Plant Reproduction and Development Chapter 38 Plant Reproduction and Development

Alternation of Generations Angiosperms and other plants exhibit alternation of generations: haploid (n) and diploid (2n) generations take turns producing each other Sporophyte: diploid plant that produces haploid spores by meiosis Gametophyte: haploid plant that produces gametes Reproduction and Development

Alternation of Generations Fertilization results in diploid zygotes, which divide by mitosis and form new sporophytes Sporophyte dominant in angiosperms Evolutionary history has reduced gametophytes in angiosperms to only a few cells, not an entire plant Reproduction and Development

Reproduction and Development

Reproduction and Development Flowers Angiosperm sporophytes produce unique reproductive structures called flowers Flowers consist of four types of highly modified leaves Sepals Petals Stamen Pistil (or carpel) Their site of attachment to the stem is the receptacle Reproduction and Development

Reproduction and Development Flower Structure Reproduction and Development

Reproduction and Development Flower Anatomy Sepals and petals are nonreproductive organs Sepals – protect the other three, the floral bud Petals – attract pollinators and act as “landing pads” Reproduction and Development

Reproduction and Development Flower Anatomy Stamen and carpels are male and female reproductive organs, respectively Stamen – consists of filament (long, thin) and anther (pollen) Carpel – consists of stigma (sticky opening), style (long tube connecting stigma to ovary), ovary (houses ovules; becomes fruit), and ovules (develops female gametes; become seeds) Reproduction and Development

Reproduction and Development Flower Anatomy Complete flowers – have all four floral organs Ex: Trillium Incomplete flowers – missing one or more of the four floral organs Reproduction and Development

Reproduction and Development Flower Anatomy Bisexual flower (perfect flower) is equipped with both stamens and carpals All complete and many incomplete flowers are bisexual A unisexual flower is missing either stamens (carpellate flower) or carpels (staminate flower) Reproduction and Development

Reproduction and Development Unisexual Flowers Monoecious plants: staminate and carpellate flowers at separate locations on the same individual plant Ex: corn ears derived from clusters of carpellate flowers; tassels consist of staminate flowers Reproduction and Development

Reproduction and Development Unisexual Flowers Dioecious plants: staminate and carpellate flowers on separate plants Ex: Date palms and Sagittaria (below) have carpellate individuals that produce dates and staminate individuals that produce pollen Reproduction and Development

Reproduction and Development Gamete Formation Development of angiosperm gametophytes involves meiosis and mitosis Reproduction and Development

Reproduction and Development Gamete Formation The male gametophytes are sperm-producing structures called pollen grains, which form within the pollen sacs of anthers The female gametophytes are egg-producing structures called embryo sacs, which form within the ovules in ovaries Reproduction and Development

Reproduction and Development Male Gamete Formation The male gametophyte begins development within the sporangia (pollen sacs) of the anther Within the sporangia are microsporocytes, each of which will from four haploid microspores through meiosis Each microspore can eventually give rise to a haploid male gametophyte Reproduction and Development

Reproduction and Development Male Gamete Formation A microspore divides once by mitosis and produces a generative cell and a tube cell Generative cell will eventually form sperm Tube cell, enclosing the generative cell, produces the pollen tube; delivers sperm to egg Reproduction and Development

Reproduction and Development Male Gamete Formation This two-celled structure (generative and tube cells) is encased in a thick, ornate, distinctive, and resistant wall: a pollen grain; an immature male gametophyte Reproduction and Development

Female Gamete Formation Ovules, each containing a single sporangium, form within the chambers of the ovary One cell in the sporangium of each ovule, the megasporocyte, grows and then goes through meiosis, producing four haploid megaspores In many angiosperms, only one megaspore survives Reproduction and Development

Female Gamete Formation This megaspore divides by mitosis three times, resulting in one cell with eight haploid nuclei Membranes partition this mass into a multicellular female gametophyte – the egg sac Reproduction and Development

Female Gamete Formation At one end of the egg sac, two synergid cells flank the egg cell Synergids attract and guide the pollen tube formation At the other end of the egg sac are three antipodal cells – no idea what they do Reproduction and Development

Female Gamete Formation The other two nuclei, the polar nuclei, share the cytoplasm of the large central cell of the embryo sac The ovule now consists of the embryo sac and the surrounding integuments (from the sporophyte) Reproduction and Development

Angiosperm Pollination The successful transfer of pollen from anther to stigma NOT fertilization: fusion of gametes Pollination leads to fertilization Cross-pollination vs. self-pollination Most angiosperms are pollinated by insects, birds, and mammals (vectors) that reward the species with food in the form of nectar Some are pollinated by wind (corn, wheat) and have small, plain, non-fragrant flowers Reproduction and Development

Angiosperm Pollination Fragrance, pattern, and colors are designed to attract the vector so it will pick up pollen and bring it to the next flower Some vectors get “tricked” Orchid flowers resemble female wasps; males attempt copulation; the more orchids the wasps “mate” with, the more pollination occurs Good example of coevolution Reproduction and Development

Reproduction and Development

Reproduction and Development

Reproduction and Development

Reproduction and Development Animal Pollinators The Scottish broom flower has a tripping mechanism that arches the stamens over the bee and dusts it with pollen, some of which will rub off onto the stigma of the next flower the bee visits Reproduction and Development

Reproduction and Development Double Pollination After pollen grain lands on stigma, the generative cell divides by mitosis into two haploid sperm cells 1 sperm fertilizes egg; forms the zygote (2n) 1 sperm fertilizes polar nuclei; forms endosperm (3n) Reproduction and Development

Reproduction and Development Double Pollination Double fertilization ensures that the endosperm will develop only in ovules where the egg has been fertilized. This prevents angiosperms from squandering nutrients in eggs that lack an embryo Reproduction and Development

Reproduction and Development Seeds After double fertilization, the embryo develops to a point and then enters a dormancy period During this time, the embryo is housed in a tough, protective coating – seed coat It will remain as the seed until germination, usually brought about by the absorption of water Seeds allow parent plants to disperse offspring and wait until environmental conditions are favorable for growth Reproduction and Development

Reproduction and Development Seeds In bean seeds (dicot), the embryo consists of an long structure, the embryonic axis, attached to cotyledons Below the point at which the cotyledons are attached, the embryonic axis is called the hypocotyl; above it is the epicotyl Tip of the epicotyl is the plumule:shoot tip with a pair of mini leaves End of the hypocotyl is the radicle, or embryonic root Reproduction and Development

Reproduction and Development Seeds Monocots have a single cotyledon called a scutellum Embryo of a grass seed is enclosed by two sheaths, a coleorhiza, which covers the young root, and a coleoptile, which cover the young shoot Reproduction and Development

Reproduction and Development Fruits Develop due to hormonal changes after fertilization Usually develop only after fertilization Designed to protect the seeds and aid in seed dispersal by wind or animals Reproduction and Development

Reproduction and Development Fruits Fruits are simply any structure related to or resulting from the ovary of a flower (Yes! That includes many of the common “vegetables”) Reproduction and Development

Reproduction and Development Seed Dispersal Fruits aid in seed dispersal based on how the fruits develop Lightweight fruits allow wind dispersal Dandelions and Maples Reproduction and Development

Reproduction and Development Seed Dispersal Floating fruits allow water dispersal Coconuts Reproduction and Development

Reproduction and Development Seed Dispersal Clingy fruits allow animal dispersal Fruits “grab” the animal (cockleburs, “jumping” cholla) Reproduction and Development

Reproduction and Development Seed Dispersal Tasty fruits allow animal dispersal Fruits entice the animal to eat it (mistletoe and birds) Animals eat the fruit and deposit the seeds (in a nice pile of fertilizer) in new places Why are unripe fruits bitter? Reproduction and Development

Reproduction and Development Seed Dispersal Explosive seed pods allow dispersal by the plant itself Impatients – get their name from their behavior Reproduction and Development