Gymnosperms

Common name:         Gymnosperms

•   dʒɪmˈnoʊspərmz/ pronunciation

Conservation status: varies

• Today gymnosperms are the most threatened of all plant groups

Etymology:   

• The term gymnosperm comes from a composite word in Greek: γυμνόσπερμος (γυμνός, gymnos, 'naked' and σπέρμα, sperma, 'seed'), literally meaning 'naked seeds'

• The name is based on the unenclosed condition of their seeds 

Cones and seed:                            

• The non-encased condition of their seeds contrasts with the seeds and ovules of flowering plants (angiosperms), which are enclosed within an ovary

• Gymnosperm seeds develop either on the surface of scales or leaves, which are often modified to form cones, or on their own as in yew, Torreya, Ginkgo

• Gymnosperm lifecycles involve alternation of generations

  • They have a dominant diploid sporophyte phase and a reduced haploid gametophyte phase which is dependent on the sporophytic phase

  • The term "gymnosperm" is often used in paleobotany to refer to (the paraphyletic group of) all non-angiosperm seed plants; in that case, to specify the modern monophyletic group of gymnosperms, the term Acrogymnospermae is sometimes used

• About 65% of gymnosperms are dioecious, but conifers are almost all monoecious

• During pollination, pollen grains are physically transferred between plants from the pollen cone to the ovule

• Pollen is usually moved 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 fertilisation are found in gymnosperms:

  • Cycads and Ginkgo have flagellated motile sperm that swim directly to the egg inside the ovule

  • Conifers and gnetophytes have sperm with no flagella that are moved along a pollen tube to the egg

    • After syngamy (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

    • The mature seed comprises the embryo and the remains of the female gametophyte, which serves as a food supply, and the seed coat

Leaves:         

• Most conifers are evergreens

•  The leaves of many conifers are long, thin and needle-like,

• Other 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

Stem & branches: 

• Conifers are perennial woody plants

• Cycads: 

  • have soft and highly parenchymatous wood, which is poorly lignified

  • their main structural support comes from an armour of sclerenchymatous leaf bases covering the stem, with the exception of species with underground stems

Roots:         

• Some genera have mycorrhiza, fungal associations with roots (Pinus)

• Some others (Cycas) small specialised roots called coralloid roots are associated with nitrogen-fixing cyanobacteria

Habit:        

• Group of seed-producing plants that includes Conifers, Cycads, Ginkgo, and Gnetophytes

Habitat:   

• There are no herbaceous gymnosperms 

• Compared to angiosperms they occupy fewer ecological niches

• Have evolved:

  •  parasites (Parasitaxus)

  • epiphytes (Zamia pseudoparasitica) 

  • rheophytes (Retrophyllum minus) - a plant that lives in fast moving water

Distribution:                  

• A majority of cycads are native to tropical climates and are most abundantly found in regions near the equator

Species:                     

• World:            12 families,   83 genera  &  1000 species

• Australia:      S,    G

• 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)

• Cycads are the next most abundant group of gymnosperms, with two or three families, 11 genera, and approximately 338 species

• The other extant groups are the 95–100 species of Gnetales and one species of Ginkgo

Additional notes:     

• Diversity and origin

  • It was previously widely accepted that the gymnosperms originated in the Late Carboniferous period, replacing the lycopsid rainforests of the tropical region, but more recent phylogenetic evidence indicates that they diverged from the ancestors of angiosperms during the Early Carboniferous

  • The radiation of gymnosperms during the late Carboniferous appears to have resulted from a whole genome duplication event around 319 million years ago

  • Early characteristics of seed plants are evident in fossil progymnosperms of the late Devonian period around 383 million years ago

  • It has been suggested that during the mid-Mesozoic era, pollination of some extinct groups of gymnosperms was by extinct species of scorpionflies that had specialized proboscis for feeding on pollination drops

  • The scorpionflies likely engaged in pollination mutualisms with gymnosperms, long before the similar and independent coevolution of nectar-feeding insects on angiosperms

  • Evidence has also been found that mid-Mesozoic gymnosperms were pollinated by Kalligrammatid lacewings, a now-extinct family with members which (in an example of convergent evolution) resembled the modern butterflies that arose far later

• Life cycle

• Gymnosperms, like all vascular plants, have a sporophyte-dominant life cycle, which means they spend most of their life cycle with diploid cells, while the gametophyte (gamete-bearing phase) is relatively short-lived

• Like all seed plants, they are heterosporous, having two spore types, microspores (male) and megaspores (female) that are typically produced in pollen cones or ovulate cones, respectively

• The exception is the females in the cycad genus Cycas, which form a loose structure called megasporophylls instead of cones

• As with all heterosporous plants, the gametophytes develop within the spore wall. Pollen grains (microgametophytes) mature from microspores, and ultimately produce sperm cells

• Megagametophytes develop from megaspores and are retained within the ovule

• Gymnosperms produce multiple archegonia, which produce the female gamete

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• Genetics

  • The first published sequenced genome for any gymnosperm was the genome of Picea abies in 2013

• Uses

  • Gymnosperms have major economic uses. Pine, fir, spruce, and cedar are all examples of conifers that are used for lumber, paper production, and resin. Some other common uses for gymnosperms are soap, varnish, nail polish, food, gum, and perfumes

Pollination from an Evolutionary Point of View

 (Source: Pollination. E. Pacini, in Reference Module in Earth Systems and Environmental Sciences, 2015)

• Gymnosperm pollination is invariably anemophilous (primary, ie wind polinated); only recently evolved genera as Ephedra and Welwitschia are pollinated by insects

• There is general agreement that early angiosperms were pollinated by Coleoptera (beetles) and Diptera (flies)

• Woody and herbaceous secondary anemophilous angiosperms may descend from zoophilic species (pollen is transferred by animals)

• Hydrophily:

  • Probably derived from anemophily

  • Hydrophilic pollen of sea grasses, characterized by submarine pollination, is 2–3 mm long & a few dozen microns wide

  • The genus Callitriche (largely aquatic plants) has terrestrial, amphibious, and submerged freshwater species; their pollen is spherical and that of submerged species is devoid of exine, as in all species with submarine pollination.

• Competition to attract pollinators is high when many entomophilous species bloom at the same time and pollinators are few

• This is avoided by different blooming periods and anemophily. Examples of entomophilous families with anemophilous members are Ranunculaceae, Thalictrum; Euphorbiaceae, Mercurialis and castor bean (Ricinus); and Asteraceae, ragweed (Ambrosia) and Artemisia

• Few entomophilous species bloom at the same time in January and February in the Northern Hemisphere Mediterranean environments, when few insects are active

• Helleborus bocconei and H. foetidus grow in similar environments and share the same pollinator, but pollen attaches to different parts of the pollinator body, so that useless pollination is avoided

• Anemophilous species of Juniperus growing in the same environments disperse pollen in different periods.