Coniferophyta Sexual Cycle - 1
Coniferophyta Sexual Cycle - 1
Conifers are Gymnosperms.
Gymnosperm means Naked Seed
Seeds develop from Ovules
An Ovule is a Megasporangium covered
by an adherent
Integument
We will use Podocarpus as an example of an Ovule because their ovules are produced at the tips of small stems and are easy to see.
 |
 |
The Ovule is diploid at first. It consists of an Integument and Nucellus. The Integument has an opening (Micropyle) at one end. One Nucellus Cell undergoes Meiosis to produce 4 Megaspores. This is the beginning of the Megagametophyte. Three Megaspores die. The remaining Megaspore undergoes Mitosis and forms the Gametophyte. The Gametophyte starts to degrade the Nucellus. This continues until most of the Nucellus is replaced by the Megagametophyte. Archegonia develop at the Micropylar end of the Megagametophyte. These produce relatively Large Eggs.
|
|
When pollen is deposited at the Micropyle it is drawn inside by a Pollination Droplet. It Germinates and produces a Pollen. Tube which grows towards the opening of the Archegonia. A Pollen Tube releases its sperm when it arrives at the Neck of an Archegonium. The Sperm do NOT have Flagella and do not need to swim through water to achieve Fertilization. This is an extremely important adaptation that helps to insure that each Ovule will get fertilized. |
 Inverted Ovule |
These plants invest a lot of energy in the Ovule. This will help to insure that a viable plant develops from each ovule. The direct method of sperm delivery to the Archegonia helps to insure that each Ovule will have at least one fertilized Egg. |
|
