Botany

Pollination and Fertilization in the Gymnosperm



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The gymnosperms are basically seed plants that are not flowering plants. As their name suggests, pollination and fertilisation of the gymnosperms involve seeds, resulting from pollen grains and ovules. There are four types of gymnosperms, the Cycadophyta, the Ginkgophyta, the Coniferophyta and the Gnetophyta. Both similarities and differences can be found in the way they are pollinated and fertilised.

General Characteristics

All seed plants are heterosporous with endosporic gametophytes, and thus they are associated with two evolutionary advances when compared with lower plants. The first one is that of the production of pollen grains and the second is the presence of ovules, which turn into seeds after fertilisation.

Pollen grains are the male spores or the microspores of the plant and they are associated with endosporic male gametophytes, or microgametophyte, meaning that the male gametes are enclosed within the spore. A pollen tube will germinate from the pollen grain to convey the sperm to the egg and pollen grain walls are made up of sporopollenin, a resistant material that preserves well and thus allow pollen fossil records to be kept. Since pollen grains of seed plants are adapted for transporting the sperm, pollination of them are usually accomplished by wind.

The ovule, on the other hand, is a multi-layered structure that is made up of reduced endosporic megagametophyte inside the megaspores, kept inside thick, fleshy megasporangium otherwise known as the nucellus. It is then protected by integuments with a small apical opening called the micropyle allowing the entry of the pollen tube. Several embryos can begin to develop within the same ovule though usually only one survives.

After fertilisation, a seed develops from the ovule. The integument becomes the seed coat and the zygote becomes the embryo, which is protected from environmental stresses and can be easily dispersed, with the ability to remain dormant until conditions become favourable for germination.

Cycadophyta

The Cycadophyta typically has male cones that are composed of microsporophylls, bearing numerous microsporangia and female cones composed of megasporophylls, with quite a few ovules. Pollens grains are deposited near the micropyle of ovules, where they germinate and produce pollen tubes, which grow through the nucellus, absorbing food along its way and as it nears the archegonium of the ovule, the tube bursts and releases a single motile sperm. Therefore, in this case the pollen tube does not bring the sperm the whole way to the egg.

Ginkgophyta

In Ginogophyta, fertilisation within ovules may not occur until after the ovules are shed from the trees. The male part of the plant is composed of stalked microsporangia while the female part of the plant is composed of paired ovules on short stalks. The micropyle secretes a sticky fluid when mature, allowing pollen grains to stick to it. Gradually, the sticky fluid dries up and the pollen grains are being drawn in via the micropyle. A pollen tube then develops and this time, two motile sperm cells are released. The pollen tube in this case also only conveys part of the way towards the egg.

Coniferophyta

An example of a genus in this phylum is the Pinus (pine trees). On a single tree, small clustered male cones are found on lower branches while larger solitary female cones are located on the upper branches. Pinus pollen takes the form of a highly reduced internal microgametophyte and has large air sacs, which gives low density to enhance dispersal by wind. In the female cones, four megaspores are produced and three of these four are aborted, with only one remaining which develops into two to three archegonia (eggs). In the case of the Conifers, the pollen tube discharges the sperm directly into the cytoplasm of the egg and though often more than one egg is often fertilised, normally only one develops.

Gnetophyta

There are three extant genera of Gnetophyta, Gnetum, Ephedra and Welwitschia. In Gnetum, the male strobilus is composed of layers of microsporangia and aborted ovules while the female strobilus is composed of whorls of ovules. There is no archegonium formed. The genus Ephedra is made up of separate male and female plants with their strobili appearing as series of bracts. In this case, archegonium is formed in the ovule. Welwitschia, on the other hand, comes with tiny male cones on short stalks and visibly larger female cones. No archegonium is formed in this case either.

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