Gymnosperms, like many plants, experience alternation of generations, which means their life cycles include both diploid and haploid stages. In the diploid stage, cells have two sets of chromosomes, and in the haploid stage they have only one. Gymnosperms take a uniquely long time to reproduce, since it often takes over a year from the time pollination occurs until fertilization is complete. Once seeds are produced, some species may hold their seeds until very specific conditions are met -- and even then, they can lie dormant for several more years before germinating.
Development of Egg and Sperm Cells in Gymnosperms
The male diploid gametophyte in the world of gymnosperms is a pollen grain with two sets of chromosomes called a microspore. A gametophyte gives rise to the gametes, or sex cells. Microspores are stored in specialized leaves called sporophylls, groups of which are formed into pollen cones. The female diploid gametophyte is called a megaspore. The sporophyll storing the megaspore makes up a single scale on a pinecone. Both the microspore and megaspore develop into haploid gametes -- egg and sperm cells -- after undergoing meiosis.
Pollination Leads to Fertilization
Haploid microspores are released into the air as pollen. When the pollen lands on an ovulate cone, a pollen tube forms and the nucleus of the sperm cell discharges through the pollen tube into the haploid female gametophyte containing the egg. Fertilization occurs when the haploid egg and sperm cells combine to form a diploid embryo, which will have one set of chromosomes from the male contributor and one set of chromosomes from the female contributor. Fertilization usually occurs more than a year after pollination.
Seed Development and Dispersal
In pines, the pine embryo is the new sporophyte. It contains a rudimentary root and some embryonic leaves called cotyledons. The female gametophyte surrounds the embryo and provides a food supply as it develops. This ovule forms the pine seed, which contains the embryo, its food supply, and a protective seed coat that forms from the integuments of the parent sporophyte. Under proper conditions, pine cone scales open to release their seeds. Some pine seeds are winged and can be dispersed by wind, while others require high heat, such as a forest fire, to open and release their seeds. Still others will readily drop the seeds once they are mature.
Germination and Plant Growth
The ripe seed must be exposed to the proper conditions in order to germinate. In some species, mature seeds may lie dormant for years, ready to germinate when they have adequate moisture, the proper temperature, adequate gas exchange and exposure to sunlight. In pines, once the seed germinates, it forms a pine seedling that grows into a mature pine tree, and the cycle starts again.