Sexual Reproduction: Plants and Fungi

Textbook Chapter: 25Plant Taxonomy and ReproductionPlant Adaptations for LandPlants evolved from aquatic photosynthetic protists, particularly from green algae, phylum ChlorophytaThe water provided as a means of nutrients and dispersal of sex cellsOn land, plants had to adapt to survive and reproduce:Plants developed cuticles and stomata to maintain moisture and water and keep from drying outPlants developed roots and stomata to obtain resources from the soil and the airPlants developed xylem and phloem for transportPlants developed dry-resistant spores and seeds for the dispersal of gametes without water

Plant PhylaPlants are classified based on whether or not they have:Vascular systemSeedsFlowersBryophytes lack vascular systemsTracheophytes have vascular systemsFerns: seedlessGymnosperms: seed non-floweringAngiosperms: seed flowering

PlantsSeededSeedlessTracheophytesBryophytesAngiospermsGymnospermsBryophytesBryophytes are non-vascular plantsThey do not contain seedsAre the most primitive plantsOnly found in moist and shady environmentsDue to lack of transport system, they are very small material exchange must occur by diffusion in moist environmentsNeeds water for reproductionNo true roots, stems, or leavesReproduces using water-proof spores

TracheophytesTracheophytes are vascular plants and contain two types of vascular tissue:Xylem: transports waterPhloem: transports food and nutrientsGrew tall due to presence of vascular systemHave specialized organs: roots, stems, leavesPteridophytes (ferns) are seedless vascular plantsGrow in moist, shady environmentsHave large leaves called frondsReproduce using spores

Seed-bearing TracheophytesDeveloped reproductive methods that do not require water, which is the seedA seed containsFully developed embryoFood supply for the embryo (cotyledon)Water-proof and dry-resistant seed coatSeed bearing structures are either cones or flowersGymnosperms and AngiospermsGymnosperms are naked seedsHave leaves called needles or scalesLeaves are designed to reduce water lost and prevent freezingMale cones produce pollen and female cones produce eggsAngiosperms are enclosed seedsHave flowers that encourage pollen transferThe flowers contain the female and male reproductive systemsEggs are ovules inside the ovary, sperm are pollens on the anthers

Angiosperms: FlowersThe identifying characteristic of angiosperms are flowersFlowers function to attract pollinators and contain the reproductive structuresA flower is a set of modified leavesThe petals are usually colorful and attract pollinatorsThe sepals are below the petals and encase the flower before it openedThe stamens are the male reproductive organs:The filament is the stalk and the anther is where the pollen is produced and is foundThe carpel is the female reproductive organ:The stigma receives pollenThe pollen tube leads the pollen down the style and into the ovaryThe ovary contains the ovules, which are the egg cells

Angiosperms: Monocots and DicotsAngiosperms can also be classified based on their number of cotyledons, monocot (one) or dicot (two)Monocot characteristics:Parallel veinsVascular bundles scatteredFloral parts in multiples of threeFibrous root systemDicot characteristics:Branched veinsVascular bundles ringedFloral parts in multiples of four or fiveTaproot system

Asexual ReproductionPlants can sometimes go through asexual reproductionVegetative clones are genetically identical plants the parent that grow from a detached part of the parent plantSome plants can produce genetically identical diploid spores that grow into genetically identical plantsSome plants grow runners they stem away from the parent and grow into a genetically identical plantSexual Reproduction of PlantsMost sexually-reproducing unicellular organisms spend most of their life cycles as haploidsPlants usually reproduce sexually, while fungi can reproduce both sexually and asexuallyPlants have alternation of generations, in which they spend parts of their life cycles as haploid gametophytes and part of the life cycles as diploid sporophytesThe identification of the alternation of generations is more apparent in bryophytes and seed-less tracheophytes than in seeded tracheophytes

Sporophytes and GametophytesThe sporophyte and gametophyte generations are both usually multicellular in multicellular plants, although the gametophyte body is usually significantly smallerIn bryophytes and seedless plants, the sporophytes and gametophytes are clearly distinguishable as two different plant bodiesBryophytes and seedless plants either spend most of their time in the gametophyte generation or equally between the twoIn seed plants, the sporophyte is the dominant generation, and is most of the plant we seeThe gametophytes in seed plants are the pollen and the ovulesSporophytes produce haploid spores through meiosis to form gametophytesGametophytes produce haploid gametes that fuse to form sporophytesLife Cycle of Non-Flowering PlantsBryophytes, seedless tracheophytes, and non-flowering plants spend about half of their life cycles as haploids and half as diploidsMulticellular diploid sporophyte undergoes meiotic cell division to produce haploid sporesThe haploid cells disperse, undergo mitotic division, differentiation, and growth to form a haploid gametophyteThe haploid gametophyte undergoes the specialization of certain cells to produce sperm and egg gametesSperm and egg fertilize to form a diploid zygoteIn seedless plants, gametes must travel through water in order to fuse, but in non-flowering seed plants, the seed offers protectionThe zygote goes through mitotic cell division, differentiation, and growth, to become a diploid adult sporophyte

Life Cycle of AngiospermsAngiosperms spend most of their life cycles in the sporophyte generationThe multicellular diploid flower produces haploid spores in the anthers and the ovules, which develop into pollen and eggs, which are the male and female gametophytes, respectivelyPollination occurs: pollen lands on the stigma of a flower, either the same one, or differentA pollen tube forms which conducts the sperm in the pollen down into the ovaryFertilization occurs, and diploid seeds with endosperms are formedThe ovary matures into fruits, which aid in seed dispersalThe seed germinates and grows into a diploid plant

Male Gametophyte Development in AngiospermsMale gametophytes begin development in the diploid sporangia of the antherThe sporangia goes through meiosis to form four haploid microsporesEach microspore divides once by mitosis to produce one generative cell and one tube cellA pair of generative cell and tube cell is encased in a thick, resistant wall, producing the pollen grainWhen the pollen grain lands on the stigma, the tube cell forms the pollen tubeThe generative cell divides by mitosis to form two sperm cellsThe sperm cells travel down the pollen tube into the ovary, where they will go through fertilization

Female Gametophyte Development in AngiospermsOne cell in the sporangium of each ovule goes through meiosis to form four haploid megasporesOnly one of the four megaspores surviveThis megaspore goes through mitosis three times, resulting in one cell with eight haploid nucleiTwo nuclei partition and form the synergid cells, which guide the pollen tube into the ovuleThree nuclei partition are called antipodal cells and have unknown functionThe other two nuclei for the polar nuclei, which share the same cytoplasm as the egg cellThe last nuclei forms the egg cell

Angiosperm Double FertilizationAfter the pollen grain lands on the stigma, the pollen tube forms and the two sperm cells are directed down into the ovaries by Ca2+In the ovaries, one sperm fuses with the egg to form the zygote, which will develop into a multicellular diploid plantThe other sperm fuses with the two polar bodies, forming a triploid cellThe triploid cell will eventually form the endosperm, which helps the seed store food for the plant embryoThis process is called double fertilization

Embryo Growth and Seed FormationAfter fertilization, the ovule develops into the seed, and the ovary develops into the fruit, containing multiple seedsAs the embryo develops, the seed starts obtaining large amounts of nutrientsThese nutrients are initially stored in the endospermAs the embryo continues to divide and grow, the cotyledons start to formAs the cotyledons grow larger, they become the primary source of nutrients for the embryo, rather than the endospermAs the cotyledons appear, the embryo elongatesThe seed stops growing when the protective seed coat developsFruitAs the seeds are developing, the ovary of the flower is developing into a fruitThe fruits main function is to aid in seed dispersal and to protect the enclosed seedsFruits are rich in sugars, which attract herbivores to eat them, who will then pass the seeds to a new are as undigested materialFruits respond to ethylene, which cause them to ripen and drop, which will also allow for seed dispersal by other methods

Seed DormancyAfter a seed matures, it is dormantA dormant seed has extremely low metabolic ratesA seed breaks dormancy and germinates into a seedling when certain conditions are met, which may or may not include:Moist and warm environment alwaysPlentiful rainAfter natural firesAfter prolonged coldAfter chemical attack through digestive systemSeed dormancy is important because it ensures that the seed will germinate at a time and place most advantageous for the seedling, preferably away from the parent and away from competitionGerminationGermination is different in monocots and dicotsIn monocots, the shoot tip breaks out of the seed and pushes upwards, towards the surfaceThe coleoptile protects the shoot tip as it pushes through the abrasive soilIn dicots, the epicotyl or the hypocotyl forms a hook, which pushes out of the groundThe hook eventually straightens out on the surface