10.3 Seed plants: gymnosperms and angiosperms  (Page 91/42)

Gymnosperms: cone bearing plants

Gymnosperms , meaning “naked seeds,” are a diverse group of seed plants and are paraphyletic. Paraphyletic groups are those in which not all members are descendants of a single common ancestor. Gymnosperm characteristics include naked seeds, separate female and male gametes, pollination by wind, and a vascular system. Gymnosperm seeds are not enclosed in an ovary; rather, they are exposed on cones or modified leaves.

Gymnosperms were the dominant phylum in Mesozoic era. They are adapted to live where fresh water is scarce during part of the year, or in the nitrogen-poor soil of a bog. Therefore, they are still the prominent phylum in the coniferous biome or taiga, where the evergreen conifers have a selective advantage in cold and dry weather. Evergreen conifers continue low levels of photosynthesis during the cold months, and are ready to take advantage of the first sunny days of spring. One disadvantage is that conifers are more susceptible than deciduous trees to infestations because conifers do not lose their leaves all at once. They cannot, therefore, shed parasites and restart with a fresh supply of leaves in spring.

The life cycle of a gymnosperm involves alternation of generations, with a dominant sporophyte in which the female gametophyte resides, and reduced gametophytes. The male and female reproductive organs can form in cones. The life cycle of a conifer will serve as our example of reproduction in gymnosperms.

Life cycle of a conifer

Pine trees are conifers (cone bearing). The male cones, or staminate cones give rise to pollen grains by meiosis. The pollen grain is the gametophyte and houses two sperm cells. In the spring, large amounts of yellow pollen are released and carried by the wind. Some gametophytes will land on a female cone. Pollination is defined as the initiation of pollen tube growth. The pollen tube grows from the pollen grain slowly, and one of the sperm cells will finally unite with an egg cell in the process of fertilization.

Female cones, or ovulate cones , contain two ovules per scale. The ovule is the female gametophyte. Upon fertilization, the diploid egg will give rise to the embryo, which is enclosed in a seed coat of tissue from the parent plant. Fertilization and seed development is a long process in pine trees: it may take up to two years after pollination.

[link] illustrates the life cycle of a conifer. The sporophyte phase is the longest phase in the life of a gymnosperm. The gametophytes are reduced in size.