Sexual Reproduction in Plants (Lecture 5)
Learning objectives
1.Key terms to know regarding Reproduction
2.Sexual reproduction in flowering plants
3.Chromosome number in sexual/asexual reproduction
4.Define Mitosis & Meiosis
5.Comparison chart of mitosis and meiosis
6.Meiosis and genetic diversity (Plants can reproduce using sexual reproduction/meiosis, which produces new varieties)
7.Alternation of generation
8.Compare reproduction by seed and by vegetative reproduction
Terms to know:
- Haploid: having a single set of chromosomes in each cell.
- Diploid: having two sets of chromosomes in each cell.
- Mitosis: cell division, which produces two genetically identical cells.
- Meiosis: reduction division, which produces four haploid reproductive cells.
Sexual reproduction
In sexual reproduction, development of new individual take place by the fusion of sex cells called male and female gametes. It is the most common type of reproduction among plants and animals.
There are two types of reproduction.
- i) Isogamy
- ii) Heterogamy
i)Isogamy:
Union of two similar gametes which can not be distinguished
into male and female gametes is called isogamy.
Fusion of such gametes is called conjugation.
It is observed in lower plants like mucor and spirogyra
ii) Heterogamy:
It is the union of two dissimilar gametes i.e. male and female gametes is known as fertilization and the zygote is called Oospore. This type of reproduction is common in flowering plants and is further divided into a) Apomixis b) Amphimixis
a)Apomixis:
It is an abnormal sexual reproduction in which embryo develops from the egg cell without fertilization and with or without meiosis. It is of various types;
- Parthenogenesis
- Apogamy
- Apospory
i)Parthenogenesis:
In this case, embryo develops directly from the egg cell or male gamete without fertilization. It gives haploid plants.
ii) Apogamy:
In this case, embryo develops directly from haploid nuclei other than egg cell i.e. it develops from synergids or antipodal cells of the embryo sac.
iii) Apospory:
In this case, embryo develops directly from the somatic cell i.e. it develops from integuments of nucleus.
b) Amphimixis:
This is normal sexual reproduction in which embryo develops from union of male and female gametes in plants.
The process of male gamete formation is known as microsporogenesis and female gamete formation as megasporogenesis in plants.
Male gametophyte or Microsporogenesis:
Female gametophyte or Megasporogenesis:
The female gametes, or eggs, are formed in the ovules, within the flower. Here, as above, a certain diploid cell undergoes meiosis; however, in this case the result is four haploid megaspores—”large spores”). Three of the megaspores die, and the remaining one undergoes mitosis several times. One of these daughter cells then functions as an egg, ready to be united with a sperm.
Gametogenesis: Female
Fertilization:
1.A grain of pollen, made up of two cells—the tube cell and the generative cell—is transferred to the female stigma.
(The pollen might be transferred by a bee, by the wind, or by some other method.)
- The tube cell germinates and grows into a pollen tube; this is a long, hollow tube that penetrates the pistil’s tissues in search of the egg.
- The generative cell divides to become two sperm cells. These two cells travel through the pollen tube and enter the ovule, which contains the egg.
- One sperm fuses with the egg to create a single diploid cell, the zygote. The zygote cell then divides, growing into the multi-celled embryo.
- The other sperm combines with the two polar nuclei, forming the triploid endosperm, a temporary food storage tissue that nourishes the developing embryo.
Seeds and Fruit:
- Successful fertilization transforms the ovule into a tiny embryo and some nourishing endosperm tissue. This little package—ovule with embryo and endosperm is now called a seed. As the seed ripens, the embryo matures. Eventually the seed develops a hard, protective seed coat.
- The transformation of ovule to seed takes place within the ovary which is now called a fruit. Botanically speaking, a fruit is a mature ovary; it is a vessel containing and protecting the plant’s developing seeds.
Once pollination occurs a tube grows from the pollen grain down through the style to the ovule
Double Fertilization:
Learning Objective 3 & 4
Mitosis & Meiosis and Comparison chart of Mitosis & Meiosis:
Cells of eukaryotes, including those of animals, plants, fungi, and protists, divide by one of two methods: mitosis or meiosis.
- Mitosis is a process of cell division that results in two genetically identical daughter cells developing from a single parent cell.
- Meiosis, on the other hand, is the division of a cell involving two fissions of the nucleus and giving rise to four gametes, or sex cells, each possessing half the number of chromosomes of the original cell.
In fungi and simple plants, meiosis produces different types of haploid cells called spores.
Most spores can develop into haploid organisms without fertilization.
Learning Objective 5
Meiosis and genetic diversity
(Plants can reproduce using sexual reproduction/meiosis, which produces new varieties)
Learning Objective 6
Alternation of generation:
- The genetic variation introduced by sexual reproduction has a significant impact on the ability of species to survive and adapt to the environment.
- Alternation of generations allows sexual reproduction to occur,
without changing the chromosome number characterizing the species.
Learning Objective 7
Compare reproduction by seed (Sexual) and by vegetative reproduction (Asexual)
| Sexual (seed) | Asexual (vegetative) |
| Cross pollination ensures variation (allows evolution) | No variations – can be advantage in commercial horticulture |
| More resistant to disease | All plants are of same species susceptible to disease |
| Seeds can remain dormant and survive unfavourable conditions | No seeds formed – no dormancy |
| Complex process | Simple process |
| Slow growth of young plants to maturity | Rapid growth |
