GYMNOSPERM LIFE CYCLE Gymnosperms go through reproduction in their male- pollen- and female- seed-cones, produced by the mature sporophyte, or the new tree. The pollen cones produce the male gametophytes, or pollen grains, and make up the male gametophyte stage in the whole life cycle. Eventually, one of the haploid nuclei in a pollen grain will divide and produce two sperm nuclei. The seed cones produce the female gametophytes, and are often larger than the pollen cones. At the base of each cone scale, their are two ovules, as shown below. This is where the female gametophytes develop- meiosis produces haploid cells that eventually develop into the female gametophytes. Later on, these gametophytes may become hundreds or even thousands of cells, and produce several large egg cells when mature.
The cycle itself generally takes about two years to fully complete. It starts in spring when the male cones release large amounts of pollen, which are carried by the wind to reach female cones. Once there, the pollen grains land on some of the sticky substance- known as pollination drop- on a female cone's scales. The substance makes sure that the pollen grains do not leave the cone.
If the pollen grain is near an ovule, the grain will split and grow into a pollen tube containing two haploid sperm nuclei. Once this pollen tube reaches the female gametophyte, one of the sperm nuclei disintegrates while the other fertilizes the egg inside the female gametophyte. If there is a point where a sperm from another pollen tube gets to the female gametophyte, more than one egg cell may be fertilized. However, only a single embryo will develop. When fertilization occurs, a diploid zygote is produced, which grows into an embryo. It is now encased inside of a seed, which are all parts of three generations in the life cycle. The outer seed coating is the old sporophyte generation, the haploid cells around the embryo are the female gametophyte, and the embryo becomes the new sporophyte plant.
EXAMPLES OF GYMNOSPERMS Some examples of gymnosperms are shown below, such as gnetophytes (left), cycads (middle left), ginkgoes (middle right), and conifers (right).
COMPARE: BRYOPHYTES & PTERIDOPHYTES WITH GYMNOSPERMS Unlike pteridophytes and gymnosperms, bryophytes are non- vascular, meaning they transport water through their xylem and products of photosynthesis through their phloem. There are also a lot of similarities between pteridophytes and gymnosperms. Both types are vascular and heterospory- they have both male and female gametophytes on the same plant. They also have a life cycle dominated by the sporophyte stage, while bryophytes are gametophyte dominant. However, gymnosperms are the only one out of the three that use pollen for reproduction as the male gametophyte- bryophytes and pteridophytes use spores. Gymnosperms also have their pollen grains transported by wind and do not require water like bryophytes and pteridophytes do. Unlike bryophytes alone, gymnosperms have true roots, so they can live in dryer conditions. Their leaves also evolved into needles, which decrease water loss and contain a resin that acts as a natural antifreeze through the winter months.