B.Sc. B.Ed. Semester – IV

Course Instructor

Dr. Gautam Kumar

Plant Biodiversity

PTS 251

‘Gymnosperm’ comes from the composite Greek ‘gymnos’ means naked and ‘sperma’ means seed, literally meaning ‘naked seeds’.

Gymnosperms are flowerless plants that produce cones and seeds.

Seeds are not encased within an ovary. The unenclosed condition of their seeds (called ovules in their unfertilized state).

They seeds sit exposed on the surface of leaf-like structures called bracts.

Gymnosperm

Gymnosperms are vascular plants of the subkingdom Embyophyta and include conifers, cycads, ginkgoes, and gnetophytes.

Gymnosperms are abundant in temperate forest and boreal forest biomes with species that can tolerate moist or dry conditions.

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Gymnosperms are believed to be the first vascular plants to inhabit land appearing in the Triassic Period around 245-208 million years ago.

The development of a vascular system capable of transporting water throughout the plant enabled gymnosperm land colonization.

Gymnosperm wood is considered softwood, unlike the hardwood of some angiosperms.

Today, there over one thousand species of gymnosperms belonging to four main divisions:

i) Coniferophyta

ii) Cycadophyta

iii) Ginkgophyta

iv) Gnetophyta

Divisions

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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 Pines, Sequoias, Firs, Hemlock, and Spruces. Conifers are an important economic source of

timber and products, such as paper, that are developed from wood.

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.

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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.

Pinus sp. (Pine) have single needle-like leaves or needle-leaf clutters along the stem.

Cupressus sp. (Cypress) have flat, scale-like leaves along the stems.

Agathis sp.have thick, elliptical leaves.

Nageia sp. have broad, flat leaves.

Agathis sp.

Pinus sp. Cupressus sp. (Cypress)

Nageia sp.

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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.

Taiga forest biome

Triangular shape of conifers

Needle-leaf of conifers Dr. Gautam Kumar Dept. of Life Sc. 6

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 and long stems that spread the large leaves out over the thick, woody trunk.

Though cycads may resemble palm trees but they are not related. Cycads can live for many years and have a slow growth process eg.

King Sago palm,

Cycas sp.

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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.

Female cone of Cycas sp.

Cycad seeds can be dangerous to pets and humans if ingested

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Ginkgophyta

Ginkgo biloba is the only surviving member of the Ginkgophyta and therefore called living fossil.

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 grow in well sunlit areas that

receive lots of water and have plenty of soil drainage.

Ginkgo biloba

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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.

They produce chemicals, including several flavinoids and terpenes with antioxidant, anti-inflammatory and antimicrobial properties.

These durable trees are fire-resistant, pest-resistant and disease-resistant.

Ginkgo male cone

Ginkgo female cone Dr. Gautam Kumar Dept. of Life Sc. 10

Gnetophyta

Gnetophyta has a small number of species (65) found within three genera: Ephedra, Gnetum and Welwitschia.

Ephedra sp.

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.

Ephedra sp.

Ripe female cones of Ephedra sp. Dr. Gautam Kumar Dept. of Life Sc. 11

Gnetum sp. 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 vascular tissue structure of these plants is also similar to that of flowering plants.

The male and female reproductive cones are contained on separate trees and often resemble flowers, though they are not.

Female cone of Gnetum sp. with seed

Male cone of Gnetum sp.

Seed of Gnetum sp. Dr. Gautam Kumar Dept. of Life Sc. 12

Welwitschia sp. 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 as well as the lack of water (1-10 cm yearly).

Male cones are brightly coloured and both male and female cones contain nectar to attract insects

Welwitschia sp.

Male cones of Welwitschia sp. Dr. Gautam Kumar Dept. of Life Sc. 13

Life cycle of gymnosperms

In the gymnosperm life cycle, plants alternate between a sexual 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 sporophtyic generation.

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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.

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Reproduction in gymnosperms

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.

Female cone

Male cone

Some gymnosperm species have male and female cones on the same tree (monoecious), while others have separate male or female cone producing trees (dioecious).

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For pollination, gametes must come into contact with one another, which 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.

Upon fertilization, the resulting zygote develops within the gymnosperm seed and forms a new sporophyte.

In cycads and ginkgoes, the flagellated sperm swim toward the egg for fertilization.

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References

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