By now you should be familiar with the alternation of generations present in all plants, and that the gametophyte (1n) and the sporophyte (2n) exist in multicellular forms. The gametophyte is the dominant form in nonvascular plants; the sporophyte is smaller and is nourished by the gametophyte as it develops. In seedless vascular plants and seed plants the sporophyte is the dominant form. This change in the dominant generation is depicted in the figure at right.
The second change in the alternation of generations is seen in the seedless vascular plants are most commonly represented by the ferns. For example, the ferns that are observed growing in the forest and the fern fronds (leaves) that are a part of floral arrangements are sporophytes (2n). The spores that are released from the undersides of fern fronds grow into gametophytes (1n), which live independently from the sporophyte. Note, the sporophyte is much larger than the gametophyte.
Seed plants represent the third modification of alternation of generations. Like the ferns, the sporophyte is the prominent generation. Unlike the ferns, however, seed plants have gametophytes that are surrounded by sporophytic tissue. This sporophytic tissue nourishes the gametophyte; therefore, the gametophyte does not live independently from the sporophyte and is even more reduced in size.
In addition to having more reduced gametophytes, seed plants use a different method to disperse their reproductive cells than do seedless vascular plants. A seedless vascular plant (e.g., a fern) releases haploid spores into the environment. If conditions are suitable, the spore grows into a mature gametophytic generation. If conditions are harsh, the spore will persist without germinating and will lie dormant until favorable conditions are present. The spore will then germinate into a gametophyte which will produce haploid gametes by mitosis. These gametes will then fuse to produce a zygote, which must live in a moist environment as it develops into an embryo and eventually a mature plant.
In the seed plants, the diploid zygote and the embryo that develops from it are contained within a seed. Many seeds are able to remain dormant until conditions are optimal for germination and growth. For example, some pine seeds actually require the heat of a fire to trigger germination. This is adaptive because just after a fire the seedling can grow quickly without competition from taller trees. Therefore, spores and seeds are similar because they both are resistant to harsh conditions.
However, spores and seeds differ in their structure and composition. Spores are unicellular, haploid, and contain little storage tissue. They are also very small and relatively simple. Seeds are multicellular, larger, and can contain a large amount of storage material.
Another major difference between the spores of seedless plants and seeds is that the haploid spores of seedless plants are released by the parent and develop independently, whereas seeds develop within the parental sporophytic tissue. Seed plants have female spores (megaspores) and male spores (microspores). The male microspore of a seed plant produces sperm within pollen grains , which are transported to the female megaspore. Seed plants have a haploid megaspore that is contained within a fleshy solid mass contained within an ovule . A seed is a fertilized mature ovule. There are also tissues in the ovule (integuments) that become the seed coat .
Therefore, while seedless vascular plants disperse their offspring via haploid spores, seed plants disperse their offspring via diploid seeds.
The sperm from seedless plants have flagella that propel them through water to reach the egg cell. This mechanism works well for plants in moist environments, however, these plants have a difficult time reproducing in drier environments. The pollen grain is an important adaptation to dry environments. Pollen is a tough structure that contains the precursor to sperm cells. The tough outer coat of the pollen grain is able to survive very harsh conditions, therefore, it can protect the sperm cells for years. When the pollen grain finally lands on the female structure of a plant, it germinates and sperm cells travel to the egg cell through a pollen tube . Thus, the male gamete is protected (rather than open to the environment, as in seedless plants). This adaptation allows seed plants to live in such dry and harsh conditions as deserts. Compare the figures below to see the difference in fertilization between seedless and seeded plants.