Sexual Reproduction
- Life Cycle of Angiosperms
- The flower - 4 modified whorls of leaves
- Variations in flower structures
- Sexual Activity in Flowers
- Development of the Embryo and Seed
- Seed Dormancy
- Seed Dispersal
- Vegetative Propagation
- Apomixis
Sporic cycle (dominant generation is the sporophyte)
Flowers are the reproductive organs of the angiosperms. A flower is the plant organ where sex occurs, seeds are formed and fruit develops. Many flowers have evolved to attract a particular type of pollinator which explains why they have such a variety of colors and fragrances.
The flower is the angiosperms evolutionary solution to reproduction in dry conditions. No external water is necessary for the transfer of the male gamete to the female gamete. In both angiosperms and gymnosperms this transfer or pollination is accomplished by wind or insects.
Flowers arise from a modified stem called the receptacle. Four types of modified leaves, are arranged in whorls or spirals from the receptacle.
The first group called the calyx is made of sepals. Sepals are usually green and rather tough to protect the more delicate parts of the flower bud.The second whorl is the corolla, composed of 3 or more petals. Petals are frequently colorful and fragrant. They also may have nectaries, glands which produce a sugary solution which is converted into honey by bees.
The sexual flower parts are stamens and carpels The male part of a flower is called the androecium. It is made of several stamens. Each stamen has an anther, balanced atop a filament. When the anther matures it splits open to release thousands of male gametophytes -- the pollen.
Cross Section of the Anther
The innermost whorl of modified leaves is the gynoecium. It consists of several carpels often fused to form a pistil. Each carpel contains at least one ovary.
Parts of a typical complete flower
- gynoecium -sum of carpels (made of) stigma, style, ovary
- androecium -sum of stamen (made of) filament anther
- corolla -sum of petals
- calyx -sum of sepals
- receptacle -base of modified leaf whorls
Incomplete flowers can be perfect, but perfect flowers cannot be complete.
Flowers may have radial symmetry (considered a primitive trait) or they can have bilateral symmetry. The bilateral symmetry --as found in orchids-- is designed to fit a specific pollinator (bee or wasp) like a key to a lock preventing the plant from self pollinating.
Some flowers like the daisy or sunflower are many individual flowers all crowded closely together. This is called an inflorescence.
Flowers are part of the sporophyte generation in angiosperms. It's the flower's job to produce the male and female gametophytes.
- male gametophyte 2 cells (pollen grain)
- female gametophyte 7 cells (embryo sac)
Male spore development begins with:microsporangia or microspore mother cell
- is diploid
- is found in a pollen sac of the anther
- undergoes meiosis
- and produces 4 microspores (haploid)
- each microspore undergoes mitosis to produce
- microgametophytes (pollen grains each with two cells)
The pollen grain has a hard bumpy outer cell wall containing 2 cells, a tube cell and a generative cell (suspended in the tube cell)
A pollen Grain
Female egg development begins with:
megasporangia or megaspore mother cell
- is diploid
- is found in the ovary
- undergoes meiosis
- and produces 1 megaspore (the other 3 die)
- the megaspore undergoes mitosis 3 times to produce the
- megagametophyte (Embryo sac composed of 8 nuclei and 7 cells)
Embryo sac or megagametophyte
Once fertilized the embryo sac or ovule composed of nucellus, integuments, and embryo will develop into the seed. The ovary --tissue immediately surround the seed or seeds-- will develop into a fruit.
Pollination is the process of moving pollen (male gametophyte) from the anther to a receptive stigma.
If the pollen and stigma are from the same flower it is self pollination. Usually cross pollination occurs because it insures variation in the offspring, a distinct evolutionary advantage.
Since self pollination is not advantageous a number of factors help prevent it such as:
Flowers have coevolved with insects for millions of years so many characteristics of flowers -- shape, color, nectar, and odor -- are designed to attract pollinators (not to be beautiful in the eyes of humans).
The variety of pollinators attracted to flowers includes: bats and moths (night), and butterflies, beetles, and flies (day).
A high degree of specialization frequently occurs between a specific flowering plant and its pollinators. In extreme cases a flower may be pollinated by a single species of insect or bird. Certain orchids have evolved to look like and even smell like a female wasp. The male wasps frantically compete for this female simulacrum and in so doing pollinates each flower it visits.Another example of a very close mutualistic symbiosis occurs between the yucca flower and the pronuba moth. Neither can survive without the other.
Grasses and gymnosperms --plants which lack attractive flowers-- make millions of pollen grains which --when released-- are light enough to float on a breeze from plant to plant. Only one pollen grain in a thousand may land in an appropriate place.
Wind pollination is a testament to how far angiosperms and gymnosperms have gone in reducing their reliance on external moisture for reproduction. It is also important to note how many pollen grains end up unfulfilled and buried in sediment.
Since their hard outer wall can be fossilized pollen gives the paleontologist an important window into the environmental conditions of ancient ecosystems.
If you wish to explore pollen tube formation follow the directions in this lab exercise.
Fertilization can not occur until the pollen grain germinates and sends a pollen tube through the style and into the egg sac. (through an opening called the micropyle
The growth of the pollen tube requires proteolytic enzymes that can digest the sporophyte tissue just ahead of the lengthening tube.
The generative cell within the pollen grain undergoes mitosis to produce two sperm cells. These cells follow the tube into the egg sac where one sperm unites with the egg to form a zygote while the other sperm joins the 2 nuclei in the central cell to form the triploid tissue called endosperm.
After fertilization the diploid zygote, endosperm nucleus, and associated maternal tissues begin mitosis and differentiation. The result is the plant embryo encased in a structure called a seed
At this stage much of the embryo is hypocotyl. An embryonic root or radicle that contains several primary tissues (procambium, protoderm, and ground meristem) is well developed. A shoot apical meristem has formed just above the cotyledons.
A Seed is made up of three parts
