Comprehensive Study Guide: Protists, Plant Terrestrialization, Plant Structure, and Seed Plant Evolution

Protista: Diversity, Phylogeny, and Ecological Roles

• General Characteristics and Taxonomy:

  • Protista is an informal name for an immense, highly diverse group of eukaryotic organisms, composed mostly of unicellular species, although colonial and multicellular forms exist.
  • Protists exhibit greater structural and functional diversity than any other eukaryotic group.
  • Unicellular protists demonstrate extreme cellular complexity because all vital biological functions—such as excretion, respiration, and digestion—must be carried out within a single cell by specialized organelles.
  • Protista is a polyphyletic group and is no longer recognized as a valid biological Kingdom.

• Nutritional Diversity:

  • Photoautotrophs: Organisms that contain plastids and synthesize organic compounds using light energy.
  • Heterotrophs: Organisms that absorb organic molecules or ingest large food particles via phagocytosis.
  • Mixotrophs: Organisms capable of combining photosynthetic autotrophy and heterotrophic nutrition depending on light and resource availability.

• Reproductive Strategies:

  • Reproduction occurs through asexual processes (such as binary fission or budding), sexual processes (such as conjugation and syngamy), or complex life cycles featuring an alternation of generations.

• Endosymbiosis and Eukaryotic Evolution:

  • Endosymbiosis: A symbiotic relationship in which one organism (the endosymbiont) lives inside the cell or cells of another organism (the host).
  • Mitochondria and Plastids: Mitochondria and plastids evolved from prokaryotes that were engulfed by ancestral eukaryotic cells.
  • Primary Endosymbiosis: An ancestral heterotrophic eukaryote engulfed a photosynthetic cyanobacterium. This event led to the origin of red algae (Red algae) and green algae (Green algae). One of the original cyanobacterial membranes was lost in descendants.
  • Secondary Endosymbiosis: Occurred when red or green algae were engulfed by heterotrophic eukaryotes on multiple independent occasions:
  • Engulfment of red algae produced Dinoflagellates and Stramenopiles.
  • Engulfment of green algae produced Euglenids (Euglenids) and Chlorarachniophytes (Chlorarachniophytes).

• The Four Eukaryotic Supergroups:

  • Excavata: Includes Diplomonads, Parabasalids, and Euglenozoans.
  • "SAR" Clade: Monophyletic supergroup named after its three major lineages: Stramenopiles, Alveolates, and Rhizarians.
  • Archaeplastida: Includes Red algae, Green algae (Chlorophytes and Charophytes), and Land plants (Land plants).
  • Unikonta: Includes Amoebozoans (Slime molds, Tubulinids, Entamoebas) and Opisthokonts (Nucleariids, Fungi, Choanoflagellates, Animals).

• Excavata: Euglenozoans:

  • A diverse clade comprising predatory heterotrophs, photosynthetic autotrophs, mixotrophs, and parasites.
  • Defining Feature: Presence of a crystalline or spiral protein rod (paraflagellar rod or PFR) inside their flagella.

• Cinetoplástidos (Kinetoplastids):
  • Possess a single large mitochondrion that contains an organized mass of DNA known as a kinetoplast (cinetoplasto).
  • Free-living species act as consumers of prokaryotes in freshwater, marine, and moist terrestrial ecosystems.
  • Parasitic species include Trypanosoma, which causes African sleeping sickness (enfermedad del sueño) and Chagas disease (enfermedad de Chagas). Observed in blood smears relative to human red blood cells ($9\,\mu\text{m}$ scale).
• Euglénidos (Euglenids):
  • Unicellular organisms featuring 1 or 2 flagella emerging from a pocket or cavity at the anterior end of the cell.
  • Characterized by a pellicle (película or periplasto), consisting of protein bands located beneath the plasma membrane, supported by dorsal and ventral microtubules. The sliding of these bands creates a flexible, contractile movement (euglenoid movement or metaboly).
  • Photosynthetic species contain chloroplasts with chlorophylls $a$, $b$, and carotenoids (reflecting a evolutionary relationship to green algae).
  • Reserve carbohydrate is stored as paramylon (paralmidón).
  • Phototaxis is mediated by a photosensitive system composed of an eyespot (mancha ocular) and a light detector/stigma (estigma), guiding positive and negative phototaxis.

• SAR Clade: Stramenopiles:
  • Characterized by flagellar dimorphism: a distinct "hairy" (hairy flagellum) flagellum paired with a shorter "smooth" (smooth flagellum) flagellum.

• Diatomeas (Diatoms):

  • Unicellular or colonial photosynthetic organisms constituting a major component of phytoplankton.
  • Photosynthetic pigments include chlorophylls $a$, $c$, and fucoxanthin (fucoxantina).
  • Energy reserves are stored as lipids and chrysolaminarin (crisolaminarina), a water-soluble polysaccharide.
  • Enclosed within a glass-like cell wall composed of two overlapping valves or frustules (frústulas) impregnated with hydrated silicon dioxide ($\text{SiO}_2$).
  • Classified morphologically into pennate (pennadas, displaying bilateral symmetry) and centric (céntricas, displaying radial symmetry).

• Algas Pardas (Brown Algae):

  • Multicellular marine organisms including the largest and most structurally complex algae (kelps).

  • Vegetative body is called a thallus (talo), differentiated in complex species into a holdfast (pie for anchoring), stipe (estípite, stem-like), and blade (lámina, leaf-like). Gas-filled air bladders (vesículas de aire) provide buoyancy (e.g., Laminaria, Fucus vesiculosus).

  • Pigments: Chlorophylls $a$, $c$, and carotenoids (including fucoxanthin, granting dark brown to olive-green coloration).

  • Storage carbohydrate: Laminarin (laminarina).

  • Economically important for producing alginate (alginato), a mucilaginous cell wall substance used as a thickener.

  • SAR Clade: Alveolates:

• Defining Feature: Presence of membrane-enclosed sacs called alveoli (alveolos) situated directly beneath the plasma membrane.

• Dinoflagelados (Dinoflagellates):

  • Mostly unicellular biflagellates whose flagella beat in two perpendicular grooves (cingulum and sulcus), inducing a spinning rotational movement ("volteretas").
  • Armored species possess cellulose plates forming an ornate theca (teca) that aids flotation (e.g., Ceratium tripos, Gonyaulax polyedra). Non-motile vegetative forms produce flagellated reproductive cells.
  • Pigments: Chlorophylls $a$, $c$, and peridinin (peridinina). Reserve material: Starch (almidón).
  • Symbioses: Live as endosymbionts in sponges, jellyfish, sea anemones, corals, and mollusks; in symbiosis, they lack thecae and exist as spherical golden cells called zooxanthellae (zooxantelas).
  • Approximately $20\%$ of species produce potent neurotoxins, causing Harmful Algal Blooms (HABs) or red tides (marea roja).
  • Many species exhibit marine bioluminescence for predator avoidance (e.g., Noctiluca scintillans, Pyrocystis fusiformis).

• Apicomplejos (Apicomplexans):

  • Unicellular animal endoparasites that spread through infectious cells called sporozoites (esporozoitos).
  • Possess an apical complex at one end of the cell containing organelles specialized for penetrating host cells and tissues (e.g., Plasmodium, the causative agent of malaria).

• Ciliados (Ciliates):

  • Diverse group of heterotrophic protists covered by short hair-like cilia (cilios) used for movement and feeding (e.g., Paramecium).

  • Exhibit nuclear dualism: large macronuclei (macronúcleos) control everyday metabolic functions, while small micronuclei (micronúcleos) function in sexual processes.

  • Conjugation (conjugación) is a non-reproductive sexual process involving the reciprocal exchange of haploid micronuclei between cells, separate from asexual reproduction via binary fission (fisión binaria).

  • Archaeplastida: Red Algae and Green Algae:

• Algas Rojas (Red Algae):

  • Mostly multicellular marine seaweeds. Some deposit calcium carbonate ($\text{CaCO}_3$) in cell walls (coralline algae / algas coralinas). Examples include Nori, Dulse (Palmaria palmata), and Bonnemaisonia hamifera.
  • Pigments: Chlorophyll $a$, carotenoids, and phycobilins (ficobilinas), which absorb blue/green light in deep waters.
  • Reserve material: Floridean starch (almidón florídeo), stored in the cytoplasm.
  • Commercially harvested for agar (agar) and carrageenan (carragenina).

• Algas Verdes (Green Algae):

  • Paraphyletic group divided into Chlorophytes (Clorofitas) and Charophytes (Carofitas).

  • Habitat: Mostly freshwater, but also marine, terrestrial (soil, snow, tree trunks), and symbiotic (lichens, protozoa, sponges, coelenterates).

  • Pigments & Reserves: Chlorophylls $a$ and $b$; store starch (almidón) inside chloroplasts.

  • Morphological diversity:

    • Unicellular motile (Chlamydomonas) and sessile (Chlorococcum).
    • Colonial motile (Volvox) and sessile (Hydrodictyon).
    • Filamentous multinucleate (Cladophora).
    • Flat parenchymatous sheets two cells thick (Ulva).
    • Siphonous / Coenocytic branched forms (Codium, Ventricaria, Acetabularia, Halimeda).
    • Complex thalli differentiated into nodes and internodes with apical growth (Chara).

  • Unikonta: Amoebozoa:

• Amoeba:

  • Unicellular organisms characterized by lobe- or tube-shaped pseudopodia (pseudópodos) used for locomotion (amoeboid movement) and capturing prey via phagocytosis (fagocitosis).
  • Includes free-living amoebas (Amoeba) and endoparasites of animals (e.g., Entamoeba).

• Hongos Limosos (Slime Molds / Mycetozoans):
  • Formerly classified under Fungi; divided into plasmodial slime molds (plasmodiales) and cellular slime molds (celulares).

Evolutionary Transition and Colonization of Land by Plants

• Timeline and Origins:

  • Cyanobacteria and protists inhabited land surfaces $1\,200\,\text{Ma}$ ($1.2\,\text{billion years ago}$); simple land plants, fungi, and animals colonized terrestrial environments around $500\,\text{Ma}$.
  • Land plants encompass approximately $290\,000\,\text{living species}$. All aquatic plants evolved secondarily from land-adapted ancestors.
  • Land plants evolved directly from a freshwater green algal ancestor belonging to the Charophytes (Carofitas).

• Homologies Shared between Charophytes and Land Plants:

  • Rings of cellulose-synthesizing proteins embedded in the plasma membrane.
  • Similarity in the structure of flagellated sperm.
  • Formation of a phragmoplast (fragmoplasto) during cell plate construction in mitotic division.
  • High nuclear and chloroplast gene sequence homologies.

• Terrestrial Adaptations:

  • Sporopollenin (esporopolenina): A durable polymer layer protecting charophyte zygotes and land plant spore walls from desiccation and mechanical damage.
  • Terrestrial Advantages: Unfiltered light, elevated atmosphere $\text{CO}_2$, and nutrient-rich soil.
  • Terrestrial Challenges: Water scarcity and absence of structural buoyancy against gravity.
  • Land plants are formally designated as Embryophytes (Embriofitas) due to embryo dependence on maternal tissue.

• Five Shared Derived Traits of Land Plants (Absent in Charophytes):

  1. Alternation of Generations (Alternancia de generaciones):
  • Life cycles alternate between two multicellular generations: a haploid ($n$) gametophyte (gametófito) that produces haploid gametes by mitosis, and a diploid ($2n$) sporophyte (esporófito) resulting from fertilization, which produces haploid ($n$) spores by meiosis.
  1. Multicellular Dependent Embryos (Embriones multicelulares y dependientes):
  • Diploid embryos are retained within female gametophyte tissues. Nutrients are passed from parent to embryo via specialized placental transfer cells (células placentarias de transferencia).

3. Walled Spores Produced in Sporangia (Esporangios con esporas de pared resistente):
   • Sporangia (esporangios) house diploid sporocytes (esporocitos) that undergo meiosis to generate haploid spores coated in protective sporopollenin.
4. Multicellular Gametangia (Gametangios multicelulares):
   • Gametes are produced inside organs called gametangia. Female gametangia, archegonia (arquegonios), produce non-motile eggs and serve as fertilization sites. Male gametangia, antheridia (anteridios), produce and release flagellated sperm.

5. Apical Meristems (Meristemos apicales):
   • Localized regions of continuous cell division located at the tips of shoots and roots that differentiate into specialized plant tissues.

• Additional Derived Terrestrial Traits:
  • Cuticle (Cutícula): Waxy epidermal covering preventing excessive water loss.
  • Stomata (Estomas): Specialized microscopic pores flanked by guard cells allowing gas exchange ($\text{CO}_2$/$\text{O}_2$) and regulating transpiration.
  • Mycorrhizae (Micorrizas): Mutualistic symbiotic relationships between fungi and early rootless land plants that facilitated mineral nutrient absorption.

• Fossil Record and Major Plant Groups:

  • Fossil spores and plant tissues date back to $470\,\text{Ma}$ and $450\,\text{Ma}$.
  • Non-Vascular Plants (Briofitas): Paraphyletic collection of small herbaceous plants ($\approx 24\,100\,\text{total species}$):
  • Phylum Hepatophyta (Liverworts / Hepáticas): $\approx 9\,000\,\text{species}$ (e.g., Marchantia polymorpha, Plagiochila deltoidea). Lack stomata.
  • Phylum Bryophyta (Mosses / Musgos): $\approx 15\,000\,\text{species}$ (e.g., Mnium, Polytrichum commune). Possess stomata.
  • Phylum Anthocerophyta (Hornworts / Antoceros): $\approx 100\,\text{species}$ (e.g., Phaeoceros). Possess stomata.
  • Briofita Life Cycle Dynamics:
  • Gametophytes ($n$) are dominant, larger, and longer-lived than sporophytes.
  • Sporophytes ($2n$) are short-lived, dependent on the maternal gametophyte, and consist of a foot (pie), seta (seta), capsule/sporangium (cápsula), and sometimes a peristome (peristoma) specialized for gradual spore release.

• Vascular Plants (Plantas Vasculares):

  • Earliest vascular fossils date to approximately $425\,\text{Ma}$.
  • Key Features:
  • Dominant, branched, independent sporophytes ($2n$).
  • True vascular transport tissue: xylem with lignified tracheids for water/mineral conduction and structural support; phloem for organic nutrient transport.
  • Well-developed absorption and photosynthetic organs: roots (raíces) and leaves (hojas).
  • Leaf Classifications:
  • Microphylls (Microfilos): Small leaves with a single unbranched vascular vein (exclusive to Lycophytes).
  • Megaphylls (Megafilos): Leaves with a highly branched vascular network (Monilophytes and Seed Plants).

• Sporophylls and Spore Systems:
  • Sporophylls (Esporofilos): Modified leaves bearing sporangia.
  • Sori (Soros): Clusters of sporangia on the underside of fern fronds.
  • Strobili (Estróbilos): Cone-like clusters of sporophylls.
  • Homospory: Production of a single spore type that develops into a bisexual gametophyte.
  • Heterospory: Production of two distinct spore types: megaspores (megaesporas) developing into female gametophytes and microspores (microesporas) developing into male gametophytes.
• Seedless Vascular Plant Lineages:
  • Phylum Lycophyta: $\approx 1\,200\,\text{species}$. Includes spike mosses (Selaginella moellendorffii), quillworts (Isoetes gunnii), and club mosses (Diphasiastrum tristachyum).
  • Phylum Monilophyta / Pteridophyta: $\approx 12\,000\,\text{species}$. Includes ferns (Athyrium filix-femina), giant horsetails (Equisetum telmateia), and whisk ferns (Psilotum nudum).

• Ecological Significance:
  • Carboniferous forests ($360{-}300\,\text{Ma}$) of giant lycophytes and ferns drawdown immense atmospheric $\text{CO}_2$, driving global cooling and glaciation. Their decay created global coal reserves (carbón mineral).

Angiosperm Structure, Tissue Organization, and Physiology

• Organ Systems of Vascular Plants:

  • Plant body is organized into two interdependent systems: the Root System (Sistema radical) and the Shoot System (Sistema del vástago/brote).
  • Roots import photosynthetic sugars from the shoot system; shoots import water and inorganic minerals absorbed by the root system.

• Roots:

  • Primary functions: Anchoring the plant, absorbing water and dissolved minerals, and storing carbohydrates.
  • Primary Root (Raíz primaria): First root to emerge from a germinating seed, branching into lateral roots (raíces laterales).
  • Taproot System (Sistema de raíz pivotante): Found in tall upright plants; consists of a thick main taproot derived from the primary root, providing strong anchorage.
  • Fibrous Root System (Sistema de raíz fibrosa): Found in small or creeping plants (and most monocots); primary root dies early, replaced by adventitious roots (raíces adventicias) arising from stems or leaves that give rise to fine lateral roots.
  • Root Hairs (Pelos radiculares): Tubular extensions of individual epidermal cells near root tips that increase surface area for absorption.
  • Modified Root Types:
  • Prop Roots (Raíces fúlcreas): Adventitious roots supporting top-heavy plants.
  • Storage Roots (Raíces tuberosas/almacenamiento): Store food and water (e.g., beets, radishes).
  • Strangling Aerial Roots (Raíces aéreas estranguladoras): Aerial roots that envelop host trees.
  • Buttress Roots (Raíces tabulares/contrafuertes): Architectural support in shallow tropical soils.
  • Pneumatophores (Pneumatóforos): Upward-growing roots designed for gas exchange in submerged, anoxic mud.

• Stems:

  • Structurally composed of Nodes (Nodos, points of leaf attachment) and Internodes (Entrenudos, stem segments between nodes).
  • Apical Bud (Yema apical): Located near the shoot tip, responsible for primary elongation growth.
  • Axillary Bud (Yema axilar): Located in the upper angle (axil) formed by a leaf and stem; retains capacity to form lateral branches, thorns, or flowers.
  • Modified Stems:
  • Rhizomes (Rizomas): Horizontal underground stems.
  • Stolons (Estolones): Horizontal surface runners facilitating asexual colonization.
  • Tubers (Tubérculos): Enlarged swollen rhizome/stolon ends specialized for food storage (e.g., potatoes).

• Leaves:

  • Principal photosynthetic organ of vascular plants, responsible for light interception, gas exchange, heat dissipation, and herbivore defense.
  • Anatomical components: Expanded blade (lámina) and a stalk called the petiole (peciolo) attaching leaf to stem at a node.
  • Venation Patterns: Monocots display parallel leaf veins; Eudicots display netted/reticulate (reticuladas) vein networks.
  • Leaf Complexity:
  • Simple Leaf: Single undivided blade attached to petiole.
  • Compound Leaf: Blade divided completely into multiple distinct leaflets (foliolos).
  • Modified Leaves: Tendrils (zarcillos for climbing), Spines (espinas for defense), Storage leaves (bulbos like onions), Reproductive leaves (hojas reproductivas producing plantlets).

• The Three Plant Tissue Systems:

  1. Dermal Tissue System (Tejido Dermal):
  • Outer protective layer. In non-woody plants, it consists of a single layer called the epidermal cells coated by a waxy cuticle (cutícula).
  • In woody plants, the periderm (peridermo) replaces the epidermis in mature stem/root zones.
  • Trichomes (Tricomas): Outgrowths of shoot epidermal cells that reduce water loss, reflect excess sunlight, or secrete chemical defenses against herbivores.
  1. Vascular Tissue System (Tejido Vascular):
  • Facilitates long-distance transport between root and shoot systems and provides structural support.
  • Xylem (Xilema): Conducts water and disolved minerals upward from roots to shoots.
  • Phloem (Floema): Transports sugars and photosynthetic products from sources to sinks.
  • Stele (Estela): Vascular arrangement; forms a central vascular cylinder in roots and divided vascular bundles (haces vasculares) in stems and leaves.
  1. Ground Tissue System (Tejido Fundamental):
  • Tissues neither dermal nor vascular. Internal to vascular tissue is called pith (médula); external to vascular tissue is called cortex (córtex). Contains cells specialized for storage, photosynthesis, support, and short-distance transport.

• Specialized Plant Cell Types:

  • Parénquima (Parenchyma Cells):
  • Possess thin, flexible primary cell walls; lack secondary walls.
  • Retain functional capability for cell division and differentiation at maturity.
  • Perform key metabolic functions: photosynthesis, starch storage, and tissue regeneration.
  • Colénquima (Collenchyma Cells):
  • Grouped in strands with unevenly thickened primary cell walls rich in pectin.
  • Provide flexible structural support to young growing shoot parts without restricting elongation growth.

• Esclerénquima (Sclerenchyma Cells):
  • Possess thick secondary cell walls heavily reinforced with lignin.
  • Dead at functional maturity; provide rigid structural support. Divided into Sclereids (esclereidas, short, irregular shape, thick secondary walls) and Fibras (fibras, long, slender strands).
• Xylem Water-Conducting Cells:
  • Tracheids (Traqueidas): Long, thin tapered cells present in all vascular plants.
  • Vessel Elements (Elementos del vaso): Wider, shorter cells aligned end-to-end with perforated end plates, forming continuous conduits called vessels (common in angiosperms).
  • Both cell types are dead at functional maturity, leaving lignified cell walls behind.
• Phloem Sugar-Conducting Cells:
  • Sieve-Tube Elements (Elementos del tubo criboso): Alive at functional maturity but lack nucleus, ribosomes, and vacuoles to reduce resistance to fluid flow. End walls contain sieve plates (placas cribosas) with pores.
  • Companion Cells (Células acompañantes): Non-conducting nucleated cells connected to sieve-tube elements via plasmodesmata; perform metabolic services for both cells.

• Meristems and Plant Growth:

  • Plants display indeterminate growth (crecimiento indeterminado) throughout life due to perpetually undifferentiated tissues called meristems (meristemos).
  • Primary Growth: Driven by Apical Meristems (meristemos apicales) located at root and shoot tips, lengthening stems and roots.
  • Secondary Growth: Driven by Lateral Meristems (meristemos laterales) in woody plants, increasing girth:
  • Vascular Cambium (Cambium vascular): Adds layers of secondary xylem (wood / madera) internally and secondary phloem externally.
  • Cork Cambium (Cambium del corcho): Replaces epidermis with tough periderm.

• Primary Growth Mechanics:

  • Shoot Growth: Leaves develop from leaf primordia (primordios foliares) beside shoot apical meristem. Axillary buds are suppressed by apical dominance (dominancia apical) exerted by active terminal buds.
  • Root Growth: Root apical meristem is protected by a root cap (cofia/caliptra). Primary growth occurs in three overlapping zones behind root cap:
  1. Zone of Cell Division (Zona de división celular): Contains root apical meristem.
  2. Zone of Elongation (Zona de elongación): Cells lengthen, pushing tip forward.
  3. Zone of Differentiation/Maturation (Zona de diferenciación/maduración): Cells mature into distinct dermal, ground, and vascular tissues.

• Plant Life Cycles and Ecological Feeding Strategies:

  • Annuals (Anuales): Complete life cycle in 1 year or less (e.g., corn, lettuce).
  • Biennials (Bienales): Require 2 years to complete cycle (vegetative growth in year 1, flowering/fruiting in year 2; e.g., carrot, celery).
  • Perennials (Perennes): Live for many years (e.g., mango, guava, oak trees).
  • Carnivorous Plants: Photosynthetic plants that trap and digest small animals/protozoans to extract essential nutrients (especially nitrogen) in nutrient-poor soils.
  • Parasitic Plants: Extract water, minerals, and photosynthetic products directly from host plant vascular systems via specialized absorbing organs called haustoria (haustorios); classified as obligate or facultative parasites.

Gymnosperm and Angiosperm Diversity, Evolution, and Reproduction

• Five Derived Traits of Seed Plants:
  1. Reduced Microscopic Gametophytes: Microscopic male and female gametophytes ($n$) develop within walls of spores retained inside sporophyte tissues ($2n$), protecting gametophytes from environmental stress.
  2. Heterospory: Megasporangia produce megaspores (giving rise to female gametophytes); Microsporangia produce microspores (giving rise to male gametophytes within pollen grains).
  3. Ovules and Egg Production: An ovule (óvulo) consists of a megasporangium ($2n$), megaspore ($n$), and protective integuments ($2n$). Gymnosperm ovules have 1 integument; Angiosperm ovules typically have 2 integuments.
  4. Pollen and Water-Independent Fertilization: Microspores develop into pollen grains (granos de polen). Pollination (polinización) is the transfer of pollen to ovule-bearing structures. Germinated pollen grains form a pollen tube (tubo polínico) that delivers sperm directly to the egg, eliminating the requirement for liquid water.
  5. Seeds as Evolutionary Units: A seed (semilla) consists of a plant embryo ($2n$), a supply of stored nutrient tissue, and a protective seed coat (cubierta protectora $2n$) derived from integuments. Seeds can remain dormant for extended periods and disperse over long distances.

• Gymnosperms ("Naked Seeds"):

  • Seeds are exposed on modified leaves (sporophylls) usually arranged in cones (strobili / estróbilos).
  • Fossil record: Appeared early ($305\,\text{Ma}$) and dominated Mesozoic ecosystems ($251{-}65.5\,\text{Ma}$). Fossil evidence demonstrates insect pollination in gymnosperms dating back over $110\,\text{Ma}$.
  • Phylum Cycadophyta (Cycads):
  • $\approx 340\,\text{species}$, 9 genera. Retain ancestral flagellated sperm. Dioecious (dioicas, separate male and female individual plants).
  • Pollinated by insects via olfactory deception (fragrance production). Highly endangered today (e.g., Zamia portoricensis).
  • Phylum Ginkgophyta:
  • Single extant species: Ginkgo biloba. Retains flagellated sperm. Dioecious; widely cultivated in temperate regions for ornamental and medicinal purposes.
  • Phylum Gnetophyta:
  • Comprises 3 morphologically distinct genera grouped by molecular data: Welwitschia mirabilis (Namib desert, produces 2 giant continuously growing leaves, lives thousands of years, dioecious); Gnetum ($\approx 35\,\text{species}$, tropical trees/lianas with angiosperm-like leaves, dioecious); Ephedra ($\approx 40\,\text{species}$, desert shrubs producing ephedrine, dioecious or rarely monoecious).
  • Phylum Coniferophyta (Conifers):
  • Largest gymnosperm phylum ($68\,\text{genera}$, $\approx 630\,\text{species}$). All species are woody (mostly trees).
  • Leaves are needle- or scale-like with thick cuticles adapted to drought. All species are wind-pollinated. Podocarpus coriaceus is the only conifer species native to Puerto Rico.

• Angiosperms (Phylum Anthophyta):

  • Most diverse and widespread plant group, defined by two major reproductive adaptations: flowers (flores) and fruits (frutos).
  • Flower Anatomy: Specialized shoot bearing 4 whorls of modified leaves (organos florales):
  • Sepals (Sépalos): Outermost non-reproductive whorl, usually green or brown.
  • Petals (Pétalos): Non-reproductive whorl, often brightly colored to attract pollinators.
  • Stamens (Estambres): Male reproductive organs consisting of a filament (filamento) topped by an anther (antera) containing microsporangia.
  • Carpels / Pistil (Carpelos/Pistilo): Female reproductive organs consisting of a sticky stigma (estigma), elongated style (estilo), and swollen ovary (ovario) containing one or more ovules/megasporangia.
  • Floral Classifications:
  • Complete (Completas): Possess all 4 floral organs vs Incomplete (Incompletas): Lacking 1 or more organs.
  • Perfect (Perfectas): Bisexual flowers containing stamens and carpels vs Imperfect (Imperfectas): Unisexual flowers.
  • Actinomorphic (Actinomorfas): Radial symmetry vs Zygomorphic (Zigomorfas): Bilateral symmetry.
  • Fruits:
  • Mature, thickened ovary wall (pericarp / pericarpo) enclosing seeds. Protect seeds and facilitate dispersal via wind, water, gravity, animal ingestion/attachment, or explosive discharge mechanisms.
  • Angiosperm Life Cycle and Double Fertilization:
  • Microspores inside anther microsporangia undergo meiosis to produce pollen grains containing two haploid cells: a generative cell (célula generativa) and a tube cell (célula del tubo).
  • Megaspore inside ovule undergoes meiosis/mitosis to form an 8-nucleate female gametophyte (embryo sac / saco embrionario).
  • Double Fertilization (Doble fecundación): Pollen tube discharges two sperm nuclei into embryo sac:
    • One sperm nucleus ($n$) fertilizes egg ($n$) to form diploid zygote ($2n$).
    • Second sperm nucleus ($n$) fuses with two polar nuclei ($n + n$) in central cell to form triploid endosperm ($3n$) nutrient tissue.

• Coevolution between Plants and Animals:

  • Reciprocal evolutionary influences between angiosperms and animal pollinators/dispersers (coevolution). Bilateral floral symmetry restricts pollinator entrance angles, increasing pollen delivery accuracy, restricting gene flow, and accelerating speciation.

• Angiosperm Diversity:

  • Basal Angiosperms: Oldest surviving lineages, including Amborella trichopoda, Nymphaea (water lilies), and Illicium (star anise).
  • Magnólidas (Magnoliids): Includes magnolias (Magnolia grandiflora), laurels, black pepper, and annonas; closely related to monocots and eudicots.
  • Monocotiledóneas (Monocots): Account for $\frac{1}{4}$ of angiosperms ($\approx 70\,000\,\text{species}$; e.g., orchids, grasses, palms). Characterized by 1 cotyledon, parallel leaf veins, scattered stem vascular bundles, fibrous root systems, monosulcate pollen (1 opening), and floral organs in multiples of 3.
  • Eudicotiledóneas (Eudicots): Account for $\frac{2}{3}$ of angiosperms ($\approx 170\,000\,\text{species}$; e.g., legumes, roses, oaks). Characterized by 2 cotyledons, netlike leaf veins, ring-arranged stem vascular bundles, taproot systems, tricolpate pollen (3 openings), and floral organs in multiples of 4 or 5.

• Human Reliance on Seed Plants:
  • Caloric Reliance: 6 crop species—Wheat (Trigo), Rice (Arroz), Maize (Maíz), Potato (Papa), Cassava (Yuca), and Sweet Potato (Batata)—supply $80\%$ of all human consumed calories.
  • Artificial Selection: Crop traits have been modified through human-directed selective breeding.
  • Secondary Metabolites (Metabolitos secundarios) and wood products derived from seed plants provide essential building materials, fuel, and pharmaceuticals.