From the microscopic world of bacteria to towering trees and complex animals, life on Earth presents a stunning diversity. Scientists have long sought to define, classify, and understand living organisms, leading to systems of nomenclature and classification. In this unit, we journey through the living world and explore the fascinating morphology of flowering plants.
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Defining Life: What Makes Organisms “Living”?
The living world is defined by unique characteristics that set living organisms apart from non-living matter. They show organization, growth, reproduction, metabolism, response to stimuli, adaptation, and evolution. Unlike inanimate objects, living beings carry genetic information that enables continuity of life from one generation to the next.
Diversity in the Living World
Life on Earth exists in an astonishing variety, ranging from simple microbes to highly evolved humans. Biologists study this diversity to understand evolutionary relationships and ecological roles. To avoid confusion in identifying species across regions and languages, the system of binomial nomenclature, introduced by Carl Linnaeus, gives every species a two-part name: the genus and the species. For example, Homo sapiens represents humans.
The Five Kingdoms of Life
Basis of Classification
Whittaker’s Five-Kingdom Classification is a landmark in biology. Organisms are grouped on the basis of their cell structure, mode of nutrition, body organization, and reproduction.
Monera: Includes bacteria and cyanobacteria. They are unicellular, prokaryotic, and reproduce asexually.
Protista: Eukaryotic unicellular organisms such as algae and protozoa. They show varied modes of nutrition and locomotion.
Fungi: Multicellular heterotrophs with chitin in their cell walls. Examples include molds and mushrooms.
Plantae: Autotrophic multicellular organisms with cell walls made of cellulose. They perform photosynthesis.
Animalia: Multicellular, heterotrophic, and lacking cell walls, this kingdom includes humans, animals, and insects.
Viruses: The Unique Entities
Viruses occupy a gray area between living and non-living. Outside a host, they behave like inert particles, but inside, they hijack the host’s machinery to replicate. Their structure typically includes DNA or RNA enclosed in a protein coat.
Morphology of Flowering Plants
Flowering plants, or angiosperms, are the most advanced group in the plant kingdom. Morphological studies help in plant identification and classification.
Roots
Roots anchor plants in the soil and absorb water and nutrients.
Dicot roots: Usually have a taproot system.
Monocot roots: Possess fibrous root systems.
Stems
Stems support branches, leaves, and flowers. They also serve as conduits for the transport of water and nutrients. Modifications like tubers and rhizomes serve as storage organs.
Inflorescence and Flower
Inflorescence is the arrangement of flowers on a stem, either in clusters (racemose) or solitary (cymose).
Flower is the reproductive unit of angiosperms, composed of sepals, petals, stamens, and carpels.
Leaves
Leaves are the main photosynthetic organs. They may be simple (a single blade) or compound (divided into leaflets). Venation patterns differ: reticulate in dicots and parallel in monocots.
Fruits and Seeds
Fruit develops from the ovary after fertilization and helps in seed dispersal.
Seeds contain the embryonic plant and stored food. Dicot seeds (like beans) have two cotyledons, while monocot seeds (like maize) have one.
General Anatomy of Plant Parts
Plant anatomy gives insight into internal structures and adaptations.
Dicot vs. Monocot Roots
Dicot root: Shows radial arrangement of xylem and phloem with secondary growth.
Monocot root: Characterized by polyarch xylem and absence of secondary growth.
Dicot vs. Monocot Stem
Dicot stem: Has vascular bundles arranged in a ring, allowing secondary thickening.
Monocot stem: Scattered vascular bundles with no secondary growth.
Dicot vs. Monocot Leaf
Dicot leaf: Shows dorsiventral anatomy with palisade and spongy mesophyll.
Monocot leaf: Is isobilateral, with mesophyll cells not differentiated.