Plant Kingdom Overview

Plant Kingdom

• The Plant Kingdom has evolved from a simple two-kingdom system to a more complex multi-kingdom system.
• The Plant Kingdom includes organisms with cell walls, traditionally including bacteria, fungi, and algae alongside land plants.
• Linnaeus's two-kingdom system classified organisms based on the presence or absence of a cell wall, grouping bacteria and fungi with land plants.
• Modern classification separates bacteria into Kingdom Monera, fungi into a separate kingdom, and algae into Kingdom Protista.
• The modern Plant Kingdom consists of five main categories: algae, bryophytes, pteridophytes, gymnosperms, and angiosperms.
• Algae are the simplest plants, often aquatic and unicellular.
• Bryophytes are land plants requiring water for reproduction ("amphibians of the plant kingdom").
• Pteridophytes are vascular land plants with spores found in moist environments like forests.
• Gymnosperms are seed-bearing plants with exposed seeds and lack fruits.
• Angiosperms are the most evolved group, with seeds enclosed within fruits and are known as flowering plants.
• Angiosperms are the most prevalent in studies due to their complexity and abundance.
• Artificial classification systems are based on external morphology (habit, leaf shape/color, and reproductive structures).
• Natural classification systems use natural affinities and consider anatomical, embryological, and phytochemical features.
• Phylogeny-based classification is the modern accepted system, using evolutionary relationships and DNA evidence.

Classification Systems

• Linnaeus introduced the first artificial classification system based on observable morphological features.
• Bentham and Hooker later developed a natural classification system that considered natural affinities, including anatomical, embryological, and chemical characteristics.
• Engler and Prantl developed a phylogenetic classification system that emphasized evolutionary relationships using genetic data.

Comparing Classification Systems

• Artificial systems are simple but inaccurate, often grouping unrelated organisms together.
• Natural systems are more accurate, considering a wider range of characteristics, but might miss evolutionary relationships.
• Phylogenetic systems are the most accurate, reflecting evolutionary history, but are complex and use advanced tools like DNA sequencing.

Key Considerations in Plant Classification

• Vegetative characters are easily influenced by the environment and are unreliable for classification.
• Sexual characters (flower parts, seeds) are genetically determined and more reliable.
• Cytotaxonomy examines chromosomes' number, structure, and behavior.
• Chemotaxonomy uses plant chemical composition (spices, medicines, pigments).
• Numerical taxonomy analyzes many characters, assigning numerical values for computer comparison.
• Phylogenetic relationships are the foundation of modern classification, based on DNA evidence.

Classification of Plants

• Numerical taxonomy is imperfect as it assigns arbitrary numerical values, leading to "fake" scores due to differing importance of characteristics.
• Plants are categorized into Algae, Bryophytes, Pteridophytes, Gymnosperms, and Angiosperms progressing from simplest to complex.

Algae

• Most algal cells are haploid.
• Algae lack true stems, roots, and leaves (thalloid body).
• They have stem-, root-, and leaf-like structures.
• Algae can form symbiotic relationships with fungi (lichens) and animals like sloths.
• Algae range from microscopic unicellular forms to large multicellular kelp.
• Algae reproduce vegetatively (fragmentation), asexually (motile zoospores), and sexually (isogamous, anisogamous, or oogamous).
• Algae are primary producers, fixing carbon dioxide during photosynthesis, contributing to about 50% of global carbon fixation.
• They are the base of aquatic food chains.
• Algae produce important substances like algin, carrageenan, and agar.
• Some, like Chlorella and Chlamydomonas, are protein-rich.

Types of Algae

• Chlorophyceae (green algae) contain chlorophyll a and b, green in color, with starch storage.
• Phaeophyceae (brown algae) contain chlorophyll a and c, carotenoids, and fucoxanthin, brown in color, with mannitol and laminarin storage.
• Rhodophyceae (red algae) contain chlorophyll a and d, phycoerythrin, and phycocyanin, red in color, and store floridean starch.

Comparing Algae

• Chlorophyceae: 2-8 equal apical flagella, cellulose cell wall with pectose.
• Phaeophyceae: 2 unequal lateral flagella, cellulose cell wall with algin.
• Rhodophyceae: No flagella in most stages, cellulose cell wall with polysulfates.

Red Algae

• Red algae are dominant in warmer marine waters.
• Red algae lack flagella on spores and gametes.
• They store food as floridean starch.
• Adapt to deep water and use violet, indigo, and blue light for photosynthesis.
• They are typically multicellular with complex post-fertilization events.

Brown Algae

• Brown algae have two unequal lateral flagella.
• They store food as laminarin. Many species display an array of structures mimicking stems, roots, and leaves.
• They use a variety of reproductive methods, including isogamous, anisogamous, and oogamous reproduction.

Green Algae

• Green algae have 2-8 equal apical flagella.
• They store food as starch and sometimes oil droplets.
• They are found in fresh, marine, and brackish waters.
• They demonstrate diverse reproduction methods, including isogamous, anisogamous, and oogamous reproduction.

Bryophytes

• Bryophytes are land plants requiring water for sexual reproduction ("amphibians of the plant kingdom").
• Primarily found in moist, shady, and hilly areas.
• Characterized by a dominant haploid gametophyte generation.
• Two forms exist: liverworts and mosses.

Life Cycle of Bryophytes

• The haploid gametophyte, a multicellular structure, is dominant.
• It produces male and female sex organs (antheridium and archegonium).
• Antheridium produces biflagellate antherozoids (mitosis).
• Archegonium produces a single egg (mitosis).
• Water is required for antherozoid movement and fertilization.
• Fertilized egg (zygote) develops into a diploid sporophyte.
• Dependent sporophyte forms a capsule, undergoes meiosis, and produces haploid spores.
• Spores germinate to form a new haploid gametophyte.

Liverworts

• Typically found in moist habitats, often with a dorsiventrally flattened gametophyte adhering to the ground.
• Reproduce asexually through gemmae cups containing gemmae.
• Gemmae are multicellular buds that develop into new gametophytes.
• Some liverworts exhibit a leafy appearance.

Mosses

• Critical for soil formation, preventing erosion, and plant succession.
• Used in packaging due to water absorption capabilities.
• Form dense mats, holding soil in place.

Lichens

• Symbiotic organisms involving fungi (mycobiont) and algae (photobiont).
• Mycobiont provides structure and absorbs water/nutrients.
• Photobiont performs photosynthesis.
• Involved in soil formation by releasing acids and breaking down rocks.
• Initiate plant succession by enhancing soil fertility.

Bryophytes

• Bryophytes are non-vascular plants, meaning they lack specialized tissues for transporting water and nutrients.
• Bryophytes exhibit a dominant gametophyte phase in their life cycle.
• Spores are the primary means of reproduction in bryophytes.
• Majority of bryophytes are found in moist environments.
• Includes different stages, namely protonema and leafy, during their life cycle.

Liverworts

• Liverworts are a type of bryophyte
• Liverworts generally have a gametophyte-dominant life cycle
• Sporophytes are dependent on the gametophyte.

Thallophytes

• Thallophytes are a type of algae
• Thallophytes have a thallus body
• Thallophytes are photosynthetic

Moss

• Moss is a type of bryophyte
• Moss has a sporophyte-dominant life cycle
• The sporophyte is an independent structure

Comparison of Bryophytes and Thallophytes

• Bryophytes are non-vascular, thallophytes are vascular.
• Bryophytes dominate at the gametophyte stage, thallophytes at the sporophyte.
• Bryophytes are generally small, while thallophytes can be larger.
• Bryophytes grow in moist conditions, thallophytes can grow in both moist and dry areas.

Pteridophytes

• Pteridophytes are vascular plants, possessing specialized tissues for water and nutrient transport.
• Sporophyte is the dominant generation in the Pteridophyte life cycle.
• Pteridophytes reproduce through spores.
• They flourish in moist and shaded environments. Examples include: Selaginella, Lycopodium, and Equisetum.
• Some pteridophytes act as soil stabilizers.
• Some are used as ornamental plants or show adaptability to moisture-rich environments and microhabitats.

Heterospory

• Heterospory is a significant advancement in pteridophytes, marking a step toward seed plants.
• Heterospory involves the production of two distinct spore types: microspores and megaspores.
• Microspores develop into the male gametophyte, and megaspores lead to female gametophytes.

Gymnosperms

• Gymnosperms are seed plants.
• Dominant generation is the sporophyte.
• Gymnosperms reproduce via seeds; seeds are not housed within an ovary or fruit (naked seeds).
• Includes large trees like redwoods and a variety of forms (trees and shrubs).
• Feature leaves especially adapted to conserve water. Often encountered in less-humid areas, employing adaptations like waxy leaves and sunken stomata for reduced water loss.
• Roots form symbiotic associations for nutrient uptake.

Reproductive Structures of Gymnosperms

• Gymnosperms reproduce via cones (male and female).
• Male cones produce microspores, developing into pollen grains (male gametophyte).
• Female cones produce megaspores, leading to the development of female gametophytes.

Important Facts of Gymnosperms

• In pteridophyte and gymnosperm life cycles, the sporophyte is the dominant stage.
• Gymnosperms showcase heterospory through their production of microspores (male) and megaspores (female), a significant evolutionary leap toward seed plants.
• The female gametophyte, housed within the megaspore (ovule), plays a vital role.
• Pollen grains, the male gametophyte, reach the female gametophyte initiating fertilization.
• A wide range of adaptations allows for survival in various environmental conditions, including aridity, cold climates, and high altitudes. These adaptations support their ecological success and role in global ecosystems.