Fungi and the Origins of Land Plants
Introduction to Fungi (Mycetes)
Definition: Fungi constitute a diverse kingdom, with 'mycetes' meaning fungus. This kingdom includes an estimated 2.2 to 3.8 million species, ranging from microscopic yeasts to large mushrooms, playing crucial roles in ecosystems as decomposers, mutualists, and pathogens.
Human Relevance:
Cryptomycetes and Microsporidians: These are basal fungal groups. Cryptomycetes are often aquatic and can be endosymbionts, while Microsporidians are obligate intracellular parasites that can cause serious human health issues, particularly in immunocompromised individuals.
Chytrids: These are aquatic fungi, many of which are parasites. Notably, species like Batrachochytrium dendrobatidis (Bd) are largely responsible for infections causing amphibian extinction worldwide, highlighting their significant ecological impact.
Focus Areas: This discussion will primarily cover Ascomycetes and Basidiomycetes, which are two of the most well-known and ecologically significant fungal phyla.
Distinguishing Fungi from Plants
Cell Wall: All fungi possess a rigid cell wall, a characteristic that historically led to their classification with plants within Botany departments.
Composition: Fungal cell walls are uniquely made of chitin, a tough, nitrogenous polysaccharide composed of repeating units of N-acetylglucosamine. This material provides structural support and protection against osmotic lysis.
Contrast with Plants: Real plants do not have chitin in their cell walls; instead, their cell walls are primarily composed of cellulose.
Animal Connection: Chitin is also a major component of the external skeletons (exoskeletons) of arthropods (e.g., insects like beetles, crustaceans) and the beaks of cephalopods. Molecular evidence, supported by genetic sequencing and ultrastructural similarities, now places fungi phylogenetically closer to animals than to plants, forming a clade called Opisthokonta.
Unicellular Forms: Unlike typical conceptions of fungi as multicellular organisms (like mushrooms), yeasts are prominent unicellular fungi. Most fungi are indeed multicellular, but they lack the complex organ and tissue organization (e.g., leaves, stems, roots, vascular systems) seen in true plants. Their body plan is typically simpler, primarily consisting of filamentous structures.
Fungal Structure and Growth
Hyphae (singular: hypha): These are branched, thread-like filamentous strands that collectively make up the main body of a fungus. Each hypha is a tubular cell or a chain of cells surrounded by a chitinous cell wall. They are the primary structures responsible for nutrient absorption. Some hyphae are septate (divided by walls called septa, often with pores allowing cytoplasmic flow), while others are coenocytic (lacking septa, forming a continuous mass of cytoplasm with multiple nuclei).
Mycelium: This term refers to the entire mass or extensive network of interconnected hyphae that constitutes the vegetative part of a fungus. While often used interchangeably with hyphae, mycelium specifically denotes the collective, often vast, and usually hidden, network that grows throughout a substrate (e.g., soil, wood, animal tissue). The mycelium's large surface area-to-volume ratio is highly efficient for absorption of nutrients.
Fruiting Structure: Structures like a toadstool or mushroom are fruiting bodies (also called sporocarps or basidiocarps/ascocarps). These are specialized, transient reproductive structures primarily associated with sexual reproduction, designed for spore dispersal. The vast majority of the organism's biomass (the mycelium) often resides concealed underground (e.g., in soil), within dead trees, or inside host organisms (animals, plants), forming an extensive, intricate network that can cover large areas.
Fungal Lifestyles and Nutrition
Fungi exhibit diverse nutritional strategies, acting as saprobes (decomposers), mutualists, or parasites/predators.
Mutualists: Mycorrhizal Fungi
Definition: This describes a crucial, close, and mutually beneficial symbiotic relationship between fungi ('myco') and the roots ('rhizal') of approximately 80\% of all vascular plants. It's an ancient association, critical for the colonization of land by plants.
Prevalence: It is thought that most, if not all, vascular plants engage in some form of mycorrhizal association, highlighting its ecological importance.
Interaction: Fungal hyphae grow extensively outwards from the root, effectively increasing the plant's root surface area by 100 to 1000 times. These hyphae wrap around plant roots (ectomycorrhizae) or extend into root cells via specialized, highly branched structures called arbuscules (characteristic of arbuscular mycorrhizal fungi, a type of endomycorrhizae).
Mutual Benefit: This is a reciprocal exchange where both partners benefit significantly. The fungus provides the plant with enhanced access to water and essential mineral nutrients (e.g., phosphates, nitrates, zinc, copper) from a larger soil volume, often breaking down complex organic matter. In return, the plant provides the fungus with readily available organic carbon compounds (sugars) produced through photosynthesis, as fungi are heterotrophic and cannot photosynthesize.
Parasites and Predators
Example: Arthrobotrys (fungus) and Nematodes (roundworms)
Mechanism: Arthrobotrys is a fascinating predatory fungus found in wet soil environments. It forms intricate, microscopic, constricting loop-like structures or sticky networks of hyphae. When a nematode (e.g., Caenorhabditis elegans, a common non-parasitic roundworm that typically feeds on bacteria in the soil) passes through one of these loops and touches the inner surface, the loop rapidly constricts (within 0.1 seconds) like a minuscule drawstring. This clamps tightly onto the worm, trapping it.
Penetration: Once the nematode is immobilized, the fungus produces specialized penetration structures called haustoria. These haustoria grow into the nematode's body, dissolving parts of its cuticle and cell walls. While similar in appearance to arbuscules, haustoria serve a parasitic function, extracting nutrients from a living host.
Outcome: The fungus then absorbs nutrients, including amino acids and sugars, from the nematode's internal tissues, effectively digesting the worm from the inside out. Eventually, only the nematode's empty cuticle (outer covering) remains. This demonstrates a rare and sophisticated predatory and parasitic fungal lifestyle, highlighting the diverse ecological roles of fungi beyond simple decomposition.
Generalized Fungal Life Cycle
Fungi (specifically Ascomycota and Basidiomycota) exhibit a unique life cycle involving haploid (n), diploid (2n), and an intermediate dikaryotic (n+n) stage. This "alternation of generations" is distinct from plants in that the multicellular stages may not both exist, and the dikaryotic phase is often prominent.
Spore Germination: Haploid spores (produced via meiosis in the sexual cycle or mitosis in asexual reproduction) germinate under suitable conditions (e.g., moisture, nutrients) to produce new haploid hyphae or a haploid mycelium. Typically, spores resulting from sexual reproduction (meiosis) give rise to genetically distinct mycelia of two compatible mating types (often designated as '+' and '-').
Plasmogamy: When two compatible haploid mycelia grow towards each other and physically encounter, their cytoplasm fuses. This process, known as plasmogamy, is the initial step in sexual reproduction and is analogous to the fusion of egg and sperm cytoplasm in animals. However, in fungi, it is immediately followed by nuclear fusion.
Dikaryotic (n+n) Stage (Heterokaryotic): Following plasmogamy, the fused cells contain two distinct haploid nuclei, one from each parent mycelium, that do not immediately fuse. This unique state, called the dikaryotic stage, is characteristic of Ascomycetes and Basidiomycetes. The dikaryotic mycelium can then proliferate significantly through mitotic divisions, with each new cell containing two haploid nuclei that divide synchronously. This stage can be short-lived or it can dominate the life cycle, forming the entire fruiting body in Basidiomycetes.
Karyogamy: After a variable delay (which can range from hours to weeks or even years, depending on the species and environmental conditions), the two haploid nuclei within certain specialized cells of the dikaryotic stage finally fuse. This nuclear fusion, known as karyogamy, results in the formation of a transient diploid (2n) nucleus. This diploid nucleus is the fungal equivalent of a zygote in other eukaryotes.
Meiosis: The newly formed diploid nucleus immediately undergoes meiosis (reduction division). This process halves the chromosome number, producing four genetically distinct haploid nuclei.
Dispersal: These haploid nuclei are then incorporated into spores (e.g., ascospores or basidiospores), which are subsequently released. Spores are typically lightweight and designed for wide dispersal by wind, water, or animals, completing the sexual cycle and allowing the colonization of new substrates.
Ascomycetes (Sac Fungi)
Characteristics: This is the largest phylum of fungi, including a vast array of species such as unicellular yeasts (e.g., Saccharomyces cerevisiae used in baking and brewing), multicellular forms like common molds (e.g., Penicillium, source of penicillin), cup fungi, powdery mildews, morels, and truffles. They are found in diverse habitats, acting as decomposers, pathogens, and mutualists.
Name Origin: The name Ascomycetes ("sac fungi") is derived from the ascus (plural: asci), a microscopic, sac-like, sexual spore-bearing structure. Within each ascus, usually eight ascospores (spores produced during sexual reproduction) are formed after meiosis and subsequent mitosis.
Fruiting Bodies: In many species, the dikaryotic phase develops into a visible, macroscopic fruiting structure called an ascocarp (e.g., cup-shaped in cup fungi, saddle-shaped in morels). However, in yeasts and some molds, the ascus is often microscopic and may not be enclosed within a complex fruiting body.
Examples:
Morels (Morchella esculenta): These are highly prized edible fungi, known for their distinctive honeycomb-like caps. They are notoriously difficult to cultivate commercially and must often be hunted in the wild, typically in spring. Caution: 'False morels' (Gyromitra esculenta), which contain toxins like gyromitrin, are poisonous and can be deadly if consumed raw or improperly prepared.
Truffles (Tuber melanosporum, Tuber magnatum): These are another group of highly expensive and sought-after edible fungi that grow hypogeously (underground), often in close mycorrhizal association with tree roots (e.g., oaks, hazelnuts). Their intense, earthy aroma, caused by volatile organic compounds, helps animals (traditionally pigs, now more commonly trained dogs) detect them for harvest.
Basidiomycetes (Club Fungi)
Characteristics: This phylum includes many of the familiar fungi we commonly recognize as mushrooms, toadstools, puffballs, shelf or bracket fungi, rusts, smuts, and stinkhorns. They are crucial decomposers of wood and other plant material, as well as significant mutualists (mycorrhizal fungi) and plant pathogens.
Name Origin: The name Basidiomycetes ("club fungi") is derived from the basidium (plural: basidia), an elongated, microscopic, club-shaped cell. This specialized structure is the site of karyogamy and meiosis, and typically produces four external basidiospores (spores produced during sexual reproduction) at its tip.
Lifestyles: Basidiomycetes exhibit diverse lifestyles. Many are important mutualists, forming ectomycorrhizal associations with trees. Others are significant plant parasites (e.g., rusts, smuts), causing considerable agricultural damage. A large number are saprobic decomposers, especially important for breaking down lignin in wood.
Examples:
Bracket Fungus (Shelf Fungi): These are often found growing horizontally as tough, woody shelves or brackets on tree trunks, both living and dead. They are important decomposers of wood.
Puffball: These fungi release vast quantities of powdery basidiospores when mature or disturbed, appearing as a puff of 'smoke'. Giant puffballs (Calvatia gigantea) can reach impressive sizes, up to the size of a basketball or larger, and are edible when young and the internal tissue is still white and firm (i.e., before spores develop).
Stinkhorns: These fungi are easily recognized by their usually phallic shape and the foul, putrid odor (like decaying meat or feces) they produce. This odor, along with a sticky, olive-green spore mass (gleba), attracts flies and other insects, which then inadvertently pick up and disperse the basidiospores, aiding in their reproduction.
Detailed Basidiomycete Life Cycle
The life cycle of a typical mushroom-forming basidiomycete emphasizes the dominance of the dikaryotic stage.
Basidiospore Germination: Haploid basidiospores (typically four produced from each basidium after meiosis, usually two of '+' mating type and two of '-' mating type) are released and germinate under favorable conditions, forming primary, haploid mycelia. These primary mycelia are often short-lived.
Plasmogamy: When two compatible primary haploid mycelia (of opposite mating types) encounter each other, their cytoplasm fuses. This process, plasmogamy, leads to the formation of a secondary mycelium.
Dikaryotic (n+n) Mycelium: The secondary mycelium is dikaryotic, meaning each cell contains two distinct haploid nuclei (one from each parent) that coexist without fusing. This dikaryotic stage is typically long-lived and vegetative dominant in Basidiomycetes, often growing extensively within the substrate for weeks, months, or even years.
Basidiocarp Development: Under appropriate environmental cues (e.g., changes in temperature, moisture, nutrient availability), the dikaryotic mycelium aggregates and differentiates to form a basidiocarp (the visible fruiting body, such as a mushroom). The entire basidiocarp is composed of dikaryotic hyphae.
Basidia Formation: Specialized club-shaped structures called basidia develop on the reproductive surfaces of the basidiocarp. In common mushrooms, these basidia line the gills underneath the cap.
Karyogamy: Inside each basidium, the two haploid nuclei finally fuse after the prolonged dikaryotic phase, forming a short-lived diploid (2n) zygote nucleus. This is the only diploid stage in the Basidiomycete life cycle.
Meiosis: The diploid nucleus immediately undergoes meiosis (reduction division) to produce four genetically unique haploid nuclei.
Basidiospore Formation and Dispersal: Each of these four haploid nuclei migrates into a small outgrowth at the tip of the basidium, where it develops into an external basidiospore. These basidiospores are then forcibly discharged or passively released and dispersed by wind, water, or animals to germinate and restart the cycle.
Fairy Rings: A natural phenomenon where basidiocarps (mushrooms) grow in a distinctive circular or arc-like pattern in open grassy areas. This occurs when a single basidiospore germinates in the center, and the resulting dikaryotic mycelium grows outwards radially and symmetrically within the soil. Each year (or season), new fruiting bodies emerge at the expanding outer margin of the mycelium, forming a ring. The inner areas of the ring may become depleted of resources or accumulate metabolic waste products, making them unsuitable for further fruiting body production, while the outward expansion continues as the mycelium seeks new nutrients.
Lichens
Definition: A lichen is a remarkable mutualistic symbiotic association that involves a close and extensive partnership between two or sometimes three organisms: a fungus (the mycobiont, typically an Ascomycete, but sometimes a Basidiomycete), and a photosynthetic partner (the photobiont), which is either a green alga (chlorophyte) or a cyanobacterium, or sometimes both.
Morphology: Lichens exhibit a wide array of growth forms, including fruticose (shrub-like or branched), foliose (leaf-like with lobes), and crustose (crust-like, growing tightly adhered to the substrate). They are highly adaptable and commonly found colonizing harsh environments such as tree bark, bare rocks, soil, and old concrete, even in extreme conditions like polar regions and deserts.
Ecological Significance: Lichens are renowned for their slow growth and extreme sensitivity to airborne pollutants, particularly sulfur dioxide (SO2). This makes them excellent biological indicators (bioindicators) of air quality. Their presence or absence and species composition can be used to assess pollution levels in an ecosystem.
Partnership: The fungus forms the structural bulk of the lichen's biomass, creating a protective thallus that encloses the photosynthetic cells. The fungal hyphae absorb water and mineral nutrients from the environment and protect the photobiont from desiccation, UV radiation, and excessive light intensity. In return, the photosynthetic partner, through photosynthesis, produces organic compounds (sugars/carbohydrates) that are transferred to and utilized by the heterotrophic fungus.
Reproduction: Lichens reproduce both sexually (fungal meiosis, leading to ascospores/basidiospores, which must then re-encounter a suitable photobiont) and asexually. Asexual reproduction is often achieved via specialized propagules like soredia (powdery clusters of fungal hyphae and algal cells) or isidia (small outgrowths of the thallus), which contain both fungal and photosynthetic material, ensuring the symbiotic relationship is passed on intact to new locations.
The Transition to Land Plants
This section discusses the pivotal evolutionary journey from aquatic algal ancestors to terrestrial plants, often referred to as 'drier plants', which involved overcoming numerous challenges associated with life on land (e.g., desiccation, gravity, dispersal of gametes).
Charophytes: Close Algae Allies
Charophytes: This is a specific lineage of freshwater green algae that are considered the closest living relatives to land plants (embryophytes). This close relationship is supported by molecular (DNA sequencing) and morphological (cellular structure, reproductive processes) evidence.
Chara: Often referred to as "stoneworts" due to calcium carbonate deposits on their surfaces, Chara is a macroscopic charophyte (up to 30 cm long or more). It possesses a complex body plan with a main axis and whorls of branches, superficially resembling a higher plant. It lives fully submerged in shallow freshwaters (ponds, lakes, ditches) and has relatively complex multicellular reproductive structures (oogonia and antheridia). While strikingly 'plant-like' in appearance and some reproductive features, it is not a true land plant as it lacks key adaptations for terrestrial life.
Zygnema: In contrast to Chara's complex morphology, Zygnema is a simpler, unbranched filamentous charophyte. Each cell typically contains two prominent, star-like (stellate) chloroplasts. Despite its simpler morphology, extensive molecular evidence from ribosomal RNA and chloroplast DNA sequences identifies Zygnema and its allies (e.g., Spirogyra, Mougeotia) as the single closest living relatives to land plants, forming a sister group to the embryophytes. This suggests that the common ancestor of land plants may have resembled a filamentous alga.
Phylogeny of Green Algae and Land Plants
Understanding the phylogenetic relationships helps clarify the evolutionary path:
Archaeplastida: This is a supergroup of eukaryotes that includes all organisms that acquired plastids (chloroplasts) through a single primary endosymbiosis event, where a heterotrophic eukaryote engulfed a cyanobacterium. This clade encompasses red algae, green algae (chlorophytes and charophytes), and land plants.
Green Algae (paraphyletic): This informal grouping traditionally includes two major clades: the Chlorophytes (e.g., Chlamydomonas, Ulva, Chlorella) and the Charophytes (e.g., Chara, Zygnema). It is crucial to note that "green algae" as a traditional category is not a monophyletic group (a clade) because it excludes the land plants (embryophytes), even though land plants evolved directly from within the charophyte lineage of green algae.
Streptophytes: This is a true monophyletic clade (a common ancestor and all its descendants). It comprises the Charophytes and the Embryophytes (land plants). This clade unites the closest algal relatives with the land plants, highlighting their shared ancestry.
Viridiplantae ('Green Plants'): This is a larger, comprehensively monophyletic group encompassing all green algae (both Chlorophytes and Charophytes) and all land plants. It represents all descendants of the common ancestor that first acquired green plastids.
Apomorphic (Derived) Traits Shared by Charophytes and Land Plants
These shared characteristics provide strong evidence for the close evolutionary relationship between charophytes and land plants, indicating they were present in their most recent common ancestor.
Phragmoplast: This is a complex system of microtubules and endoplasmic reticulum fragments that forms in the equatorial plane of the dividing cell during late anaphase and telophase of mitosis. It plays a critical role in the construction of the new cell plate or cross wall during cytokinesis. The phragmoplast guides vesicles filled with cell wall materials (e.g., cellulose) from the Golgi apparatus to the center of the dividing cell, directing their deposition outwards (centripetally) to form the new cell wall. This mechanism ensures proper cell division and multicellular organization.
Multicellular Gametangia: (Found in some charophytes and all land plants) These are specialized, complex, multicellular structures that are responsible for the production and protection of gametes (sperm and eggs). The male gametangium is called an antheridium (produces sperm), and the female gametangium is called an archegonium (produces eggs). Unlike single-celled algal gametangia, these multicellular structures provide a degree of protection against desiccation for the delicate gametes, an important adaptation for life in transitional environments.
Apical Meristems: (Found in some charophytes and all land plants) These are regions of perpetually embryonic, undifferentiated 'stem cells' located at the tips of shoots and roots (in land plants) or at the growing points of charophytes. The cells within apical meristems continuously divide through mitosis and then differentiate, allowing for indeterminate growth (continuous elongation) and the formation of all the various tissues and organs of the plant body. This ability to grow indefinitely from specific points is fundamental to the modular construction of plants.
Apomorphic Traits Exclusive to Land Plants (Kingdom Plantae / 'Drier Plants')
These defining characteristics evolved in the lineage leading to land plants after their divergence from charophytes, representing key adaptations that allowed them to colonize and thrive in terrestrial environments.
Dependent Embryo: This is a defining feature of embryophytes (land plants). It refers to a multicellular diploid sporophyte (the plant 'baby') that is uniquely retained and protected within the tissues of the female gametophyte (the maternal parent). The maternal gametophyte nourishes the developing embryo via specialized placental transfer cells, providing essential nutrients and protection against environmental stresses. This parental protection is crucial for the embryo's survival and differentiates land plants from their algal ancestors, whose zygotes are typically released and free-living.
Cuticle: This is a significant adaptation for preventing desiccation on land. The cuticle is a waxy, non-living, extracellular layer composed of cutin (a polyester of fatty acids) and waxes, which covers the exposed epidermal surfaces of stems and leaves. It forms a hydrophobic barrier that minimizes water loss through evaporation. Roots, which are typically subterranean and responsible for water uptake, generally lack a cuticle. While essential for water retention, the cuticle also necessitated the evolution of stomata for gas exchange.
Alternation of Generations (with multicellular gametophyte and sporophyte): This is a life cycle pattern where there are two distinct, multicellular stages: a haploid gametophyte (which produces gametes by mitosis) and a diploid sporophyte (which produces spores by meiosis). While some algae (e.g., Laminaria) also exhibit alternation of generations, the defining feature in land plants is that both the gametophyte and sporophyte stages are multicellular and typically morphologically distinct. This life cycle allows for both sexual recombination
KEY TERMS:
Hypha/Hyphae: Branched, thread-like filamentous strands that collectively make up the main body of a fungus, responsible for nutrient absorption. They can be septate (divided by walls) or coenocytic (lacking septa).
Mycelium/Mycelia: The entire mass or extensive network of interconnected hyphae that constitutes the vegetative part of a fungus, typically hidden within a substrate. Its large surface area-to-volume ratio is highly efficient for nutrient absorption.
Haustorium/Haustoria: Specialized penetration structures produced by parasitic fungi that grow into a host's body to extract nutrients. While similar in appearance to arbuscules, haustoria serve a parasitic function.
Mycorrhizae: A crucial, close, and mutually beneficial symbiotic relationship between fungi ('myco') and the roots ('rhizal') of approximately 80\% of all vascular plants, critical for their colonization of land.
Arbuscule: Specialized, highly branched structures formed by fungal hyphae that extend into plant root cells, characteristic of arbuscular mycorrhizal fungi (a type of endomycorrhizae), facilitating nutrient exchange between fungus and plant.
Ascomycota: The largest phylum of fungi, known as "sac fungi" due to the ascus, a microscopic, sac-like, sexual spore-bearing structure. Includes yeasts, molds, cup fungi, morels, and truffles.
Basidiomycota: The phylum of fungi known as "club fungi," including many familiar mushrooms, toadstools, puffballs, and shelf fungi. Their name is derived from the basidium.
Basidium: An elongated, microscopic, club-shaped cell in Basidiomycetes where karyogamy and meiosis occur, typically producing four external basidiospores at its tip.
Basidiospore: Haploid spores produced externally from a basidium during sexual reproduction in Basidiomycetes, typically four per basidium.
Basidiocarp: The visible fruiting body of a Basidiomycete, such as a mushroom, entirely composed of dikaryotic hyphae, designed for spore dispersal.
Mating Type (+) or (-): Designations for genetically distinct haploid mycelia that are compatible for sexual reproduction.
Dikaryotic (n+n): A unique stage in fungal sexual life cycles (characteristic of Ascomycetes and Basidiomycetes) where fused cells contain two distinct haploid nuclei, one from each parent mycelium, that coexist without fusing.
Plasmogamy: The fusion of cytoplasm from two compatible haploid mycelia, the initial step in fungal sexual reproduction, analogous to the fusion of egg and sperm cytoplasm in animals.
Karyogamy: The fusion of the two haploid nuclei within a dikaryotic cell, occurring after plasmogamy, resulting in the formation of a transient diploid (2n) nucleus.
Lichen: A remarkable mutualistic symbiotic association involving a fungus (mycobiont, typically an Ascomycete) and a photosynthetic partner (photobiont, either a green alga or a cyanobacterium, or both).
Cuticle: A waxy, non-living, extracellular layer composed of cutin and waxes that covers the exposed epidermal surfaces of stems and leaves in land plants, forming a hydrophobic barrier to minimize water loss.
Alternation of Generation: A life cycle pattern in land plants (and some algae) involving two distinct, multicellular stages: a haploid gametophyte (which produces gametes by mitosis) and a diploid sporophyte (which produces spores by meiosis).
Diplobiontic: Information not available in the provided notes.
Heteromorphic: Information not available in the provided notes.
Homospory: Information not available in the provided notes.
Heterospory: Information not available in the provided notes.
Multicellular Gametangia: Specialized, complex, multicellular structures (e.g., antheridium for sperm, archegonium for eggs) found in some charophytes and all land plants, responsible for the production and protection of gametes.
Phragmoplast: A complex system of microtubules and endoplasmic reticulum fragments that forms in the equatorial plane of the dividing cell during late anaphase and telophase of mitosis in charophytes and land plants, critical for cell plate formation.
Embryo: A defining feature of embryophytes (land plants), referring to a multicellular diploid sporophyte that is uniquely retained, protected, and nourished within the tissues of the female gametophyte.
Apical Meristem: Regions of perpetually embryonic, undifferentiated 'stem cells' located at the tips of shoots and roots (in land plants) or growing points of charophytes, allowing for indeterminate growth and tissue differentiation.