10- Flowers and Reproduction

Reproduction

  • Asexual

    • fragmentation

      • most common methods of asexual reproduction

  • Flower Anatomy

    • flowers have

      • carpels: (pistils) that contain one or more ovules

      • stamens: that produce pollinators (male)

      • petals: that attract pollinators

      • sepals: protect the flower

    • a flower is a stem with leaf-like structures

    • complete flowers have all four floral appendages: sepals, petals, stamens, carpels

    • incomplete flowers lack at least one appendage

    • sepals

      • outermost floral appendage

        • modified leaves that surround maturing flower parts

        • protect the flower buds as it develops

        • may be colorful

        • sepals together are referred to as the calyx

    • petals

      • located above the sepals on the receptacle

        • leaf-like but contain pigments other than chlorophyll

        • attract pollinators

        • absent in wind-pollinated species

    • stamens

      • occur above the petals

        • known as the androecium

        • two parts: anther and its supporting filament

        • diploid anther cells (microsporocytes) undergo meiosis to produce four microspores

        • microspores form a resistant cell wall and become pollen

    • carpels

      • collectively are the gynoecium

        • stigma: catches the pollen grains

        • style: elevates the stigma

        • ovary: where the megaspore are produced

      • within the ovary are placentae, bearing small structures called ovules

    • the ovules have a central mass of parenchyma called nucellus

      • there are megasporocyte undergoes meiosis to produce four megaspores (three degenerate)

    • ovule develops into seed after it is fertilized

      • surrounding ovary develops into a fruit

  • Evolution of Carpels

    • most obvious conversion of gymnospermous sporophylls into stamens and carpels, resulting in the formation of flowers

      • in most angiosperms carpels, the edges of the sporophyll primordia crowd against each other and grow shut, sometimes closing completely that no sign of a seam remains (closed carpel)

  • Double Fertilization

    • in the transition from gymnosperms to angiosperms, fertilization evolved such that the second sperm cell of the pollen tube fuses with the polar nuclei of the megagametophyte

      • produces the endosperms nucleus

  • Gametophyte

    • microgametophytes develop from microspores by mitosis

      • in angiosperms, the microgametophyte consist of at most three cells

      • located within the original pollen cell wall

    • microspore nucleus divides into:

      • large vegetative cell

      • small generative cell, which divides to form two sperm cells

    • pollen lands on stigma and germinates

      • produces a pollen tube that penetrates into the stigma and makes its way to the ovule

    • a megametophyte is produces through division of the megaspore nucleus

      • it divides to form eight cells, the embryo sac

      • most nuclei migrate to opposite ends

      • walls form around the nuclei

    • the large eight-nucleate megaspore becomes a megagametophyte with seven cells

      • one is binucleate

      • the seven cells are:

        • one large central cell with two polar nuclei

        • three small antipodal cell

        • an egg apparatus consisting of two synergids and an egg

  • Fertilization

    • the pollen tube

      • penetrates the nucellus

      • reaches the egg apparatus

      • enters one synergrid

    • the two sperm are released

      • one sperm moves through the synergid, loses it protoplasm along the way, to fuse with the egg nucleus

    • in angiosperms the second sperm nucleus migrates into the central cell

      • undergoes karyogamy with both polar nuclei

      • establishes a large triploid endosperm nucleus

    • called double fertilization because both sperm nuclei undergo fusions

    • endosperm nucleus undergoes rapid mitosis

  • Embryo and Seed Development

    • the zygote grows and forms a short stalk like suspensor

      • pushes embryo deep into the endosperm

    • division of the zygote form the embryo

      • develops into one or two cotyledons

    • the embryo elongates and a short axis is established

      • radicle: embryonic root

      • epicotyl: embryonic stem

      • hypocotyl: root/shoot junction

    • cotyledons store nutrients during and after germination

    • in monocots the endosperms remains in the mature seed

  • Seed Development and Cross Pollination

    • as the ovule develops into a seed the ovary matures into a fruit

    • three layers form

      • exocarp: outer layer usually skin or peel

      • mesocarp: middle layer usually flesh

      • endocarp: innermost layer can be tough or thin

    • entire fruit wall is pericarp

    • cross pollination: by pollen from a different individual

    • self pollination: by pollen from the same flower or another on the same plant

    • results of each are similar to those of sexual vs asexual reproduction

    • mechanisms have evolved to

      • decrease the probability of self pollination

      • increase the chances of cross pollination

    • self pollinating may be inhibited by compatibility barriers

    • monoecious and dioecious species

      • imperfect flowers: lack either stamens and carpels

      • perfect flower: has both stamens and carpels

      • dioecious: some produce only staminate flowers and others produce only carpellate flowers

        • ensures cross pollination

      • monoecious: located on the same plant as carpellate flowers

    • coevolution

      • a flower adapts for visitation by a particular animal and the animal for efficient exploitation of the flower

      • most flowers are radically symmetrical

  • Pollination

    • wind pollinated usually have no petals and reduced or absent sepals

    • inflorescence: smaller flowers with fewer ovules may grouped together

      • determinant: apex is converted to a flower which tends to open before the lower flowers

      • interdeterminate: lowest or outermost flowers open first and new flowers are being initiated at the apex

  • Fruit Types and Seed Dispersal

    • fruit are adaptations that result in the protection and distribution of seeds

    • tough fruits are well-protected but expensive to produce

      • seed must still be able to break out to germinate

      • if the animals are to disperse seeds, the part of the fruit must be edible or otherwise attractive

      • seed and embryo must be protected from consumption

    • true fruits

      • containing only ovarian fruits

    • accessory (false) fruits

      • non-ovarian tissue is present

    • simple fruit

      • develops from a single ovary

    • aggregate fruit

      • separate carpals of one gynoecium fuse

    • multiple fruit

      • individual fruits of an inflorescence fuse

    • dry fruit: inedible to animals

      • dehiscent fruit: break open releasing the seeds

      • indehiscent fruit: fruits do not

    • fleshy fruit: eaten to distribute fruits

    • in grasses, seed and fruit are fused together

      • there is little protection and no attraction for animals

    • fruits of beans and peas are called legumes

      • form from a single carpel

    • wind-dispersed seeds are lightweight

      • they often have wings or parachutes that carry then in wind

    • fruits and seeds that are transported by water must:

      • be buoyant and resist mildew and rot

    • animals often disperse seed by interacting with the fruit

    • pomes (apples and pears) develop from inferior ovaries and are accessory fruits

    • drupes (peaches and cherries) provide maximum attraction to animals with minimum danger to the seed