Kingdom Plantae: The Plant Kingdom

Introduction

The Plant Kingdom, also known as Kingdom Plantae, encompasses all eukaryotic, multicellular, photosynthetic organisms. They are characterized by the presence of a cell wall made of cellulose, and they are autotrophs, meaning they produce their own food through photosynthesis. The study of plants is called botany.

(a) Algae

Algae are simple, thalloid, autotrophic, and largely aquatic (both freshwater and marine) organisms. They exhibit a great diversity in form and structure, ranging from unicellular (e.g., Chlamydomonas) to colonial (e.g., Volvox), filamentous (e.g., Spirogyra), and massive plant bodies (e.g., kelps).

Characteristics of Algae

Classification of Algae

Algae are classified into three main classes based on their pigment composition:

1. Chlorophyceae (Green Algae)

• Pigments: Chlorophylls a and b, and carotenoids.
• Stored Food: Starch.
• Cell Wall: Cellulose.
• Flagella: 2-8, equal, apical.
• Habitat: Freshwater, brackish water, saltwater.
• Examples: Chlamydomonas, Volvox, Spirogyra, Chara.

2. Phaeophyceae (Brown Algae)

• Pigments: Chlorophylls a and c, carotenoids, and xanthophylls (fucoxanthin).
• Stored Food: Laminarin and mannitol.
• Cell Wall: Cellulose and algin.
• Flagella: 2, unequal, lateral.
• Habitat: Primarily marine.
• Examples: Ectocarpus, Dictyota, Laminaria, Sargassum, Fucus.

3. Rhodophyceae (Red Algae)

• Pigments: Chlorophylls a and d, and phycobilins (phycoerythrin).
• Stored Food: Floridean starch.
• Cell Wall: Cellulose, pectin, and polysulphate esters.
• Flagella: Absent.
• Habitat: Primarily marine, especially in warmer areas.
• Examples: Polysiphonia, Porphyra, Gracilaria, Gelidium.

Economic Importance of Algae

1. Food Source: Many species of algae, such as Porphyra (laver), Laminaria (kelp), and Sargassum, are consumed as food in various parts of the world.
2. Agar: Agar, a gelatinous substance obtained from red algae like Gelidium and Gracilaria, is used in laboratories as a culture medium and in the food industry as a gelling agent.
3. Algin: Algin, obtained from brown algae, is used as an emulsifier and thickener in the food, cosmetic, and textile industries.
4. Diatomaceous Earth: The siliceous cell walls of diatoms accumulate over billions of years to form diatomaceous earth, which is used in polishing, filtration of oils and syrups, and as an insecticide.
5. Single-Cell Protein (SCP): Algae like Chlorella and Spirulina are rich in protein and are used as food supplements.

(b) Bryophyta

Bryophytes are small, non-vascular plants that are often referred to as the "amphibians of the plant kingdom" because they require water for sexual reproduction. They are commonly found in moist, shady areas.

General Characteristics of Bryophytes

• Habitat: Damp, humid, and shaded localities.
• Body: Thallus-like, prostrate or erect, and attached to the substratum by unicellular or multicellular rhizoids. They lack true roots, stem, and leaves.
• Vascular Tissue: Xylem and phloem are absent.
• Dominant Phase: The main plant body is the haploid gametophyte.
• Reproduction: The male sex organ is the antheridium, and the female sex organ is the archegonium. Water is essential for fertilization. The zygote develops into a multicellular sporophyte, which is dependent on the gametophyte.

Distinctive Features of Liverworts and Mosses

Life Cycle of Funaria (A Moss)

The life cycle of Funaria shows a clear alternation of generations between the haploid gametophyte and the diploid sporophyte.

Gametophytic Generation (n):

1. Spore Germination: A haploid spore germinates to form a filamentous, green, and branched structure called the protonema.
2. Bud Formation: Buds develop on the protonema, which grow into the mature, leafy gametophyte.
3. Gametophyte: The gametophyte is the dominant, photosynthetic, and independent phase. It bears the sex organs.
4. Sex Organs: The antheridia (male) and archegonia (female) are produced at the apex of the leafy shoots.
5. Fertilization: Antherozoids (male gametes) swim in water to reach the egg in the archegonium and fertilize it.

Sporophytic Generation (2n):

1. Zygote: The diploid zygote is the first cell of the sporophytic generation.
2. Sporophyte: The zygote develops into a sporophyte, which is differentiated into a foot, seta, and capsule. The sporophyte is dependent on the gametophyte for nutrition.
3. Spore Formation: Within the capsule, diploid spore mother cells undergo meiosis to produce haploid spores.
4. Spore Dispersal: The spores are released from the capsule and germinate to form a new gametophyte.

Graphic Outline of the Life Cycle of Funaria

      Spore (n) -> Protonema (n) -> Gametophyte (n) -> Antheridia/Archegonia (n) -> Gametes (n)
                                                                                    |
                                                                                Fertilization
                                                                                    |
                                                                                Zygote (2n)
                                                                                    |
                                                                                Sporophyte (2n)
                                                                                    |
                                                                                Spore Mother Cells (2n)
                                                                                    |
                                                                                  Meiosis
                                                                                    |
                                                                                Spores (n)

Economic Importance of Bryophytes

1. Pioneer Species: Bryophytes are important pioneer species that colonize bare rocks and soil, making them suitable for the growth of higher plants.
2. Soil Conservation: Mosses form dense mats on the soil, which help to prevent soil erosion.
3. Peat: Sphagnum, a moss, provides peat, which is used as a fuel and as a packing material for the trans-shipment of living material because of its water-holding capacity.
4. Indicator Species: Some bryophytes are sensitive to pollution and can be used as indicators of air and water quality.

(c) Pteridophyta

Pteridophytes are the first terrestrial plants to possess vascular tissues (xylem and phloem). They are commonly known as "ferns" and are found in cool, damp, shady places.

Characteristics of Pteridophytes

• Habitat: Cool, damp, shady places.
• Body: The main plant body is a sporophyte, which is differentiated into true roots, stem, and leaves.
• Vascular Tissue: Xylem and phloem are present.
• Dominant Phase: The sporophyte is the dominant, photosynthetic, and independent phase.
• Reproduction: The sporophyte produces spores in sporangia, which are borne on leaf-like appendages called sporophylls. The spores germinate to form a small, multicellular, free-living, mostly photosynthetic thalloid gametophyte called the prothallus. The prothallus bears the antheridia and archegonia. Water is required for fertilization.

Classification of Pteridophytes

Pteridophytes are classified into four classes:

1. Psilopsida: e.g., Psilotum
2. Lycopsida: e.g., Selaginella, Lycopodium
3. Sphenopsida: e.g., Equisetum
4. Pteropsida: e.g., Dryopteris, Pteris, Adiantum

Life Cycle of a Fern (A Typical Pteridophyte)

The life cycle of a fern also shows alternation of generations.

Sporophytic Generation (2n):

1. Sporophyte: The dominant plant body is the diploid sporophyte, which is differentiated into roots, rhizome (underground stem), and leaves (fronds).
2. Sporangia: On the underside of the leaves, there are clusters of sporangia called sori.
3. Spore Formation: Inside the sporangia, diploid spore mother cells undergo meiosis to produce haploid spores.

Gametophytic Generation (n):

1. Spore Germination: The haploid spores germinate to form a heart-shaped, free-living, and photosynthetic gametophyte called the prothallus.
2. Sex Organs: The prothallus bears both antheridia and archegonia on its ventral surface.
3. Fertilization: The antherozoids swim in a thin film of water to the archegonia and fertilize the egg.

Development of a New Sporophyte:

1. Zygote: The diploid zygote develops into an embryo, which grows into a new sporophyte. The young sporophyte is initially dependent on the gametophyte for nutrition, but it soon becomes independent.

Graphic Outline of the Life Cycle of a Fern

      Spore (n) -> Prothallus (Gametophyte) (n) -> Antheridia/Archegonia (n) -> Gametes (n)
                                                                                    |
                                                                                Fertilization
                                                                                    |
                                                                                Zygote (2n)
                                                                                    |
                                                                                Sporophyte (2n)
                                                                                    |
                                                                                Spore Mother Cells (2n)
                                                                                    |
                                                                                  Meiosis
                                                                                    |
                                                                                Spores (n)

Homospory and Heterospory

• Homospory: The production of only one type of spore, which is of the same size and shape. The gametophyte is monoecious (bisexual). Example: Lycopodium, Dryopteris.
• Heterospory: The production of two types of spores: smaller microspores (male) and larger megaspores (female). The gametophytes are dioecious (unisexual). Example: Selaginella, Salvinia.

Economic Importance of Pteridophytes

1. Ornamental Plants: Ferns are widely grown as ornamental plants for their delicate and graceful foliage.
2. Soil Binders: Pteridophytes bind the soil and prevent soil erosion.
3. Medicinal Uses: An extract of the rhizomes of Dryopteris is used as a vermifuge (to expel tapeworms).
4. Biofertilizers: The water fern Azolla has a symbiotic relationship with the nitrogen-fixing cyanobacterium Anabaena, and it is used as a biofertilizer in rice fields.

(d) Progymnosperms

Progymnosperms are an extinct group of plants that are considered to be the ancestors of modern gymnosperms. They exhibited characteristics of both pteridophytes and gymnosperms.

General Characteristics of Progymnosperms

• Habit: They were trees with well-developed secondary growth.
• Leaves: They had fern-like foliage.
• Reproduction: They reproduced by spores, like pteridophytes, but they also showed the initial stages of seed development.
• Vascular System: They had a eustele (a vascular cylinder with collateral or bicollateral vascular bundles), which is a characteristic of gymnosperms and angiosperms.

Graphic Outline of the Life Cycle of a Typical Progymnosperm

The life cycle of a progymnosperm is not fully understood due to the fossil nature of the group. However, it is believed to be similar to that of heterosporous pteridophytes, with the development of a rudimentary seed habit.

Economic Importance of Progymnosperms

As an extinct group, progymnosperms have no direct economic importance today. However, they are of great evolutionary significance as the ancestors of seed plants.

(e) Gymnosperms

Gymnosperms are a group of seed-producing plants that have naked seeds, meaning their ovules are not enclosed within an ovary.

General Characteristics of Gymnosperms

• Habit: They are mostly evergreen trees or shrubs.
• Roots: They have a well-developed taproot system. In some, the roots have a symbiotic association with fungi (mycorrhiza, e.g., Pinus) or nitrogen-fixing cyanobacteria (coralloid roots, e.g., Cycas).
• Stem: The stem is woody, erect, and branched.
• Leaves: The leaves are simple or compound, and they are well-adapted to withstand extremes of temperature, humidity, and wind.
• Reproduction: The main plant body is the sporophyte. They are heterosporous, producing haploid microspores and megaspores. The male and female gametophytes are dependent on the sporophyte. The male gametophyte is the pollen grain. The female gametophyte is the archegonium. Pollination is usually by wind (anemophily). Fertilization does not require water. The zygote develops into an embryo, and the ovule develops into a seed.

Graphic Outline of the Life Cycle of Pinus (A Typical Gymnosperm)

Sporophytic Generation (2n):

1. Sporophyte: The mature Pinus tree is the diploid sporophyte.
2. Cones: It bears male cones (microsporangiate strobili) and female cones (macrosporangiate strobili).
3. Microspores: The male cones produce microspores (pollen grains) by meiosis.
4. Megaspores: The female cones contain ovules, and within each ovule, a megaspore mother cell undergoes meiosis to produce four haploid megaspores, of which one survives.

Gametophytic Generation (n):

1. Male Gametophyte: The pollen grain is the male gametophyte. It contains two male gametes.
2. Female Gametophyte: The functional megaspore develops into the female gametophyte, which bears two or more archegonia.

Pollination and Fertilization:

1. Pollination: The pollen grains are carried by the wind to the ovules.
2. Fertilization: The pollen tube grows towards the archegonium, and one of the male gametes fuses with the egg to form a diploid zygote.

Seed Formation:

1. Embryo: The zygote develops into an embryo.
2. Seed: The ovule develops into a seed, which contains the embryo, stored food (endosperm), and a protective seed coat. The seed is naked (not enclosed in a fruit).

Graphic Outline of the Life Cycle of Pinus

      Pollen Grain (n) -> Male Gametes (n)
                                          |
      Megaspore (n) -> Female Gametophyte (n) -> Egg (n)
                                          |
                                      Fertilization
                                          |
                                      Zygote (2n)
                                          |
                                      Embryo (2n)
                                          |
                                      Seed (2n)
                                          |
                                      Sporophyte (2n)
                                          |
                                      Male/Female Cones (2n)
                                          |
                                        Meiosis
                                          |
                                  Microspores/Megaspores (n)

Economic Importance of Gymnosperms

1. Timber: Gymnosperms, such as pine, cedar, and fir, are a major source of timber for construction, furniture, and paper production.
2. Resins: Resins are obtained from various conifers and are used in the production of turpentine, varnish, and sealing wax.
3. Edible Seeds: The seeds of some gymnosperms, such as pine nuts (Pinus gerardiana), are edible.
4. Medicinal Uses: The drug ephedrine, which is used to treat asthma and bronchitis, is obtained from Ephedra.

(f) Angiosperms

Angiosperms, or flowering plants, are the most diverse and widespread group of plants. They are characterized by the presence of flowers and fruits.

General Characteristics of Angiosperms

• Habit: They range in size from tiny, aquatic plants like Wolffia to tall trees like Eucalyptus.
• Flowers: The flower is the reproductive structure of angiosperms. It consists of four whorls: calyx, corolla, androecium (stamens), and gynoecium (pistil or carpel).
• Ovules: The ovules are enclosed within the ovary.
• Pollination: Pollination can be by wind, water, insects, or other animals.
• Double Fertilization: Angiosperms exhibit double fertilization, where one male gamete fuses with the egg to form the zygote, and the other male gamete fuses with the two polar nuclei to form the triploid primary endosperm nucleus (PEN).
• Fruit: After fertilization, the ovary develops into a fruit, and the ovules develop into seeds. The fruit protects the seeds and helps in their dispersal.

Graphic Outline of the Life Cycle of a Typical Angiosperm

Sporophytic Generation (2n):

1. Sporophyte: The mature flowering plant is the diploid sporophyte.
2. Flowers: It produces flowers, which contain the male (stamens) and female (pistil) reproductive organs.
3. Microspores: The anthers of the stamens produce microspores (pollen grains) by meiosis.
4. Megaspores: The ovules in the ovary produce megaspores by meiosis.

Gametophytic Generation (n):

1. Male Gametophyte: The pollen grain is the male gametophyte.
2. Female Gametophyte: The embryo sac is the female gametophyte. It contains the egg cell and the polar nuclei.

Pollination and Fertilization:

1. Pollination: The pollen grains are transferred from the anther to the stigma of the pistil.
2. Double Fertilization: The pollen tube grows down the style to the ovule. One male gamete fertilizes the egg to form the zygote (2n), and the other male gamete fuses with the two polar nuclei to form the primary endosperm nucleus (3n).

Seed and Fruit Formation:

1. Embryo: The zygote develops into an embryo.
2. Endosperm: The primary endosperm nucleus develops into the endosperm, which provides nourishment to the developing embryo.
3. Seed: The ovule develops into a seed.
4. Fruit: The ovary develops into a fruit.

Graphic Outline of the Life Cycle of an Angiosperm

      Pollen Grain (n) -> Male Gametes (n)
                                          |
      Embryo Sac (n) -> Egg (n) + Polar Nuclei (n+n)
                                          |
                                  Double Fertilization
                                          |
                                Zygote (2n) + Endosperm (3n)
                                          |
                                      Embryo (2n)
                                          |
                                      Seed (2n)
                                          |
                                      Fruit (2n)
                                          |
                                      Sporophyte (2n)
                                          |
                                        Flower (2n)
                                          |
                                        Meiosis
                                          |
                                  Microspores/Megaspores (n)

Economic Importance of Angiosperms

Angiosperms are of immense economic importance to humans.

1. Food: They are the primary source of food for humans and other animals, providing cereals, pulses, fruits, and vegetables.
2. Fodder: Many angiosperms are used as fodder for livestock.
3. Fuel, Fiber, and Timber: They provide wood for fuel and construction, and fibers for clothing and other purposes.
4. Medicines and Spices: Many angiosperms are a source of medicines, spices, and other commercially important products.

Monocots and Dicots

Angiosperms are divided into two classes: monocotyledons (monocots) and dicotyledons (dicots).