buds, which may be terminal or axillary. Buds develop into either stem branches or flowers.

Parts of stem

Node: is part of a stem where a leaf or bud is fixed. Internode: is the regions between two successive nodes. Terminal or apical buds: are located at the apex or tip of the stem. Lateral or axillary buds: are found on the sides of the stem, usually in the leaf axil. Adventitious buds: develops at other parts of plant other than the axil and apex. Leaf scar:  mark left on the stem where a leaf was attached. Bud scale scar: mark on the stem where a bud scale was attached. Lenticel: a breathing pore in the skin or bark of a stem.

N/B: When the terminal bud sprouts and its scales fall off, a growth rings or girdle are formed. The portion of a stem between two sets of growth rings indicates one season's growth.

Functions of Stem

1.    It supports leaves in position to receive maximum light
2.    It supports flowers and fruit for effective pollination and dispersal
3.    It transports water and solutes from the roots to the leaves.
4.    It transports manufactured food from the leaves to other parts of the plant
5.   Green stems are photosynthetic.
6.   Serve as food storage organs.

7.   It takes part in vegetative reproduction. 

Modification of Stem

Modification of Stem for Climbing

1. Tendrils: Plants with weak stems produce slender, spirally coiled, sensitive organs called tendril for climbing.  E.g., cucumber, watermelon and grapes. 

2. Twiners: These plants have long and slender stems with branches. They climb by twining bodily around trees and shrubs. E.g., Ipomoea, yam. 

3. Thorns and prickles: These are hard, woody, pointed structures meant for protection and also allow plants to adhere to other plants. E.g.  Citrus and Bougainvillea

Modification of Stem for photosynthesis

Phylloclade: These are flattened or cylindrical chlorophyll containing stems which carry out photosynthesis. The leaves of such plants are reduced to scales or spines to decrease the loss of water by transpiration. Example: Opuntia and Euphorbia. 

Modification of Stem for Water Storage

Some plants store large quantities of water in their stems which enables them to withstand period of drought. Such plants have reduced leaves or no leaves at all to decrease the loss of water by transpiration. E.g., Opuntia and Cactus (Euphorbia).

Underground Stem Modifications               

Most storage stem grows underground. They are modified for storage of food and vegetative reproduction. The underground stems are of four types namely rhizome, tuber, bulb and corm.

Modification for food storage: Food is stored to enable the plant to survive unfavorable conditions such as drought. E.g.  potato, ginger, canna lily and cocoyam

Modification for vegetative propagation: Stems of some plants are modified for reproduction through cutting. A piece of adult stem, with several nodes, is cut from the parent and half-buried in the soil. The buds grow to produce new plants using food reserved in the stem. E.g. cassava, sugar cane, yam, cocoyam, ginger etc. 

Leaf     

The leaf is a lateral, generally flattened structure borne on the stem. The principal function of a leaf is to absorb sunlight for the manufacture of food in a process called photosynthesis. A typical leaf consists of the following parts;

Petiole: It is the stalk or the part which connect the leaf to the stem. 

Blade or Lamina: It is the large, broad, flattened portion of the leaf. It plays a vital role in absorbing sunlight. 

Midrib: It is the large, center, main vein running down the center of the leaf.

The leaf base: it’s the part of leaf petiole that is attached to the stem.

The angle formed between the petiole and the stem is called the leaf axil. A bud or cluster of buds is usually located in the axil. 

Sessile Leaf: this is unstalked leaf and do not possess a petiole. 

Petiolate Leaf: is also known as stalked leaf; this is the leaf that has a petiole.

Leaf Venation

Venation is the arrangement of veins in a leaf. The prominent vein is referred to as mid-vein or primary vein. Branches from the main vein are secondary veins, which give rise to tertiary veins. There are basically four types of leaf venation:

1.    Parallel venation: veins are arranged parallel to one another from the base to the apex.  This occur in monocotyledonous plants, e.g. grasses

2. Reticulate venation: the veins are arranged in network. This is characteristics of dicot plants. E.g. Hibiscus   

3. Pinnate venation: the veins extend from a midrib to the leaf edge e.g. apple.

4. Palmate venation: the veins radiate fan-shaped from the petiole e.g. grapes.

Leaf Types

a. Simple leaf: It has a single lamia (an undivided blade). The shape of the leaf may be of lobes but the lobes do not reach the main vein or the midrib. Simple leaf may be either pinnate or palmate E.g. Hibiscus

b. Compound Leaf: The leaf blade is fully subdivided into multiple leaflets. The middle vein of a compound leaf is called a rachis.

1. Palmate Compound Leaf: leaflets are attached to the tip of the petiole.

2. Trifoliate: a palmate compound leaf which consists of three leaflets. E.g. beans.   

3. Pinnate Compound Leaf: It consists of leaflets laterally arranged along the midrib. E.g., cassia

4. Bipinnate Compound Leaf: the leaflets are also divided into leaflets and the leaflets are arranged along a secondary vein. E.g., Flamboyant

Leaf Margins

The edges of leaf lamina may be smooth, lobed or toothed. 

1. Entire: has smooth margin (no teeth nor lobes on the margin).

2. Serrate: has small teeth on the margin which point toward the apex; saw-like. 

3. Double serrate: has teeth which have smaller teeth on them.

4. Lobed: margin divided into rounded lobes and sinuses.

Phyllotaxy  

This is the arrangement of the leaves on the stem or branch. This is usually of three types:

Alternate: a single leaf arises at each node, e.g. Mustard, Rose Balsam.

Opposite: a pair of leaves arise at each node and lie opposite to each other e.g. guava. 

A subopposite arrangement is a condition in which the leaves are not spaced far enough apart to be considered alternate nor are they perfectly opposite one another.

Whorled: If more than two leaves arise at a node and form a whorl, it is called whorled e.g.  Alstonia.

Leaf Shapes

1.     Ovate: egg shaped with a broad base

2.   Linear: very long and thin, with the sides parallel

3.   Oblong: tapered to both ends, but with the sides more or less parallel

4.   Cordate: heart-shaped

5.   Elliptic: shaped like an ellipse, tapered at both ends and with curved sides.

6.   Oval: broadest at the middle   Etc.

Function of a Leaf

1.   It contains the pigment chlorophyll which require for photosynthesis.

2.    Stomata of the leaves help in gaseous exchange. 

3.   It aids in the removal of excess water during transpiration process.

4.   It serves to store food materials e.g. onion. 

5.    It takes part in vegetative propagation.

Modification of Leaves

Modification of leaves for protection

Bud scales: are tough, overlapping, waterproof leaves that protect buds from frost, desiccation, and pathogens. E.g. canna lily, onion

Spines: some leaves are modified to form defensive structures (spines) e.g. Opuntia 

Modification of leaves for Climbing

Tendrils: leaves lamina or leaflets are reduced to slender, spring like structures that allows plant to cling to other objects. E.g. Grapes, Sweet Pea.  

Modification for storage

Bulb: leaves are modified as storage organs. e.g. onion

Succulent leaves: Leaves are thick and used for water storage.

Modification for water conservation

Scale Leaves: most desert plants leaves are reduced to form scales. The scales are thin, dry, membranous, stalk-less which minimize the rate of transpiration. E.g. Cedar

Spines: reduce water loss e.g. cactus

Modification for reproduction 

Some plants have modified colorful leaves that attract pollinators and aid in reproduction. E.g. Sugar Maple, Red Maple.

Bryophyllum:  leaves possess adventitious buds and undergo vegetative propagation.

Modification for trapping insects

Insectivorous (Carnivorous) plants: e.g. Pitcher Plants, Sundew, Bladderwort, Venus flytrap, Butterwort. Carnivorous plants have green leaves and are able to manufacture their own food. Usually grow in places with little nitrogenous salts and use insects and other small animals as source of nitrogen. They have modified leaves with bright color/scents/sugary liquids for attracting and trapping insects. The plant secretes enzymes to digest the insect and absorb soluble nitrogenous compounds.

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