Archegoniate Study Document

Archegoniate Introduction: The group of plants having archegonia, i.e., the female reproductive organ, are called Archegoniate. Each plant of this group is an archegoniate. The group Archegoniate comprises these divisions: Bryophyta, Pteridophyta, and Gymnosperms. Unifying Features of Archegoniate: Archegoniate are archegonium-bearing plants and include both fossil and living forms. They are largely land plants consisting of non-flowering (Bryophytes and Pteridophytes) and flowering (Gymnosperms) plants. They probably originated from green algae and gradually occupied the land habitat by developing innovative structures and methods of dispersal. All archegoniate are aposporous. These land plants have probably emerged from an algal ancestor. Fossil records of plants indicated that most primitive forms of land were probably archegoniate. The Archegonium is the female sex organ and is found in almost all members of archegoniate plant groups except a few (Angiosperms, the Gnetales). The male sex organ is called an Antheridium. Archegonium is multicellular and is a flask-shaped vegetative structure. Archegoniates are bestowed with female reproductive organs, known as archegonia. Each archegonium is a sac-like container, housing the egg cell and borne on the female gametophyte. It is a flask-shaped structure consisting of the following parts: A basal swollen venter embedded in the gametophytic tissue. An expanded and swollen basal portion, known as the venter. An upper elongated slender part called the neck. The venter is roughly round and bears a large and a few sterile venter canal cells. The neck usually consists of four to six rows of neck canal cells. The canal cells and the egg cell are easily protected by sterile cells formed by the cells of the expanded venter. In this way, the egg cell becomes part of the archegonium. An important aspect found during the evolution of archegonia is the separation of the compact cell region called the egg cell and the archegonium. The gametophytic female sex cells transform to produce a female gametophyte. The jacket cell wall (archegonium) is present at the interface of the archegonium and the gametophytic tissue. The archegonium cells further divide and transfer nutrients to the growing embryo after fertilization. The Antheridium: The male reproductive structure contains a male gametophyte (a male sex organ or body). Antheridium is a stalked spherical to pear-shaped structure and consists of outer sterile jacket cells that protect the internal androcytes. The sperm mother cells (spermatogenous cells) are formed within the antheridium. Each spermatogenous cell divides diagonally to form biflagellated sperm cells (male sex cells). The slightly twisted sperm, after reaching maturity, are released by the opening of the apical channel of the antheridium. This allows them to be carried to the archegonium by water. After entering the archegonium through the neck, the antherozoids fertilize the egg cell within the archegonium. Fertilization of the egg cell leads to the formation of a diploid zygote. Life Cycle: The life cycle of the plant continues by regular alternation between the sporophyte and gametophyte generations. All Bryophytes and Pteridophytes exhibit this alternation of generations. The life cycle alternates between the gametophyte and sporophyte phases. Both phases are multicellular. In Gymnosperms, it is diploid. Heterospory: An evolutionary characteristic feature of all higher archegoniates. They produce two types of spores: different in size, the microspores and megaspores which ultimately develop into the male gametophyte and female gametophyte, respectively. True Bryophytes and some early Pteridophytes are homosporous. Fertilization in Gymnosperms: The process of fertilization takes place by siphonogamy. For the first time among the archegoniates, Gymnosperms have adopted the method of fertilization where male pollen grains (microspores) form a tube (siphon) and the male gamete moves through the tube to reach the egg cells inside the archegonium. Soon after fertilization, the zygote develops into an embryo and matures into ovules, transforming into seeds. The formation of seeds for the first time takes place in Gymnosperms among the archegoniate. Zygote Development: The zygote, a product of fertilization of male and female gametes, is the beginning point of the sporophytic generation. The zygote develops and provides a location for permanent retention of the embryo. The zygote is the subsequent evolutionary development into an embryo leading to the development of the sporophyte. The zygote provides the basis for the establishment of an embryophyte. Transition to Land: A main feature and characteristic feature of the lower Archegoniates is that the sporophyte remains dependent on the gametophyte. The sporophyte develops freely in direct contact with the soil, water, and light. Origin of Land Plants: There are Summary Assumptions: Bryophytes are the first and most primitive archegoniates derived from various algal ancestors (descended from charophytes through pteridophytes evolution derived from chlorophyceae (green algae). Algal ancestor was Ulvophyceae. Current Paleobotanical, Cytological, Physiological, and Molecular Data: Indicate that the green algae and archegoniate embryophytes share a common ancestor, both being unicellular and biflagellate. Evidence of Various Features: Shows that the archegoniates descended from a last common ancestor shared with the Coleochaete or from a direct descent from charophytes (Chlorophyceae). The chlorophyll and carotenoid pigments share many features and form a close group of algae. Therefore, the idea (assumption) is that the first land plants evolved from an ancestor resembling charophytes. The resemblance between the charophytes' algae and embryophytes are mentioned in the following features: Genetic Features: Both are multicellular and eukaryotic. Both are photosynthetic and hence autotrophic. The land plants (archegoniates) are primarily made up of cellulose, but charophytes have a hemicellulose-rich cell wall. Rosette Cellulose Synthesizing Complex (RCSC): In land plants and charophytes, the plasma membrane of both possess rosette-shaped arrays of proteins that synthesize cellulose microfibrils in linear homologous line (though a common ancestry between charophytes and land plants). The rosette synthesizing system evolved independently of cellulose synthesis in other green algae. Enzymes in Peroxisomes: In both charophytes and land plants, peroxisomes are typically found to be associated with chloroplasts. The enzymes in the peroxisomes help minimize the loss of organic products due to photo-respiration. Other algae groups do not have such particular peroxisome enzymes. DNA and RNA Sequences: The DNA and RNA sequence of land plants and charophytes support their close relationship. Cell division features and certain details of cell division are common between charophytes and land plants. Flagellated Sperm: The details of flagellated sperm of both resemble each other. Structure of their flagellated sperm. Transition to Land Habitat: Tendency Created by Aerial Lower Green Algae Plants: Most primitive life on land surface, ancestors' Coleochaete or terrestrial charophyceae. Primitive green plants, specific structures that help in proper water balancing. Primitive Features of Water Storing System: Derived from moisture required for land plants and the aerial environment. Pre-adaptation of Primitive Green Algae: Adaptations to land environment. Plants Migrated from Water to Land: Different adaptations that evolved in such plants to overcome the land habitat environment. Conduction of Water in Primitive Land Plants: Helped in solving the water balance problem, absorption of water by diffusion. (i) Protection: They were protected against grazing. Excessive absorption and storage of water, food, and dry seasons helped these plants to develop cuticles to prevent excessive loss and resist decay. Development of simple water-conducting tissues in the form of elongated cells. (ii) Primitive Plants: Primitive plants were covered with a layer of cutin to help against excessive water loss and desiccation. Problems Faced: In moving to the land surface, they faced a number of life-sustaining challenges from water and land. They developed cuticles, which are waterproof. The gametes and offspring were protected by a jacket of sterile cells to solve the problem of desiccation. Further Adaptations: Migration of plants from aquatic to terrestrial regions developed a variety of adaptations to overcome these life-sustaining challenges. Such plants developed mechanisms to check water loss and desiccation. Some plants developed cuticles, which provided an impermeable barrier to water loss. Reproduction: Primitive plants had to move onto the land surface to reproduce. Early embryophytes carried a protective cuticle and jacket cells to maintain their body structure. The formation of the sterile layer over the surface of the plant may not have been difficult because it already had a protective outer layer. Bryophytes: Early embryophytes developed a method of reproduction in moist areas by producing flagellated sperm cells and non-motile egg cells. Development of Specialized Structures: These structures helped them to maintain water balance and protect reproductive organs. Siphonogamy: The process of siphonogamy evolved in Gymnosperms and later in Angiosperms. The Gymnosperms adapted to the dry land habitat by the development of pollen grains that could be carried by the wind to reach the archegonium. Therefore, transition to land habitat was primarily a slow process because of the process called adaptation, which took a lot of time in a natural way. The transitioning of plants from water to land led to specialization of the plant body by developing new organs to retain and absorb water and adapt to external spores and disperse mechanisms. Spores become resistant to survive in a terrestrial climate. Adaptation to Land Habitat: Bryophytes: The pioneers among plants migrating to the terrestrial habitat. Some bryophytes are amphibious, while others grow in aquatic habitats, showing a wide variety of water. For sexual reproduction, bryophytes need water, but they can survive longer periods without it. Bryophytes developed gametangia and sporangia to protect reproductive cells. As plants migrated to land, the following adaptations evolved: Structure: Plants developed elaborate root systems to provide strong anchorage and absorb water from the soil. Fully developed vascular tissues helped in transport. Leaves: As plants migrated from aquatic to terrestrial habitats, they developed well-developed tissues for transport, growth, and reproduction. For example, bryophytes, which grow in humid soil, close to the source of water, do not require vascular tissue. Roots: Developed from root-like structures to provide strong anchorage and absorb water from the soil. Structure of Plants: Tracheophytes have extremely branched body plans, many times several meters high. They possess well-developed vascular systems to help in transport and continuous supply of water. Photosynthesis: Plants increased their photosynthetic area to manufacture enough organic material. Water Loss: To reduce water loss by transpiration, leaves developed cuticles, hairs, and trichomes on their surfaces. Leaves and other plant parts evolved mechanisms of transpiration to leave the body cool and prevent overheating. Supporting Tissue: Developed various supporting tissues, such as sclerenchyma, to provide mechanical strength to plants. (iii) Developed Archegonium: Which retains the egg even after fertilization, forming the zygote, is received in the secure environment. It is immediately after it is formed in the early forest of Archegoniate plants. They made adaptations for their embryos. They retained the embryo inside the archegonium until it was necessary to protect and nourish it during adverse conditions. This adaptation helped to protect the embryo during early development. (iv) Bryophytes and Most of the Pteridophytes: Are homosporous. Their gametophyte is free-living and monoecious, exposed to the environment. Because of the terrestrial environment, some Pteridophytes and Spermatophytes produce heterosporous gametophytes. These types of gametophytes (which form male gametophytes and megaspores which give rise to female gametophytes) were protected and retained within spores until they germinated into new gametophytes. This adaptation ensured the survival and reproduction of the species. (v) Seed Habit: The Spermatophytes produce seeds, and this is an important step toward adaptation to land terrestrial life. Seeds develop in a period of dormancy, which allows them to germinate under favorable conditions. Moreover, seed coats provide sufficient food to sustain the developing young sporophytes. Thus, Plants Migrating to Land Adapted Themselves: In many ways to survive, grow, and spread on land. Alternation of Generation: Hofmeister: Observed that the life cycle of different groups of organisms is similar. All of them show alternation of generation, marked by periodic reduction and duplication of the number of chromosomes. Three Primitive Groups of Plants: (i.e., algae, fungi, and bryophyte) The plant body is a haploid (n) phase, and the diploid (2n) phase is represented by a zygote. In these plants, meiosis occurs in the zygote, and it forms spores by mitosis. This type of life cycle is called "haplontic." Bryophytes and Tracheophytes: Alternation of generations involves two multicellular phases: Gametophyte: A multicellular haploid plant. Sporophyte: A multicellular diploid plant. Alternation: The plants, during their life cycle, have a sporic meiosis. Meiosis occurs in spore mother cells. The diploid sporophyte undergoes meiosis to produce haploid spores, which then grow into haploid gametophytes. This type of alternation is known as "intermediate alternation of generation." Alternation of Generation: Generations in which the Gametophyte and Sporophyte: Are morphologically related or similar except for reproductive structures. Algal and Fungal: There can be an alternation of two morphologically dissimilar generations called "Heteromorphic" (also called Isomorphic alternation of generation). Bryophytes: Have a distinct (persistent) gametophytic generation as the dominant phase. Their gametophytic generation is also known as the plant kingdom, and both phases of the life cycle are terrestrial. The gametophyte possesses rhizoids and is green in nature. Rhizoids serve the double function of anchoring and absorbing water and minerals. Sporophytes are dependent on the gametophyte for sustenance. Pteridophytes and Spermatophytes: However, the sporophyte forms the independent and dominant phase in pteridophytes. After the initial stages of development, the sporophyte is attached to the gametophyte but soon becomes independent of the gametophyte and attains a much larger size. Spermatophytes: The gametophyte phase is highly reduced and is dependent on the sporophyte. Gymnosperms: The archegonia are much reduced and are embedded in the female gametophyte (present inside the ovule). The archegonia are not found in other members of the gymnosperms and in angiosperms. In other members of the gymnosperms and in angiosperms, embryo sacs carry out the same functions as archegonia. Reduction in Gametophytic Phase: Also, a corresponding reduction in the gametophytic phase of the archegoniate as they spread towards the land. Maximum reduction of the gametophytic phase is in the angiosperms, where the gametophyte is represented by only a few small-celled structures within the ovule. Evolution of Sporophyte: A vast reverse condition is seen in the evolution of the sporophyte. It is very small, ephemeral, and from its origin, it is independent upon the gametophyte and produces only a limited quantity of spores. In pteridophytes and gymnosperms, the sporophyte is the dominant generation, it bears well-evolved roots, stems, and leaves. Types of Alternation of Generation: There are three types of alternation generation found in the life cycle such as: Haplobiotic Diplobiotic Types of Alternation of Generation: Haplobiontic: It is a life cycle involving only one multicellular generation. It is observed to be of two types: Haploid Type: This type is also known as the haplontic life cycle. It includes only multicellular generations of haploid plants. The plants do not produce a diploid phase. It is seen in homosporous and a few heterosporous forms. Diploid Type: It is also known as the diplontic life cycle. In this type, the only multicellular generation is diploid. It is observed in some algae. Diplobiotic: Haplodiplontic life cycle includes an alternation of generations. The two generations are: Sporophyte: It is the diploid multicellular generation. Sporophyte produces haploid spores through meiosis. Gametophyte: It is the haploid multicellular generation. Gametophyte produces gametes through mitosis. Haplo-Diplontic: This is an intermediate condition between diplontic and haplontic types. Here the gametophyte and sporophyte phases are multicellular. But they differ in their dominant phase. It is also called a diploid-haploid life cycle. Significance of Alternation of Generation: Variation: Variations are produced during meiosis. Survival: Better chance of survival. Adaptation: Better adapted to the environment. New Varieties: Development of new varieties.