NOTES FOR BIOLOGY 1002
SECTIONS 004, 005, 006
Spring 2006
DR. STEVEN POMARICO
CHAPTER 31
PLANT REPRODUCTION
Coevolution of Flowers and Pollinators.
Wind pollinated flowers are inconspicuous and unscented.
This group produces LOTS of pollen because it’s a low probability of a hit for an individual pollen grain (oaks and maples).
Animal pollinated flowers have several modifications in order to
Attract animal pollinators.
Frustrate undesirable visitors.
Ensure cross-fertilization.
Bee pollinated flowers
Brightly colored (white, yellow, or blue) with ultraviolet pattens (See fig 31.2)
These flower are tubular and produce their nectar (the food enticement) at the bottom of a short tube.
Butterfly pollinated flowers
Also brightly colored with have long tubular flowers.
Moth pollinated flowers
Light colored and sweet smelling so they’re easier to find in the dark.
Typically night flowering plants.
Fly pollinated flowers
Smell like rotting flesh or dung
Hummingbird pollinated flowers
Very deep tubular flowers of red or orange colors that produce large amounts of nectar.
These flowers usually lack fragrance (birds attracted by color not scent) and don’t have a landing zone.
Copulatory pollinated flowers
Sex is the enticement.
Some orchids mimic female wasps in scent and shape, attracting males that attempt to copulate with the flower picking up pollen in the process.
Plant Life Cycles
Alternation of generations revisited (See fig 31.3)
2 distinct multicellular adult forms
sporophyte: diploid plant which produces haploid spores
gametophyte: haploid plant which produces gametes
Two types of spores are seen in the flowering plants. They are heterosporous
Megaspores - gives rise to the female gametophyte
Microspores - gives rise to the male gametophyte
The two different gametophytes produce two types of gametes
female gametophyte is an embryo sac containing an egg cell
male gametophyte is a pollen grain (sperm)
After fertilization the zygote develops into the seed (plant embryo), which will grow into a new sporophyte.
The Flower Structure
The parts of the flower evolved from leaves.
Complete flowers have 4 main parts (See fig. 31.3)
-Sepals
-Petals
-Stamens
-Carpel
The stamens are the male reproductive part and consist of the filament (or stalk), which hold the anther.
The anther contains the pollen grains (male gametophyte)
The carpel is the female reproductive part and is a vase shaped structure made up of the stigma, the style, and the ovary.
The stigma is the top of the vase. It is a sticky structure designed to catch the pollen.
The style is the neck of the vase. The pollen must “grow” through the style to reach the egg.
The ovary is the bottom of the vase. The ovary contains the ovules, which become seeds. The outer layers of the ovary become the fruit.
Incomplete flowers lack one or more of the 4 floral parts.
Gametophyte Development in Flowering Plants
The gametophytes are haploid and develop within sporophyte flowers
They are very small compared to the sporophyte and cannot live on their own
Pollen develops within the pollen sacs of anthers (See fig 31.6)
1) Microspore mother cell develops inside the pollen sacs
2) Diploid microspore mother cells divide by meiosis to form 4 haploid microspores from each microspore mother cell.
3) Each microspore divides by mitosis to make an immature pollen grain containing:
a. tube cell.
b. generative cell (inside the tube cell)
4) The generative cell goes through mitosis to form two sperm cells
This is now the mature pollen grain
Embryo sac development (See fig 31.6)
The outer layer of the ovule is the integument
The tissue of the ovule is diploid.
The series of events leading to the female gametophyte are
1) The megaspore mother cell develops within the ovule that is within the
ovary of the carpel.
2) The megaspore mother cell divides by meiosis to form four haploid megaspores
3) Three of the four megaspores degrade.
4) The remaining one of the four goes through 3 rounds of mitosis BUT NOT CYTOKINESIS which results in 8 nuclei in one cell.
5) The nuclei are distributed 3 on each end of the megaspore and 2 in the center.
At this point cytokinesis occurs forming 7 (NOT 8) cells.
-3 cells on each end of the embryo sac (one is the egg)
-one larger central cell with 2 polar nuclei
The larger center cell becomes the primary endosperm cell.
Pollination and Fertilization (See fig 31.6)
1) Pollination starts when pollen from an anther lands on a stigma
2) The pollen grain grow a tube down through the style towards the ovary
3) The 2 sperm cells from the generative cell move down the tube to the ovary where a double fertilization occurs
Double Fertilization
One sperm fuses with the egg cell to form the zygote
The other sperm fuses with the polar nuclei in the primary endosperm cell making this triploid endosperm (3 sets of chromosomes) tissue
Development of Seeds and Fruits
The seed develops from the ovule
The integuments become the seed coat
The zygote becomes the embryo.
The primary endosperm becomes the endosperm that acts as food for the new plant.
The walls of the ovary turn into the flesh of the fruit.
The main function of the fruit is to aid in seed dispersal
-wind dispersal (See fig 31.9)
-mechanical dispersal (see fig 24-23)
-water dispersal
-animal dispersal
Sexual versus Asexual Reproduction
Many plant species can reproduce both sexually and asexually.
Asexual reproduction can occur via fragmentation
Fragmentation can be an artificial process (cuttings) or a natural process.
The parent plant sends out horizontal stems (runners) that can lead to completely new plants
Asexual reproduction is natural cloning where all the offspring are genetically identical to the parent plant.
Sexual offspring combine the genes from two parents and are genetically different from their parents