External Morphology and Plant Part Modifications Study Guide

Learning Objectives for Plant External Morphology

• Explanations of the structure for modified roots, stems, and leaves are documented to distinguish them from standard morphology.

• The importance of modified roots, stems, and leaves in biological functions and human life is established.

• Procedures for drawing accurate figures of modified plant parts are provided.

• Knowledge is applied to realize the practical significance of these modifications in daily life.

Modifications of the Main Root

• The primary function of a root is to entangle the tree within the soil. However, roots undergo modifications for special functions, particularly food storage.

• Roots modified for food storage lack nodes, internodes, and leaves. Any leaves visible above the soil grow from small stems situated directly above the roots.

• There are four distinct types of main root modifications based on shape:

• Fusiform Roots: These roots become thick and juicy due to food storage. The middle portion is characteristically thick, while the two ends gradually taper. Example: Radish.

• Conical Roots: These store food resulting in a main root that is thick and juicy. The upper side is broad, and the lower side tapers gradually toward the bottom. Example: Carrot.

• Napiform Roots: The upper portion of the root becomes significantly rounded due to food storage, while the lower part becomes suddenly thin. Example: Turnip.

• Tubercular Roots: The main root swells irregularly due to the storage of food, resulting in no particular or defined shape. Example: Mirabilis (Sondhya Maloti).

Modified Adventitious Roots

• Adventitious roots are modified to perform special functions categorized into food storage, mechanical support, and physiological activities.

Modifications for Food Storage

• Root Tuber: These grow from nodes near the soil level and become irregularly swollen. They attain an indefinite shape for future food use. Example: Sweet potato.

• Fasciculated Roots: These function similarly to root tubers but form a fascicle or bunch. All roots within the bunch swell up due to food storage. Example: Asparagus and Dahlia.

• Nodulose Roots: These roots are characterized by swelling specifically at their tips for food storage. Example: Mango Ginger.

• Moniliform Roots: These adventitious roots swell and constrict alternately, resembling a string of beads. Example: Bitter gourd.

Modifications for Mechanical Support

• These modifications assist plants in standing straight, climbing, or floating.

• Prop Roots: These grow from the stem of branches and extend downward into the soil. They eventually thicken to resemble pillars that bear the load of branches. Example: Banyan.

• Stilt Roots: Found in plants with weak stems that cannot stand erect. Roots develop from the lower part of the main stem and grow tangentially until they reach the ground to provide support. Example: Screw pine (Keya).

• Climbing Roots: These grow from the nodal regions of weak stems and help the plant climb by clinging to a support or another plant. Example: Betel vine.

Modifications for Physiological Functions

• Aerial Roots: These roots absorb moisture from the environment. Example: Orchid.

• Parasitic Roots (Sucking Roots): Found in parasitic plants lacking chlorophyll. These roots penetrate the host plant's body to extract food. Example: Gold creeper (Dodder).

• Respiratory Roots (Pneumatophores): Found in saline and muddy soil in coastal regions. Branch roots grow upward from the main root and feature small pores on their surface to aid in respiration. Example: Sundori, Goran.

• Reproductive Roots: Certain roots take part in the process of reproduction. Example: Sweet potatoes, Teasle gourd (Kakrol).

Modified Stems

• Stems typically grow above soil level and bear leaves, flowers, and fruits, but they modify their position and structure for special functions.

Underground Modified Stems

• These stems grow beneath the soil surface to survive adverse conditions, store food, and aid in vegetative propagation.

• Tuber: Round and swollen stems that bear nodes, internodes, scale leaves, and axillary buds. Cavities at the leaf axils of scale leaves are called "eyes," from which new plants grow in favorable conditions. Example: Potato.

• Rhizome: These grow parallel to the soil surface or erect. They exhibit clear nodes, internodes, scale leaves, and roots at the nodes. Example: Ginger, Turmeric.

• Bulb: The stem is very small, round, and convex (elevated in the middle). Nodes and internodes are compressed. Thick, juicy scale leaves cover the stem, and the base produces profuse fibrous roots. Example: Onion, Garlic.

• Corm: Large, more or less round stems with prominent nodes and internodes. Axillary buds from scale leaves grow laterally to produce daughter corms. Example: Amorphophallus (B. ol).

Sub-aerial Modified Stems

• These consist of weak stems lying on or under the soil surface, used for vegetative propagation.

• Runner: Weak stems arising from the axillary buds of lower nodes that run along the ground. Example: Hydrocotyle (Thankuni), Oxalis (Amruli), Cynodon (Durba).

• Stolon: Horizontal long branches growing from the base of the plant. The branches curve, and nodes that touch the soil produce roots and new plants. Example: Arum (Man Kochu).

• Offset: Characterized by thick and short internodes in aquatic plants, giving the stem a dwarf appearance. Example: Pistia (Topapana), Eichhornia (Kochuripana).

• Sucker: Axillary buds that grow laterally beneath the soil surface to produce new plants. Example: Chrysanthemum (Chandramallika), Bamboo.

Aerial Modified Stems

• These stems remain above the soil but change to perform functions like food synthesis, self-defense, or climbing.

• Phylloclade: Flat, green stems that can manufacture food. The leaves are modified into spines for self-defense and to reduce water loss. Example: Cactus (Phonimonosha).

• Thorn: Axillary buds modified into stout, sharp structures for protection. Example: Wood apple (Bel), Lawsonia (Mehendi), Maina Kata.

• Stem Tendril: Thread-like, thin, and spirally coiled structures that help climbers cling to support. Example: Passion flower, Harjora.

• Bulbil: Axillary buds that store food and become round, ball-like structures instead of developing into branches. Example: Dioscorea (Chupri aloo).

Modified Leaves

• Leaves change their normal structure to perform specific biological roles.

• Leaf Tendrils: The apex of the leaf or the whole leaflet is modified into a spring-like structure to help the plant cling to support. Example: Wild pea.

• Storage Leaves: Thick and juicy leaves used for storing food and water. Example: Onion, Garlic, Aloe vera.

• Fly Traps: Leaves modified into pitchers or bags to capture insects. When an insect enters, the lid closes, and the plant absorbs nutrients from the insect. Example: Pitcher plant, Dodder (aquatic plant).

• Reproductive Leaves: Buds grow from the leaf margins, develop roots, and eventually detach to become independent plants. Example: Bryophyllum (Patharkuchi).

• Leaf Spines: Leaves modified into sharp spines for protection. Example: Lemon.

• Scale Leaves: Thin, dry, or juicy leaves found on underground stems. Juicy scale leaves (such as in onions) store food and protect the apical bud.

Questions & Discussion

1. In which plant are Moniliform roots found?

• a. Dahlia

• b. Mango ginger

• c. Sweet Potato

• d. Bitter Gourd

• Answer: d. Bitter Gourd

2. What are the characteristics of a Rhizome?

• i. Definite nodes and internodes.

• ii. Nodes and internodes are compressed.

• iii. Stay in horizontal position under the soil.

• Answer: i and iii are correct (based on standard description of ginger/turmeric).