PLANT KINGDOM

Whittaker's Five Kingdom Classification: Whittaker proposed the Five Kingdom classification in 1969, which included Monera, Protista, Fungi, Animalia, and Plantae as the broad classifications of living organisms.

Changes in Plant Kingdom Classification: Over time, our understanding of the plant kingdom has evolved. Fungi, members of Monera, and Protista with cell walls are now excluded from Plantae, even though they were previously classified under the same kingdom.

Description of Plantae: This chapter focuses on the classification within Kingdom Plantae, also known as the plant kingdom. It discusses Algae, Bryophytes, Pteridophytes, Gymnosperms, and Angiosperms.

Classification within Angiosperms: The chapter explores the classification within angiosperms to understand the concerns that influenced classification systems. Early systems relied on superficial morphological characters, while natural classification systems considered internal features like ultra-structure, anatomy, embryology, and phytochemistry.

Phylogenetic Classification: Presently, phylogenetic classification systems based on evolutionary relationships between organisms are widely accepted. These systems assume that taxa sharing a common ancestor belong to the same group.

Additional Sources for Classification: In addition to evolutionary relationships, modern classification incorporates information from other sources. Numerical Taxonomy assigns numbers and codes to observable characteristics, considering hundreds of characters equally. Cytotaxonomy uses cytological information, such as chromosome number and structure, while chemotaxonomy analyzes chemical constituents for classification.

3.1 ALGAE

Algae are chlorophyll-bearing, simple, thalloid, autotrophic organisms that are primarily aquatic, occurring in both freshwater and marine environments. They can also be found in other habitats such as moist stones, soils, and wood. Some algae have symbiotic associations with fungi (lichen) or animals.

Forms and Sizes: Algae exhibit a wide range of forms and sizes. They can be colonial, like Volvox, or filamentous, like Ulothrix and Spirogyra. Some marine algae, such as kelps, form massive plant bodies.

Reproduction: Algae reproduce through vegetative, asexual, and sexual methods. Vegetative reproduction occurs through fragmentation, where each fragment develops into a new thallus. Asexual reproduction involves the production of spores, with zoospores being the most common type. These motile spores germinate and give rise to new plants. Sexual reproduction occurs through the fusion of two gametes. Gametes can be flagellated and similar in size (isogamous) or non-flagellated but similar in size (as in Spirogyra). Fusion between dissimilar-sized gametes is called anisogamous, while fusion between a large non-motile female gamete and a smaller motile male gamete is termed oogamous.

Importance of Algae: Algae play a crucial role in the ecosystem and have various uses for humans. They contribute significantly to carbon dioxide fixation through photosynthesis and increase the level of dissolved oxygen in aquatic environments. Algae serve as primary producers, providing energy-rich compounds as the foundation of food cycles for aquatic animals. Many species of marine algae, such as Porphyra, Laminaria, and Sargassum, are used as food. Certain algae, particularly brown and red algae, produce hydrocolloids like algin and carrageen, which have commercial applications. Agar, obtained from Gelidium and Gracilaria, is used in microbiological culture and food products. Chlorella, a protein-rich unicellular alga, is used as a food supplement, even by astronauts.

Classification: Algae are classified into three main classes: Chlorophyceae (green algae), Phaeophyceae (brown algae), and Rhodophyceae (red algae). This classification is based on shared characteristics and evolutionary relationships among algae species.

3.1.1 Chlorophyceae

Green Algae: Green algae belong to the class Chlorophyceae and are commonly referred to as green algae. They exhibit a wide range of plant body types, including unicellular, colonial, and filamentous forms.

Grass Green Color: Green algae are typically characterized by their grass green color, which is attributed to the dominance of chlorophyll a and b pigments. These pigments are localized within specific chloroplasts.

Diversity of Chloroplasts: Green algae display a diversity of chloroplast shapes, which can be discoid, plate-like, reticulate, cup-shaped, spiral, or ribbon-shaped, depending on the species. Many species have pyrenoids within the chloroplasts, which serve as storage bodies containing protein and starch. Some green algae may store food in the form of oil droplets.

Cell Wall Composition: Green algae usually possess a rigid cell wall composed of an inner layer of cellulose and an outer layer of pectose. This cell wall provides structural support and protection.

Vegetative Reproduction: Vegetative reproduction in green algae commonly occurs through fragmentation, where a portion of the algae breaks off and develops into a new individual. They can also reproduce through the formation of various types of spores.

Asexual Reproduction: Asexual reproduction in green algae involves the production of flagellated zoospores within specialized structures called zoosporangia. These zoospores are motile and can give rise to new individuals.

Sexual Reproduction: Green algae exhibit considerable variation in the type and formation of sex cells during sexual reproduction. It can be isogamous (similar-sized and flagellated gametes), anisogamous (dissimilar-sized but flagellated gametes), or oogamous (large non-motile female gamete and small motile male gamete).

Commonly Found Green Algae: Some well-known examples of green algae include Chlamydomonas, Volvox, Ulothrix, Spirogyra, and Chara. These species represent the diversity of green algae in terms of their characteristics and reproductive strategies.

3.1.2 Phaeophyceae

Brown Algae: Brown algae belong to the class Phaeophyceae and are primarily found in marine habitats. They exhibit a wide range of sizes and forms, ranging from simple branched, filamentous forms like Ectocarpus to highly branched forms like kelps, which can reach heights of up to 100 meters.

Pigment Composition: Brown algae possess chlorophyll a, chlorophyll c, carotenoids, and xanthophylls. The amount of the xanthophyll pigment fucoxanthin present in brown algae determines their color, which can range from olive green to various shades of brown.

Storage of Food: Brown algae store food as complex carbohydrates, typically in the form of laminarin or mannitol.

Cell Wall and Protoplast: The vegetative cells of brown algae have a cellulosic wall that is usually covered on the outside by a gelatinous coating of algin. The protoplast of these cells contains plastids, a centrally located vacuole, and a nucleus.

Plant Body Structure: The plant body of brown algae is typically attached to the substrate by a holdfast. It consists of a stalk called the stipe and a leaf-like photosynthetic organ called the frond.

Vegetative Reproduction: Vegetative reproduction in brown algae commonly occurs through fragmentation, where a fragment of the algae develops into a new individual.

Asexual Reproduction: Asexual reproduction in most brown algae is achieved through the production of biflagellate zoospores. These zoospores are pear-shaped and have two unequal laterally attached flagella.

Sexual Reproduction: Brown algae can exhibit isogamous, anisogamous, or oogamous sexual reproduction. Gametes, which are pyriform (pear-shaped), bear two laterally attached flagella. Union of gametes may occur in water or within specialized structures called oogonia in oogamous species.

Commonly Found Brown Algae: Some common examples of brown algae include Ectocarpus, Dictyota, Laminaria, Sargassum, and Fucus. These species represent the diversity of brown algae in terms of their characteristics, reproductive strategies, and ecological significance.

3.1.3 Rhodophyceae

Red Algae: Red algae, belonging to the class Rhodophyceae, are so named due to the predominance of the red pigment r-phycoerythrin in their bodies. They are commonly referred to as red algae.

Marine Habitat: The majority of red algae are found in marine environments, with higher concentrations occurring in warmer areas. They inhabit both well-lit regions near the water's surface and deeper parts of the ocean where there is less penetration of light.

Thallus Structure: The red algae typically have multicellular thalli, and some species exhibit complex body organization. Their thalli are predominantly red in color due to the abundance of r-phycoerythrin pigment.

Food Storage: Red algae store their food as floridean starch, which has a structure similar to amylopectin and glycogen.

Vegetative Reproduction: Red algae commonly reproduce vegetatively through fragmentation. Fragments of the algae can develop into new individuals.

Asexual Reproduction: Asexual reproduction in red algae occurs through the production of non-motile spores.

Sexual Reproduction: Red algae exhibit oogamous sexual reproduction, involving non-motile gametes. After fertilization, complex post-fertilization developments take place.

Common Members: Some commonly found red algae include Polysiphonia, Porphyra, Gracilaria, and Gelidium. These species represent the diversity of red algae in terms of their characteristics, reproductive strategies, and ecological roles.

3.2 BRYOPHYTES

Bryophytes, including mosses and liverworts, are commonly found in moist shaded areas, such as hillsides. They prefer damp, humid, and shaded environments.
They are often referred to as the amphibians of the plant kingdom because they can live in soil but require water for sexual reproduction.
Bryophytes play a significant role in plant succession on bare rocks and soil. They are among the first organisms to colonize rocks, decomposing them and creating a suitable substrate for higher plants.
The plant body of bryophytes is more differentiated than that of algae. It is thallus-like, either prostrate or erect, and attached to the substratum by rhizoids (unicellular or multicellular structures). Bryophytes lack true roots, stems, or leaves but may possess structures resembling them.
The main plant body of a bryophyte is haploid and is called the gametophyte. It produces multicellular sex organs. The male sex organ is called antheridium, which produces biflagellate antherozoids (sperm cells). The female sex organ is called archegonium, which produces a single egg.
Fertilization occurs when an antherozoid fuses with the egg, resulting in the formation of a diploid zygote. The zygote develops into a multicellular sporophyte, which remains attached to the photosynthetic gametophyte and derives nourishment from it.
Some cells in the sporophyte undergo reduction division (meiosis) to produce haploid spores. These spores germinate and develop into new gametophytes.
Bryophytes have limited economic importance. However, some mosses provide food for herbaceous mammals, birds, and other animals. Certain moss species, like Sphagnum, are used for peat, which has been historically used as fuel and packing material due to its water-holding capacity.
Mosses, along with lichens, are important pioneers in rock colonization and have ecological significance in decomposing rocks and preventing soil erosion.
Bryophytes are classified into two main groups: liverworts and mosses.

3.2.1 Liverworts

Liverworts commonly grow in moist and shady habitats such as streamsides, marshy areas, damp soil, tree bark, and deep in the woods.
The plant body of a liverwort is thalloid, meaning it is flat and ribbon-like, as seen in species like Marchantia. The thallus is dorsiventral, with one surface closely attached to the substrate.
Leafy liverworts have tiny leaf-like appendages arranged in two rows on stem-like structures.
Asexual reproduction in liverworts occurs through the fragmentation of thalli (the thallus) or the formation of specialized structures called gemmae. Gemmae are green, multicellular, asexual buds that develop in small receptacles known as gemma cups on the thalli. These gemmae detach from the parent body and germinate to form new individuals.
Sexual reproduction in liverworts involves the production of male and female sex organs, which may occur on the same thallus or different thalli. The male sex organ is called an antheridium, which produces motile sperm cells. The female sex organ is called an archegonium, which contains a single egg.
After fertilization, a sporophyte develops. The sporophyte is differentiated into a foot, seta (stalk), and capsule. Within the capsule, meiosis occurs, resulting in the production of haploid spores.
These spores are released from the capsule and can germinate to develop into free-living gametophytes, completing the life cycle of liverworts.

3.2.2 Mosses

The dominant stage in the life cycle of mosses is the gametophyte, which consists of two stages: the protonema stage and the leafy stage.
The protonema stage develops directly from a spore and is a creeping, green, branched, and often filamentous stage.
The secondary protonema gives rise to the leafy stage as a lateral bud. The leafy stage consists of upright, slender axes with spirally arranged leaves. It is in this stage that the sex organs are produced.
Mosses are attached to the soil through multicellular and branched rhizoids.
Vegetative reproduction in mosses occurs through fragmentation and budding in the secondary protonema.
Sexual reproduction in mosses involves the production of sex organs called antheridia (male) and archegonia (female) at the apex of the leafy shoots.
After fertilization, the zygote develops into a sporophyte, which consists of a foot, seta (stalk), and capsule.
The sporophyte in mosses is more elaborate than that in liverworts. The capsule of the sporophyte contains spores.
Spores are formed through meiosis within the capsule. Mosses have mechanisms for spore dispersal.
Common examples of mosses include Funaria, Polytrichum, and Sphagnum.

3.3 PTERIDOPHYTES

The dominant stage in the life cycle of pteridophytes is the sporophyte, which is a multicellular, well-differentiated phase.
Pteridophytes have free-living, mostly photosynthetic thalloid gametophytes called prothallus, which require cool, damp, shady places to grow.
The gametophytes of pteridophytes bear male and female sex organs called antheridia and archegonia, respectively.
Water is required for the transfer of male gametes (antherozoids) released from the antheridia to the archegonia for fertilization.
Fusion of the male gamete with the egg present in the archegonium results in the formation of a zygote.
The zygote develops into a multicellular, well-differentiated sporophyte, which is the dominant phase of pteridophytes.
Most pteridophytes produce homosporous spores, where all the spores are of the same kind. However, some genera like Selaginella and Salvinia are heterosporous, producing two kinds of spores: macrospores (large) and microspores (small).
The megaspores and microspores germinate and give rise to female and male gametophytes, respectively. In heterosporous pteridophytes, the female gametophytes are retained on the parent sporophytes for varying periods.
The zygotes develop into young embryos within the female gametophytes, which is a precursor to the seed habit considered an important step in evolution.
Pteridophytes are classified into four classes: Psilopsida (e.g., Psilotum), Lycopsida (e.g., Selaginella, Lycopodium), Sphenopsida (e.g., Equisetum), and Pteropsida (e.g., Dryopteris, Pteris, Adiantum).

3.4 GYMNOSPERMS

Gymnosperms are plants in which the ovules are not enclosed by an ovary wall and remain exposed before and after fertilization. They have naked seeds.
Gymnosperms include medium-sized to tall trees and shrubs, with examples like the giant redwood tree Sequoia.
The roots of gymnosperms are generally tap roots, and some genera form associations with fungi (mycorrhiza) or have specialized roots called coralloid roots associated with nitrogen-fixing cyanobacteria (Cycas).
The stems of gymnosperms can be unbranched (Cycas) or branched (Pinus, Cedrus).
Gymnosperms have leaves that may be simple or compound. They are well-adapted to withstand extreme temperatures, humidity, and wind. Conifers, for example, have needle-like leaves that reduce surface area, and their thick cuticle and sunken stomata help reduce water loss.
Gymnosperms are heterosporous, meaning they produce haploid microspores and megaspores. These spores are produced within sporangia borne on sporophylls, arranged spirally to form lax or compact strobili or cones.
The male cones or strobili bear microsporangia on microsporophylls and produce pollen grains as highly reduced male gametophytes.
The female cones or strobili bear megasporangia on megasporophylls, which contain ovules. The megaspore mother cell within the ovule differentiates and undergoes meiosis to form four megaspores. One of the megaspores develops into a multicellular female gametophyte.
The male and female gametophytes in gymnosperms do not have an independent free-living existence like in bryophytes and pteridophytes. They remain within the sporangia retained on the sporophytes.
Pollen grains are released from the microsporangia and are carried by air currents. They come in contact with the opening of the ovules and the pollen tube grows towards the archegonia within the ovules for fertilization.
After fertilization, the zygote develops into an embryo and the ovules develop into seeds. These seeds are not covered and remain exposed.

3.5 ANGIOSPERMS

Angiosperms, or flowering plants, have specialized structures called flowers where pollen grains and ovules are developed.
Unlike gymnosperms, the ovules in angiosperms are enclosed within the ovary of the flower.
Angiosperms produce seeds that are enclosed within fruits, which are mature ovaries.
Angiosperms are a diverse group of plants that can be found in a wide range of habitats.
They vary in size from tiny plants like Wolffia to tall trees like Eucalyptus, which can reach heights of over 100 meters.
Angiosperms provide various benefits to humans, including food, fodder, fuel, medicines, and other commercially important products.
Angiosperms are divided into two major classes: dicotyledons (dicots) and monocotyledons (monocots).
Dicotyledons have seeds with two cotyledons (embryonic leaves), and their floral parts are usually in multiples of four or five.
Monocotyledons have seeds with a single cotyledon, and their floral parts are usually in multiples of three.