Plant Science 1203L Laboratory 5 - Sexual Reproduction (Pollination and Double Fertilization)
Today s lab is about sexual reproduction in plants. As with stem or root structure there are numerous definitions and terms to be aware of. Sexual reproduction in higher plants is similar to that in most of nature. Generally, the male gamete (sex cell) is transferred to the female gamete. In order for the gametes to be produced they had to undergo a change in chromosome number from 2N to N through the process of meiotic division. A significant difference between animal and plant sexual reproduction is the double fertilization process that occurs in plants. Double fertilization produces both and embryo and the nutritional tissue that are both packaged into the seed.
Terms and Definitions Gametes are the haploid (N) sexual reproductive cells in plants and are contained within the pollen as the sperm and the egg in the ovule.
Pollen contains the male gamete (sperm) while the ovules hold the female gamete (egg). Pollen is produced in the male flower structure known as the stamen which is composed of two parts, the filament and anther. The filament is a stalk like structure that supports the anther. The anther is composed of an elongated sac-like structure with four chambers in which pollen grains are produced. Each pollen grain will initially have two nuclei, one a generative cell and the other is a pollen tube nucleus. Upon germination the generative cell divides to produce two sperm for double fertilization. The tube nucleus directs the growth of the pollen tube while the two sperm will lag behind until the tube reaches the ovule.
Germinated pollen grain Pollen grains showing Anther in cross section generative and tube nuclei
The cell wall of pollen is composed of two layers, a inner layer termed the intine and an outer layer termed the exine (composed of a double layer of sexine and nexine) that surround and protect the generative cell and the tube nucleus. The shape or morphology of the pollen outer wall is very characteristic of a particular plant species and has been used by archeologists to determine what plants might have been present or used by ancient cultures.
Layers forming pollen CW Pollen from the same species Pollen from different species Pollination is the transfer of the male gamete (pollen) to the receptive surface (stigma) of the female pistil. Note! This simply is the movement of the male gamete to the female part of the flower and should not be confused with actual double fertilization. By receptive stigma we mean a compatible stigmatic surface that is from the same species of plant. Pollen landing on a receptive stigma will be stimulated to germinate and produce a pollen tube that will grow down through the stigma and style to the ovary where the female gametes are located. Pollen landing on the sigma of a different species would find the surface to be non-compatible (non- receptive) and will not germinate.
Types of Pollination - Some plants require that pollen from a different plant of the same species be transferred to their sigma for pollen germination to occur. This is termed cross pollination and usually assures genetic diversity in the offspring that are produced. Many plants (and crops such as rice or wheat) can utilize the pollen from the same flower or flowers of the same plant to achieve fertilization. This is termed self pollination. Finally when man intervenes and intentionally transfers pollen to a stigma it is termed artificial pollination.
Pollinating Agents (Vectors)- Wind can carry pollen great distances. Every spring, summer and fall pollen is often present in air samples depending on what species is blooming. Water can also act to move pollen to a receptive stigma. Insects such as the common honey bee as well as some other insects (butterflies or wasps) are responsible for carrying pollen from one plant to another. Animals such as birds and even bats can move pollen from plant to plant. Some plant species and many crops require cross pollination by bees or they do not produce. Bees receive nectar and often gather pollen for food while their movements scatter pollen from plant to plant. This shows how there are often tight associations between plant and animal commu nities. Egg/Ovule/Ovary - The pistil is the female flower structure that is composed of three identifiable external parts: the ovary, style and stigma. The ovary is an enlarged structure at the base which holds the ovules (eventual seed). Inside each ovule is a egg cell (gamete). Going upwards from the ovary is a stalk like structure termed the style. The tip of the style has a special surface known as the stigma that receives pollen during pollenation. The stigma has many finger-like projections coated with materials to which pollen sticks upon transfer.
Ovary in cross section Cross section of ovule Pollen on stigma surface
Some of these substances stimulate the pollen to undergo what is also termed germination. When the pollen germinates it produces a pollen tube that grows down through the style and into the ovary where it is directed to one of the many ovules usually present. Upon germination the generative cell divides and produces two sperm. The style also contains food materials that are possibly utilized by the pollen tube for its growth.
Below is a diagram that shows formation of male and female gametes. Double fertilization - Plants use a double fertilization mechanism (as opposed to single fertilization in animals). Double fertilization is the fusion of one of the two sperm with an egg to form the embryo followed by fusion of the other sperm with the polar nuclei of the egg to form the food storage tissue called endosperm. The pollen grain germinates and sends a pollen tube down through the style, stigma and into the locule of the ovary. The tube follows the funiculus around to the micropyle opening and into the egg sac where the sperm are released. One sperm fuses with the egg forming the embryo (2N) while the other sperm fuses with the two polar nuclei (N) forming a 3N nucleus that will form endosperm.
Cross section of a plant ovule
Answer the questions on the answer sheet and turn in at the next lab meeting. Laboratory 5 - Sexual Reproduction Answer Sheet Name______
1. Pollen and the egg are gametes. What are their chromosome number compared to non-reproductive plant cells?
2. What is the chromosome number of the endosperm tissue after fertilization?
3. Pollination is the same as fertilization (T or F). Defend your answer!
4. Which type of pollination would tend to show greater genetic diversity in the offspring?
5. Crops such as soybean, almonds and peaches can self pollinate and need no pollinating agents for assistance. (T or F). Defend you answer!
6. List 3 possible pollinating agents: a.______
b.______
c.______
7. Double fertilization require 2 separate pollen grains to participate. (T or F). Defend your answer!