How Gynosperms Are Developed For Water Conservation Biology Essay


When we look at a coniferous tree, or admire the age of a Great Redwood, we subconsciously recognise a botanic group known as the gynosperms. The group includes the largest tree and the oldest tree and the slowest growing plant on earth. It includes the enigmatic Ginko tree which is a sole species survivor from the Jurassic age when dinosaurs roamed the Earth share belong to this group. It also includes the cycad ferns that can be found in tropical and subtropical climes.

Despite their wide variety of forms, the gynosperms share a common feature. Rather than produce flowers, these plants produce naked seeds. The seeds can be exposed in a wide variety of structures including the pine cone with which we are all familiar. The group name Gynosperm is actually derived from a Greek word that means naked seed.

The gynosperms evolved some time between the arrival of primitive plants such as ferns and the mosses and the flowering plants that we know and love. Historically the gynosperms are first seen in the fossil record some 370 million years ago during the Devonian era. Although scientists are still debating whether the gynosperms and flowering plants share a common ancestor, what they have found is intriguing. Despite their wide variety of forms, and the individual success of some species of some species such as the conifers that have spread in large numbers, the gynosperms are not represented by a great many species. Less than 1,000 species of gynosperms are known, and of those over 50% are conifers, 3% Cycads and only one is a Ginko. The gynosperms, particularly the conifers, are remarkable because they have been so successful, having spread in vast coniferous forests across the globe with comparatively little evolutionary change. The conifers that we recognise today are believed to have evolved in the Triassic era some 240 million years ago when conditions were much drier than in previous times.

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The conifers might be regarded as the highest form of development among the gynosperms. Biology and geography text books often discuss how conifers have adapted to the severe drought conditions that are found within the taiga of northern Canada and Siberia. During winter when the ground is frozen the conifer has little access to liquid water and is exposed to drying winds. The conifer has developed tough narrow leathery leaves to endure the drying winds.

Less well known are the water conservation features than are common to all gynosperms. Although the gynosperms are found in many climatic zones including temperate forests and subtropical and tropical zones where water is plentiful these features help explain how it is that the gynosperms have been able to colonise two of the most arid conditions on earth, the taiga and the Namibian desert.

Crucially, the gynosperms do not require water for fertilization and they produce seeds. The male plant releases pollen which is transported by wind or insect. When the pollen lands on the female pine cone fertilization occurs when the sperm penetrates the egg. A seed may take up to three years to mature on the female plant. The seed is packed with foodstuffs which will support a young plant in harsh conditions. It also enables the plant to postpone germination if conditions are harsh.

The gynosperms also have a very advanced system of vascular bundles that allow water and food transport across the plant. These systems are markedly advanced compared with their predesecors among the ferns and mosses. The gynosperms deposit lignin within the cell walls. This protects against water loss from individual cells and provides strength to the extent that large tree structures are possible. Those conifers that endure very harsh conditions, such as the larch which is able to resist very harsh conditions in Siberia are also able to deposit an anti-freeze into the bundles to enable fluids to continue to flow during the winter.

Perhaps the most remarkable gymnosperm is the Welwitschia Mirabilis from the division Gnetophyta. These are extremely long lived and slow growing plants that survive in the extremely arid conditions of the Namib desert in which the rainfall is less than 10mm (0.4 inches) per year. The plant produces just two leaves which continue to develop for a thousand years and has a deep tap root. The leaves contain special cells that can extract moisture from the coastal sea fogs. The remarkable existence of the Welwitschia is such that it celebrated on the Namibia coat of arms.

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Welwitschia is notable for its survival in the extremely arid conditions in the Namib, sometimes deriving moisture from the coastal sea fogs.

It has less than 10 mm (0.4 inches) of rain annually and is almost completely barren.


Vegetative reproduction

Seed development takes a long time - 3years are all heterosporous: the microspores are shed as pollen, whereas the megaspore germinates in the strobilus to produce the female gametophyte. The archegonia in this gametophyte get fertilized by sperm from the male gametophyte and the zygote grows to produce an embryo which is enclosed in a seed coat of tissue from the parent plant.

Oldest living things


Coniferous forests

 Ginkgo (one species)


 Cycas group (one genus in one family)

 Zamia group (one or two families)


 Pine-like conifers

 Pine group (one family)

 Sciadopitys (one species in one family)

 Podocarps (one or two families)

 Araucarias (one family)

 Cephalotaxus (one genus in one family)

 Cypress group (one or two families)

 Yew-like conifers (one family)


 Gnetum (one genus in one family)

 Ephedra (one genus in one family)

 Welwitschia (one species in one family)

Gymnosperms are woody plants, either shrubs, trees, or, rarely, vines (some gnetophytes). They differ from flowering plants in that the seeds are not enclosed in an ovary but are exposed within any of a variety of structures, the most familiar being cones

Gymnosperms include many of the largest trees on earth. Ginkgo can grow very large. Among the conifers, Sequoiadendron giganteum is the largest of all trees

Facts about gymnosperm: evolution, as discussed in community ecology: A period of extensive glaciation and drought: The Permian Period:

...(251 to 65.5 million years ago) and possibly even the flowering plants (see angiosperm: Paleobotany and evolution). By the end of the Permian, gymnosperms (seed plants whose seeds lack a covering) such as ginkgoes and early conifers had appeared. By the Early Triassic they had become widespread in drier environments that other plants could...

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The term "gymnosperm" comes from the Greek word gymnospermos (γυμνόσπερμος), meaning "naked seeds", after the unenclosed condition of their seeds (called ovules in their unfertilized state). Their naked condition stands in contrast to the seeds or ovules of flowering plants (angiosperms) which are enclosed during pollination. Gymnosperm seeds develop either on the surface of scale- or leaf-like appendages of cones, or at the end of short stalks (Ginkgo).

The gymnosperms and angiosperms together comprise the spermatophytes or seed plants. By far the largest group of living gymnosperms are the conifers (pines, cypresses, and relatives), followed by cycads, Gnetales (Gnetum, Ephedra and Welwitschia), and Ginkgo (a single living species).


[edit] Classification

Further information: Spermatophyte

In early classification schemes, the gymnosperms (Gymnospermae) were regarded as a "natural" group. There is conflicting evidence on the question of whether the living gymnosperms form a clade[1] although according to some recent analyses of molecular data the living gymnosperms do appear to be monophyletic.[2]. The fossil record of gymnosperms includes many distinctive taxa that do not belong to the four modern groups, including seed-bearing trees that have a somewhat fern-like vegetative morphology (the so-called seed ferns or pteridosperms.)[3] When fossil gymnosperms such as Bennettitales, Caytonia and the glossopterids are considered, it is clear that angiosperms are nested within a larger gymnosperm clade, although which group of gymnosperms are their closest relatives remains unclear.


[edit] Diversity and origin

It is widely accepted [4] that the gymnosperms originated in the late Carboniferous Period. Early characteristics of seed plants were evident in fossil progymnosperms of the late Devonian period around 380 million years ago. It has been suggested that during the mid-Mesozoic period, pollination of some extinct groups of gymnosperms were by extinct species of scorponflies that had specialized proboscis for feeding on pollination drops. The scorponflies likely engaged in pollination mutualisms with gymnosperms, long before the similar and independent coevolution of nectar-feeding insects on angiosperms.[5][6]

Conifers are by far the most abundant extant group of gymnosperms with six to eight families, with a total of 65-70 genera and 600-630 species (696 accepted names).[7] Conifers are woody plants and most are evergreens.[8] The leaves of many conifers are long, thin and needle-like, others species, including most Cupressaceae and some Podocarpaceae, have flat, triangular scale-like leaves. Agathis in Araucariaceae and Nageia in Podocarpaceae have broad, flat strap-shaped leaves.

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Cycads are the next most abundant group of gymnosperms, with about 130 species. The other extant groups are the 75 - 80 species of Gnetales and one species of Ginkgo.

Life cycle

Gymnosperms are spore-bearing plants (sporophytes), with a sporophyte-dominant life cycle; as in all other vascular plants the gametophyte (gamete-bearing phase) is relatively short-lived. Two spore types, microspores and megaspores, are generally produced in pollen cones or ovulate cones, respectively. A short-lived multicellular haploid, gamete-bearing phase (gametophyte) develops inside the spore wall. Pollen grains (microgametophytes) mature from microspores, and ultimately produce sperm cells; megagametophyte tissue develops in the megaspore of each ovule, and produces multiple egg cells. Thus, megaspores are enclosed in ovules (unfertilized seeds) and give rise to megagametophytes and ultimately to egg cells. During pollination, pollen grains are physically transferred between plants, from pollen cone to the ovule, being transferred by wind or insects. Whole grains enter each ovule through a microscopic gap in the ovule coat (integument) called the micropyle. The pollen grains mature further inside the ovule and produce sperm cells. Two main modes of fertilization are found in gymnosperms. Cycads and Ginkgo have motile sperm that swim directly to the egg inside the ovule, while conifers and gnetophytes have sperm with no flagella that are conveyed to the egg along a pollen tube. After fertilization (joining of the sperm and egg cell), the zygote develops into an embryo (young sporophyte). More than one embryo is usually initiated in each gymnosperm seed. Competition between the embryos for nutritional resources within polyembryonic seeds produces programmed cell death to all but one embryo. The mature seed comprises the embryo and the remains of the female gametophyte, which serves as a food supply, and the seed coat (integument).[9]