Gymnosperms: Definition, Examples, and Reproduction

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Coniferophyta

The Coniferophyta division contains conifers, which have the greatest variety of species among gymnosperms. Most conifers are evergreen (retain their leaves throughout the year) and include some of the largest, tallest and oldest trees on the planet. Examples of conifers include pines, sequoias, firs, hemlock, and spruces. Conifers are an important economic source of lumber and products, such as paper, that are developed from wood. Gymnosperm wood is considered softwood, unlike the hardwood of some angiosperms.

The word conifer means "cone-bearer," a distinct characteristic common to conifers. Cones house the male and female reproductive structures of conifers. Most conifers are monoecious, meaning that both male and female cones can be found on the same tree.

Another readily identifiable trait of conifers is their needle-like leaves. Different conifer families, such as Pinaceae (pines) and Cupressaceae (cypresses), are distinguished by the type of leaves present. Pines have single needle-like leaves or needle-leaf clutters along the stem. Cypresses have flat, scale-like leaves along the stems. Other conifers of the genus Agathis have thick, elliptical leaves, and conifers of the genus Nageia have broad, flat leaves.

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Conifers are conspicuous members of the taiga forest biome and have adaptations for life in the cold environment of boreal forests. The tall, triangular shape of the trees allows snow to fall from the branches more readily and prevents them from breaking under the weight of the ice. The needle-leaf conifers also have a waxy coat on the leaf surface to help prevent water loss in the dry climate.

Cycadophyta

The Cycadophyta division of gymnosperms include cycads. Cycads are found in tropical forests and subtropical regions. These evergreen plants have a feather-like leaf structure and long stems that spread the large leaves out over the thick, woody trunk. At first glance, cycads may resemble palm trees, but they are not related. These plants can live for many years and have a slow growth process. The King Sago palm, for example, may take up to 50 years to reach 10 feet.

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Unlike many conifers, cycad trees either produce only male cones (produce pollen) or female cones (produce ovules). Female cone-producing cycads will only produce seeds if a male is within the vicinity. Cycads rely mainly on insects for pollination, and animals aid in dispersal of their large, colorful seeds.

The roots of cycads are colonized by the photosynthetic bacteria cyanobacteria. These microbes produce certain poisons and neurotoxins that accumulate in the plant seeds. The toxins are thought to provide protection against bacteria and fungal parasites. Cycad seeds can be dangerous to pets and humans if ingested.

Ginkgophyta

Ginkgo biloba are the only surviving plants of the Ginkgophyta division of gymnosperms. Today, naturally-growing ginkgo plants are exclusive to China. Ginkgoes can live for thousands of years and are characterized by fan-shaped, deciduous leaves that turn yellow in autumn. Ginkgo biloba are quite large, with the tallest trees reaching 160 feet. Older trees have thick trunks and deep roots.

Ginkgoes thrive in well sunlit areas that receive lots of water and have plenty of soil drainage. Like cycads, ginkgo plants produce either male or female cones and have sperm cells that use flagella to swim toward the egg in the female ovule. These durable trees are fire-resistant, pest-resistant, and disease-resistant, and they produce chemicals thought to have medicinal value, including several flavinoids and terpenes with antioxidant, anti-inflammatory, and antimicrobial properties.

Gnetophyta

The gymnosperm division Gnetophyta has a small number of species (65) found within three genera: Ephedra, Gnetum, and Welwitschia. Many of the species from the genus Ephedra are shrubs that can be found in desert regions of the Americas or in the high, cool regions of the Himalayan mountains in India. Certain Ephedra species have medicinal properties and are the source of the decongestant drug ephedrine. Ephedra species have slender stems and scale-like leaves.

Gnetum species contain some shrubs and trees, but most are woody vines that climb around other plants. They inhabit tropical rain forests and have broad, flat leaves that resemble the leaves of flowering plants. The male and female reproductive cones are contained on separate trees and often resemble flowers, though they are not. The vascular tissue structure of these plants is also similar to that of flowering plants.

Welwitschia has a single species, W. mirabilis. These plants live only in the African desert of Namibia. They are very unusual in that they have a large stem that remains close to the ground, two large arching leaves that split into other leaves as they grow, and a large, deep taproot. This plant can withstand the extreme heat of the desert with highs of 50°C (122°F), as well as the lack of water (1-10 cm yearly). Male W. mirabilis cones are brightly colored, and both male and female cones contain nectar to attract insects.

Gymnosperm Life Cycle

In the gymnosperm life cycle, plants alternate between a sexual phase and an asexual phase. This type of life cycle is known as alternation of generations. Gamete production occurs in the sexual phase or gametophyte generation of the cycle. Spores are produced in the asexual phase or sporophyte generation. Unlike in non-vascular plants, the dominant phase of the plant life cycle for vascular plants is the sporophtye generation.

In gymnosperms, the plant sporophyte is recognized as the bulk of the plant itself, including roots, leaves, stems, and cones. The cells of the plant sporophyte are diploid and contain two complete sets of chromosomes. The sporophyte is responsible for the production of haploid spores through the process of meiosis. Containing one complete set of chromosomes, spores develop into haploid gametophytes. The plant gametophytes produce male and female gametes which unite at pollination to form a new diploid zygote. The zygote matures into a new diploid sporophyte, thus completing the cycle. Gymnosperms spend most of their life cycle in the sporophyte phase, and the gametophyte generation is totally dependent upon the sporophyte generation for survival.

Gymnosperm Reproduction

Female gametes (megaspores) are produced in gametophyte structures called archegonia located in ovulate cones. Male gametes (microspores) are produced in pollen cones and develop into pollen grains. Some gymnosperm species have male and female cones on the same tree, while others have separate male or female cone producing trees. In order for pollination to take place, gametes must come into contact with one another. This typically occurs via wind, animal, or insect transfer.

Fertilization in gymnosperms occurs when pollen grains contact the female ovule and germinate. Sperm cells make their way to the egg inside the ovule and fertilize the egg. In conifer and gnetophytes, sperm cells have no flagella and must reach the egg via the formation of a pollen tube. In cycads and ginkgoes, the flagellated sperm swim toward the egg for fertilization. Upon fertilization, the resulting zygote develops within the gymnosperm seed and forms a new sporophyte.

Key Points

• Gymnosperms are flowerless, seed-producing plants. They belong to the subkingdom Embophyta. 
• The term "gymnosperm" literally means "naked seed."  This is because the seeds produced by gymnosperms are not encased in an ovary. Instead, gymnosperm seeds sit exposed on the surface of leaf-like structures called bracts.
• The four main divisions of gymnosperms are Coniferophyta, Cycadophyta, Ginkgophyta, and Gnetophyta. 
• Gymnosperms are often found in temperate forest and boreal forest biomes. Common types of gymnosperms are conifers, cycads, ginkgoes, and gnetophytes.

Sources

Asaravala, Manish, et al. “Triassic Period: Tectonics and Paleoclimate.” Tectonics of the Triassic Period, University of Califonia Museum of Paleontology, www.ucmp.berkeley.edu/mesozoic/triassic/triassictect.html.

Frazer, Jennifer. “Are Cycads Social Plants?” Scientific American Blog Network, 16 Oct. 2013, blogs.scientificamerican.com/artful-amoeba/are-cycads-social-plants/.

Pallardy, Stephen G. “The Woody Plant Body.” Physiology of Woody Plants, 20 May 2008, pp. 9–38., doi:10.1016/b978-012088765-1.50003-8.

Wagner, Armin, et al. “Lignification and Lignin Manipulations in Conifers.” Advances in Botanical Research, vol. 61, 8 June 2012, pp. 37–76., doi:10.1016/b978-0-12-416023-1.00002-1.​