Morphology of Flowering Plants (Part B): STEM - PDF

IBL-109 SEMESTER-1 LECTURE-5 SCHOOL OF AGRICULTURE SEACOM SKILLS UNIVERSITY PREPARED BY – MS. ARITRI PAL...

IBL-109 SEMESTER-1 LECTURE-5 SCHOOL OF AGRICULTURE SEACOM SKILLS UNIVERSITY PREPARED BY – MS. ARITRI PAL MORPHOLOGY OF FLOWERING PLANTS ( PART-B ) : STEM : The stem of a plant has several functions, including: Support: The stem provides support for the plant's leaves, flowers, fruits, and branches. Transportation: The stem transports water and nutrients from the roots to the leaves, and sugars from the leaves to the rest of the plant. Elevation: The stem keeps the leaves in the light and provides a place for the plant to keep its flowers and fruits. Protection: The stem protects the plant. Vegetative propagation: The stem is involved in the vegetative propagation of plants. Production of new living tissues: The stem helps in the production of new living tissues. Types of Modifications with Examples - Modified stems can be categorized into three types: Underground – Rhizome, Bulb, Corm, Tuber Subaerial – Runner, Sucker, Offsets, Stolon Aerial – Tendrils, Thorn, Bulbils, Cladode, Phylloclade 1. Underground Stem Modifications As the name suggests, these are present underground. They serve various purposes such as storage of food, propagation and perennation. They can be identified from roots by the presence of nodes and internodes. They help in propagation through asexual reproduction and help the plant survive a period of dormancy. They provide protection from grazing by animals and environmental stress. Rhizome – It runs parallel to the ground, the upper portion can produce scaly leaves and the bottom portion can give rise to adventitious roots. They contain, nodes, internodes and buds. They store food.Examples: Ginger, turmeric.Banana contains rootstock, which is rhizome growing vertically. Bulb – Stem is reduced and surrounded by scale leaves, modified bulbs can be seen.Examples: Tulips, Lilies, Daffodils, Onion, Garlic Corm – It is a short upright stem, covered with scaly leaves. It is hard as compared to bulbs.Examples: Colocasia, Yam, Saffron Tuber – Here the terminal of the stem is fleshy and stores food. The stem contains nodes and internodes but due to swelling, they are not very distinct. Presence of scaly leaves at nodes can be seen as eyes or scars.Examples: Potato, Artichokes 2. Subaerial Stem Modifications These types of stems are present on the ground or partially underground. They are present mostly in plants, which are short-lived and have a weak and herbaceous stem. The stem is used for faster propagation. Runner – It originates from the elongation of the basal internode and runs horizontally over the ground. It carries bud and anchors the soil with the help of root development at nodes. It gives rise to a new plant.Examples: Grasses, Cynodon, Oxalis Stolon – A short and weak aerial branch arises from the main axis it bends down and touches the ground forming roots. A new plant develops there.Examples: Mint, Strawberry Sucker – Sucker arises from the underground portion of the stem, it grows horizontally under the soil and then comes above the ground. It develops adventitious roots and shoots with leaves and forms a new plant.Examples: Chrysanthemum, banana, pineapple. Offsets – They are found mostly in aquatic plants. A lateral branch arises having short internodes. At nodes, rosettes of leaves develop at the upper portion and roots below.Examples: Pistia, Eichornia 3. Aerial Stem Modification In some plants, the aerial part of the stem and buds get modified to perform various functions such as support, climbing, photosynthesis and vegetative propagation. Tendrils – These are coiled and a delicate part, which are used for climbing. They develop from the axillary bud.Examples: grapevines, watermelons, pumpkin, cucumber Thorns – These are developed from axillary buds. They are pointed, woody and hard, spiny structures, which provide protection from grazing animals.Examples: Bougainvillea, lemon Bulbils – It is a modified axillary bud, it stores food and becomes fleshy. It detaches from the mother plant and develops into a new plant.Examples: Dioscorea Cladode and Phylloclade – These are modified stems, which are green and perform photosynthesis. They become fleshy. Leaves are reduced to spines. These are commonly found in xerophytic plants to reduce water loss.Examples: Opuntia, Euphorbia, Asparagus :LEAF: Venetion: It is the arrangement of veins and also the various veinlets in the leaves. Different plants show different types of venation. Usually, there are two types of venation: Reticulate venation: In a reticulate venation, the veinlets are arranged in a random fashion to form a complex network of veinlets. Ex: Dicotyledonous plants like a rose plant. Parallel venation: In this fashion, the veinlets are arranged parallel to each other. Ex: In monocotyledons like paddy. Structure of a Leaf Leaves are thin, flat organs responsible for photosynthesis in the plants. It develops laterally at the node. It is an important part of the shoot system and it originates from shoot apical meristems. The structure of a leaf is described below in detail : Parts of a Leaf: Generally, leaf base, petiole, and lamina, together form the main parts of a leaf. Leaf Base: This is the part where a leaf attaches to the stem. Leaf base has two small leaf-like structure called stipules. In plants like paddy, wheat, and other monocotyledons, this leaf base is wide and masks the stem. Petiole: Petiole is the long, thin, stalk that links the leaf blade to the stem. Lamina: Also known as leaf blade. It is the green, flat surface of the leaves. It consists of a small branched vein and veinlets. The vein that runs along the middle of the lamina is called midrib. Midrib divides the surface of the lamina into two. These veins and veinlets give rigidity to the leaf blade and help in the transportation of water and other substances. Types of Leaves: There are two broad categories of leaves – simple and compound, which are further classified into different groups based on their shape, size, their arrangements on the stem, leaves of flowering and non-flowering plants, and various other physical attributes. The two different types of leaves found in a plant are: Simple Leaf When a single lamina is connected to the main stem by a petiole, the leaf is said to be simple. A simple leaf may be incised to any depth but not down to the midrib or petiole. Eg., Guava leaves Compound Leaf A compound leaf is a leaf made up of two or more leaflets. In a compound leaf, the midrib of the leaf is branched into different leaflets and is connected by a single petiole. For eg., Pea, palm leaves. Palmately Compound Leaf In a palmately compound leaf, the leaflets are attached at the tip of the petiole. Eg., Silk cotton. These can be differentiated into: Unifoliate: These type of leaves have only one leaflet. Eg., Citrus Bifoliate: These leaves have two leaflets. Eg., Balanites Trifoliate: These leaves have three leaflets emerging from the same point. Eg., Oxalis Quadrifoliate: These leaves have four leaflets arising from the same point. Eg., Marsilea Multifoliate: This type of leaf has many leaflets arising at a common pint. Eg., Bombax Pinnately Compound Leaf In a pinnately compound leaf, the midrib of the leaf is divided into numerous leaflets and all connected by a common axis. Eg., Neem. These can be further differentiated into: Pinnate: A compound leaf that has an axis on each side of the midrib is known as a pinnate leaf. Unipinnate: The leaf with leaflets on each side of the axis. Eg., cassia Bipinnate: Here, a secondary axis bearing the leaflet is produced by the central axis. Eg., Acacia Tripinnate: Here, a tertiary axis bearing leaflets emerges from the secondary axis. Eg., Moringa Decompound: Leaf with more than three pinnate. Eg., old leaves of coriander Parapinnate: A leaf without a terminal leaflet. Eg., Cassia Imparipinnate: Leaf with an odd terminal leaflet. Eg., Pea Phyllotaxy: The patterns of arrangement of leaves on the stem are called Phyllotaxy. Plants show three types of phyllotaxy- alternate, opposite and whorled types of phyllotaxy. When only a single leaf develops at each node alternatively, it is an alternate type of phyllotaxy.E.g. China rose. When a pair of leaves develops at each node opposite to each other, it is called opposite phyllotaxy.E.g. Guava plants. When more than two leaves develop at the nodes to form a whorl of leaves, it is called whorled phyllotaxy. E.g. Alstonia. Modification Of Leaves: We know leaves are specialised to perform photosynthesis. In addition, they also have other significant roles to play, such as support, storage of food, defence, etc. For each of these functions, they have been modified into different forms. For example, tendrils of peas, spines of cacti, onion bulb, leaves of insectivorous plants, etc. are different modified leaves. Let us have a detailed look at some of the modification of leaves: Storage Leaves The xerophytic plants and plants belonging to the Crassulaceae family have thick and succulent leaves that store water in their tissues. The parenchymatous cells of these leaves have large vacuoles filled with hydrophilic colloid. This modification helps the plant to resist desiccation. Leaf Tendrils Leaf tendrils exist in plants with weak stems. The leaves get modified into thread-like structures called tendrils. These tendrils climb a nearby stick or wall and provide support to the plant. For eg., In Lathyrus aphaca, the whole leaf is modified into tendrils. The upper leaflets of Pisum sativum get modified into tendrils. Leaf Spines A few plants have their leaves modified into needle-like structures known as spines. The spines act as defensive structures. They also reduce water loss due to transpiration. For eg., in Opuntia, the leaves are modified into spines. Scale Leaves These are thin, membranous structures, without stalks, brownish or colourless in appearance. They protect the auxiliary bud present in their axil. Scale leaves in onion are fleshy and thick and store food and water. Casuarina and Asparagus also contain sale leaves Leaflet Hooks In some plants, the terminal leaflets of leaf get modified into hook-like structures that help them in climbing. Eg., Bignonia unguiscati. Leaf Roots In a few plants, one of the leaves present at the nodes gets modified into adventitious roots which helps them to float over the water surface. Eg., Salvinia Phyllode In some plants, the petiole becomes flattened, taking the shape of a leaf and turns green in colour. This is known as phyllode. For eg., Australian Acacia. Insectivorous Leaves: Few plants require nitrogen for their development. In such plants, the leaves are modified to catch and digest insects. Few of the modifications are mentioned below: Leaf Pitcher- In a few plants like Nepenthes, the leaf-lamina is modified into a pitcher-like structure. The insect is digested into the inner walls of the pitcher which secretes a digestive fluid into the pitcher cavity. Leaf Bladder- In such plants, the segments of the leaves are modified into bladders. These plants are found in water. The inner wall is provided with digestive glands which helps in digesting the trapped insect. For eg., Utricularia In Drosera– The lamina possesses numerous hair with a sticky globule at its tip containing digestive enzymes. The moment an insect sits on the lamina, the hair covers the insect completely. Functions of Leaves: The leaves perform the following functions: Photosynthesis Photosynthesis is the primary function of leaves. They convert carbon dioxide, water, and UV light into glucose through the process of photosynthesis. Transpiration Transpiration is the removal of excess water from the plants into the atmosphere. This occurs by the opening of stomata present in the leaves. Guttation Removal of excess water from the xylem at the edges of the leaves when the stomata are closed is known as guttation. Storage Leaves are a site of photosynthesis. Therefore, they store water and nutrients. The succulent and thick leaves particularly adapt to water storage. Defence Some leaves are modified into spines to protect them from being damaged or eaten by animals. For eg., Opuntia.